CALIFORNIA AGRICULTURAL EXTENSION SERVICE CIRCULAR 61 April, 1932 POTATO PRODUCTION IN CALIFORNIA D. R. PORTER Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. THE COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA Digitized by the Internet Archive in 2011 with funding from University of California, Davis Libraries http://www.archive.org/details/potatoproduction61port POTATO PRODUCTION IN CALIFORNIA D. It. PORTERi INTRODUCTION The potato is one of our staple food crops, and although it is gen- erally conceded to thrive best in regions having a cool growing season, high acre yields are, as a matter of fact, produced in some of the interior valleys in California where the average air temperature dur- ing the growing season is relatively high. Such large acre yields are the direct result of proper culture, fertile soil, good seed, and disease control. TABLE l California Potatoes: Acreage, Yield per Acre, and Carlot Shipments, 1920 TO 1930 California acreage Yield per acre in 100-pound sacks Carlot shipments, California Per cent of California Year Area in acres Per cent of U. S. total shipments California United States from Delta region 1920 70,000 17.8 84 66 10,058 46 9 1921 74,000 18.7 84 55 9,000 45 7 1922 76,000 17.6 78 63 8,215 56.3 1923 52,000 16 3 90 65 6,765 56 4 1924 46,000 13 7 96 76 6,247 63.8 1925 43,000 13 8 95 63 6,169 65.2 1926 43,000 13.8 96 68 7,354 63.1 1927 52,000 14 9 91 69 7,323 69.1 1928 56,000 14 6 83 76 7,410 65 3 1929 35,000 10 4 90 67 7,769 78.5 1930 35,000 10.3 99 68 8,281 69.6 In 1929 and 1930, approximately 35,000 acres of potatoes were grown in California. As indicated in table 1, the annual acreage has decreased by 50 per cent since 1920. This decrease has been brought about by substitution of other crops, by failure to control successfully certain diseases and insect pests, and possibly by the development, in certain sections, of cultural practices which make possible high acre yields and low cost per unit of production. In Los Angeles County carlot shipments dropped from 1,532 in 1920, to 172 in 1928, and from 2,780 to 349 in Sacramento County. Similar decreases are recorded for the counties of Contra Costa, Santa Cruz, Yolo, Inyo, Orange, Humboldt, and Santa Barbara. At one time the Salinas Val- ley and the American River bottomland near Sacramento had many thousands of acres in potatoes. 1 Assistant Olericulturist in the Experiment Station. California Agricultural Extension Service [Om.61 In 1930 the California potato acreage was 10.3 per cent of that of the United States ; the acre yield was 99 one-hundred-pound sacks per acre, while the average yield for the United States was only 68. TABLE 2 California Potatoes: Percentage of Total Carlot Shipments by Months from Southern, Central, and Northern Districts, 1928 to 1930, Inclusive Month Southern district 1928 1929 1930 Central district 1928 1929 1930 Northern district 1928 1929 1930 January February... March April May June July August September October November. December. Total 1.4 2.9 5.6 1.2 14 30 4 45.2 113 6 0. 5 51 38.8 4 4 100.0 100 100.0 100 100.0 4 6 5 9 4 7 4 22.4 27.6 2 8 7.8 7.7 4.7 12.2 100.0 100.0 100.0 10.5 13 100.0 TABLE 3 California Potatoes: Monthly Shipments 1927 to 1930, Inclusive Month 1927 1928 1929 1930 Average, 1927-1930, inclusive January February March April May June July August September. October November.. December.. Total for each year.. cars 412 263 103 109 151 882 939 1,273 1,041 745 744 661 7,323 cars 688 558 282 127 445 974 607 678 794 828 711 718 7,410 836 534 406 79 137 614 932 1,052 986 897 690 607 7,769 cars 644 510 509 148 810 1,286 545 884 1,016 777 652 500 8,281 cars 645 466 325 116 386 939 756 972 959 812 699 622 per cent 6.1 4.2 1.5 5.0 12.2 9.8 12.7 12.5 10.5 9.1 8.1 Although the average yield per acre in California in 1930 was 99 sacks, much higher yields are recorded by growers in certain sections, particularly in the Delta region, in Kern County, and in certain areas in southern California, In these districts, yields of 200 to 300 sacks 1932 J Potato Production in California 5 per acre are not uncommon. Although the data in table 1 show that carlot shipments have decreased since 1920, they are not an accurate index of the importance of the crop, because an increasing proportion moves to local market by motor trucks. Potatoes are shipped from California during every month (tables 2 and 3). As is indicated in table 2, heaviest shipments are made from the southern district in June and July ; from the central district from May to November ; and from the northern district from July to January. The winter crop of 'new' potatoes from San Diego County and the Colma section begins to reach the market in February. Move- ment from the Shafter district usually begins late in May, whereas active movement from the Delta region usually begins in late July or in early August. In 1927 and 1928, the respective shipments to points out of the state were 1,044 and 1,043 carloads. The heaviest shipments were made to Texas, Oregon, Washington, and Utah; but smaller ship- ments were made as far east as Syracuse, New York, and Shreveport, Louisiana. THE POTATO DISTRICTS OF CALIFORNIA A number of more or less isolated districts have developed exten- sive potato production within the state, each district having certain advantages as to soil, climate, or shipping season. Cultural methods, varieties, and time and methods of marketing differ widely in the different districts. The rather exacting requirements of the potato plant as to soil and climate make it unwise to attempt commercial potato production in new or untried districts except in a small or experimental way until conditions have been found suitable for the crop, and until the season of maturity and the marketing facilities have proved to be such that the product can be disposed of advantageously. The principal districts are discussed briefly below. Several small districts in San Diego County specialize in winter potato production. Seed harvested in August is planted in November, and the crop is harvested from February to April. British Queen, because of its vigorous growth during the cool winter months, has been the principal variety used. White Rose is also being planted. The crop of this district is usually dug when only partly developed. The product is marketed in lugs and shipped by express or by truck to California markets, or sacked for long distance shipment in carlots. In the Colma district, adjoining San Francisco on the south, potatoes are grown both as winter and early summer crops. Approxi- mately 2,000 acres constitute the winter crop, which is planted from 6 California Agricultural Extension Service [ 0ib - 61 November to February and dug from March 15 to July. About 90 per cent of the winter crop is of the variety Garnet Chili, perhaps 10 per cent being Bliss Triumph. Seed is usually obtained from Oregon, and 200 sacks per acre is considered a good yield. The mild winter climate on the hillsides near the ocean permits the culture of potatoes without damage from frost; but the lower, more level land is usually planted later, in order to escape possible frost damage. Such a crop is machine planted and dug, whereas the crop on the hillsides is planted, cultivated, and dug by hand because the land is usually too rolling to permit machine culture. The earlier harvests are of partly grown tubers, which are sold in nearby markets in lugs ; but, as the crop matures, shipments are made into the Pacific North- west or the Middle West (Chicago, Omaha, Denver, and Salt Lake City) in 50 or 100-pound bags. The early summer crop is planted with the varieties Garnet Chili and British Queen; but the acreage is much less than that of the winter crop, and the price received for summer potatoes is usually less than that for winter. Such virus diseases as mosaic, leafroll, and spindle tuber are common to certain poor seed stocks. Late blight is likely to cause damage during late March and April, and control is often attempted by spraying with bordeaux mixture. The installation of vacuum fumigation plants for tuber-moth control has aided in expanding carlot shipments into states having quarantine against this insect. Growers generally realize that the tuber quality of Bliss Triumph is higher than that of Garnet Chili, but believe that the former requires more water to produce a satisfactory yield than the latter. The Triumph matures earlier than the Garnet Chili and is very similar in tuber characteristics. Growers in the Colma section might wisely experiment with the Bliss Triumph variety, for certified seed may now be obtained from a few California growers as well as from the more northern states. The Shafter-Wasco district, in Kern County, has developed rapidly in recent years. The sandy soils and warm spring weather are favorable for the production of standard varieties for shipment in early summer. White Rose is the variety most extensively used, seed being planted in January and February and maturing in May, June, and July. Late plantings are exposed to injury by heat or by tuber moth. Shipments are mostly sacked, in carlots, although there are some extra early shipments in lug boxes. 1932] Potato Production in California 7 Some loss is caused by common scab, rhizoctonia, nematodes, and tuber moth; and some of the poorer seed stocks which have severe infection with certain virus diseases often produce a very low yield. In Marin and Sonoma counties the crop is grown on rather rough land, without irrigation. Most of the commercial crop is of the British Queen variety, which is well adapted to this coastal district but not popular on the large markets. Climatic conditions favor high quality, especially for seed purposes; but soil conditions appear to be respon- sible for the rather low yield. In Marin County a new seedling potato known as Farley's Marin is being grown to some extent. Its type resembles the Burbank, and it is a better boiler than British Queen, yielding about as well, and finding a ready market locally. In San Bernardino County most of the commercial acreage is grown on sandy loam soil in the vicinity of Chino, Colton, and San Bernardino. White Kose is the most widely used variety, being planted both as a spring and as a fall crop. In 1924 the acreage totaled 859, while in 1929 it was 1,574. In Riverside County a large per cent of the annual crop is pro- duced in the vicinity of Temecula, Elsinore, Perris, Moreno, and Riverside. White Rose is the leading variety, 687 acres being grown in 1928, and 1,602 in 1929. Most of the crop is planted in early spring, and the late crop is used mainly for seed. Seed disinfection for the control of common scab and rhizoctonia is a common practice. Some damage is caused by the flea beetle and tuber moth. Carlot shipments from Los Angeles County declined from 2,226 in 1921 to 172 in 1928, while the area planted decreased from 9,973 acres in 1919 to 2,774 in 1929. Much of the crop in this county is consumed locally, moving to market in lug boxes by motor truck. White Rose is the leading variety. In 1929 there were grown in San Joaquin and Contra Costa counties (comprising the Delta district) 12,885 acres, or approxi- mately 40 per cent of the state acreage. High soil fertility, adequate irrigation, and close planting combine to make possible high acre yields. The soil used for potatoes in the Delta region consists largely of reclaimed peat land. Although exploitation of the raw land by some growers has led to a decline in yields, the more successful growers, who apply intelligent methods of production, have been able to increase the acre yields materially. In some cases growers have found it necessary to devise specially constructed machinery to handle the crop. Irrigation methods in the Delta are unique. The "islands" are surrounded by levees which protect the land from 8 California Agricultural Extension Service [Cm. 61 overflowing water from the canal or river. Water is siphoned over the levee through 18 or 24-inch pipes into permanent head ditches, and is distributed from the head ditches into smaller temporary lateral ditches which run parallel with the potato rows. These laterals are located about 60 feet apart and are dug with a special ditching machine after the crop is planted. Usually they are about 8 inches wide and 2 feet deep. When the laterals are filled, the water percolates through the soil, reaching the middle of the checks in a day or two, after which the level of the water is held about constant in the ditches, about 12 to 18 inches below the soil surface, for a period of about 60 days. Thus the potato plants have an abundant and continuous supply of water, their response being rapid growth and heavy set of tubers. When the crop is nearly mature the water is drawn off, a procedure allowing the top soil to dry out somewhat and making conditions more favorable for maturation and harvesting. Electrically driven centrifugal pumps, located on the levee, lift the water from the field back into the river or canal when drainage is desired. White Rose and Burbank are the most extensively planted varie- ties. The seed is either grown locally as a fall crop or imported from Minnesota or Oregon. The amount of seed used per acre ranges from 10 to 20 sacks; the rows are 28 to 36 inches apart; and the distance between hills is from 9 to 12 inches. Acid phosphate and potash fertilizers are usually applied to the soil, either before planting or as a side dressing when the plants are small. That environmental conditions in the Delta are conducive to high acre yields is apparent when one learns that in two instances approximately 1,000 bushels of No. 1 potatoes were harvested from a measured acre of land. One of the most pressing problems in the Delta is the prevention of loss in yield because of the many virus diseases, scab and black scurf. Another is to determine whether or not good seed can be produced locally or whether such seed can more profitably be imported. Investigation of these two problems is under way at present. Other smaller districts include Butte and Humboldt counties, where British Queen, Bliss Triumph, and White Rose are grown ; and Modoc, Lassen, and Inyo counties, on the east side of the Sierras, where a considerable quantity of the Netted Gem variety is grown for shipment. 1932] Potato Production in California VARIETIES Only a few really distinct varieties are grown in California, though a confusing array of names is applied to them. Below are given in some detail the characteristics of the leading varieties. The best known name is recommended for varieties that seem to be identical. General adoption of such standard varietal names will help to avoid confusion in buying seed. The importance of knowing the character- istics of the leading varieties should be emphasized if for no purpose other than that of recognizing and discarding varietal mixtures from seed stocks. White Rose, the main variety for southern and central California, is a medium-early variety, being earlier than the Burbank or British Queen, but later than Irish Cobbler or Idaho Rural. The plants are - of medium size, erect, with many well developed lateral branches from the main stem, a feature which gives the plant a spreading appearance late in the season. The stems are bright green in color. The flowers are white. The tubers are white, have few eyes, and are rather long and flat, being rounded at the apical end, but often pointed at the stem end. The White Rose can be distinguished from the Burbank in that the tuber is flatter and wider in the central part, while the Burbank is usually more cylindrical and of about the same width throughout its length. Soil conditions, however, often determine the shape and smoothness of any tuber. When grown under favorable soil and climatic conditions, the White Rose has smooth, evenly shaped tubers of attractive appearance for market. Experiments indicate that the natural rest period for the tubers of this variety is relatively short, a fact, making it well adapted to the practice of growing two crops a year. In Wisconsin and Minnesota, White Rose is generally known as Late Pride and many carloads of seed are shipped to California from these states under the latter name. Burbank was long the standard variety in the Delta district, but is now giving way to the White Rose type. It is still grown exten- sively in some of the coastal and interior districts of northern Cali- fornia, Three strains are grown : Low Top Burbank, Pride of Mult- nomah, and High Top Burbank. The tubers of these strains are alike, but the tops increase in size in the order in which they are mentioned. The plants are more erect and have larger, coarser leaves than the White Rose. The flowers are white. The tubers are long, slightly flattened or cylindrical, have a large number of eyes, and 10 California Agricultural. Extension Service [Cm. 61 are slightly pointed at both apical and basal ends. Burbank is a late variety. Russet Burbank is a late-maturing variety more generally known as Netted Gem. It is little grown in California except at the higher elevations east of the Sierras, tho-ugh it grows very well in other sections. It has been found to produce especially well in the peat soils of the Delta district, though it has not yet been grown there on a large scale. The plants and tubers resemble the Burbank variety ''■-■' - > V Fig. 1. — On the left, abnormal tuberization characteristic of the British Queen variety when grown in warm sections. On the right, formation of vegetative sprouts from nearly mature tubers. This occurs on many varieties when the moisture supply is irregular and the temperatures are very high as the crop matures. (From Cir. 287.) except that the tubers are covered with a dense, brownish, corky netting. This netting is less marked when the potatoes are grown in peat or muck soils. A serious defect in both the Netted Gem and the smooth-skinned varieties of Burbank is the tendency for the tubers to produce knobby second growth when raised on heavy, uncongenial soils or with an irregular moisture supply. Garnet Chili is probably the oldest variety now raised in America. In California it is grown for early market in the Colma district near San Francisco. In time of maturity it is relatively late, but when 1932] Potato Production in California 11 planted in the fall it produces a fair crop in early spring and hence, so far as the market is concerned, is considered an early potato. The plants are tall, erect, and rather spindling" in habit of growth. The stems and flowers are purplish. The tubers are slightly elongated or nearly round, and tend to have sunken eyes. The skin is smooth and of a light pink color early in the season but, as maturity approaches, becomes rougher and darker. British Queen is a standard variety for coastal districts from San Francisco north to Washington. It is more strictly limited by its climatic requirements than most other varieties. Only in a constantly cool, humid climate will the plant and tubers develop normally. In the interior valleys and in the warmer parts of the south, tubers do not develop satisfactorily (fig. 1). Under irrigated conditions in warm sections, the plants may grow rapidly all season with the forma- tion of very few marketable potatoes. The plant is a relatively vigorous grower, tall, erect, and dark green. The blossoms are usually profuse and are purple in color. The tubers are slightly elongated, slightly flattened, and smooth, with both stem and apical ends somewhat square and blocky. SELECTION OF SEED? Much of the success of growing a crop of potatoes depends upon securing 'good seed.' Just what constitutes good seed potatoes is, however, difficult to define. Few commercial potato growers in California produce their own seed, and perhaps in the majority of cases it is best that they do not. The larger part of the potato acreage is planted with seed purchased from another grower, from a dealer, or from a seedsman. Seed so obtained is generally of unknown history and quality, except as to its appearance when received, and sometimes as to the reputation of the seed grower and the region where the seed was grown. Appearance of the seed is practically of no value in determining the quality of seed potatoes, but the last two factors are important. The production of high-grade potatoes is coming to be more and more a specialized business, and the need for some assurance as to freedom from infectious diseases has lead to the development of certified seed production. 2 Additional information on potato growing in the West may be found in Farmers' Bulletin 1639, "Potato Production in the Far Western States." U. S. Dept. Agriculture, Washington, D. C. 12 California Agricultural Extension Service [Cir. 61 FACTORS AFFECTING SEED QUALITY Appearance: — Although seed tubers of a certain definite type, smooth, good sized, and attractive in appearance are often selected, none of these factors have any correlation with producing qualities. The appearance and shape of tubers depends on the variety and on the soil and climate of the locality where they were grown. Naturally, tubers are preferred that are free of knobby second growth, rots, and tuber -borne diseases such as scab and rhizoctonia. Region of Production, — There is a firmly rooted idea that seed from northern districts is more vigorous than that from more southern localities. Certainly seed potatoes grown under cool, humid climatic conditions are better than those grown in hot sections ; but seed grown in southern California as a fall crop, in the cool season of the year, may equal or surpass in producing quality seed grown in the North- west. Although climate has, no doubt, some direct effect on the vigor of seed potatoes, it also has been found that high temperatures mask the symptoms of certain diseases, especially mosaic, so that it is not possible to rogue out the diseased plants from the seed fields where the temperature is high and the sunlight is intense. Thus the climatic factor may have much to do with the elimination of diseases, such as mosaic, which under severe climatic conditions weakens the plants without producing visible symptoms. There is also another factor in location. Some districts which are relatively free from aphis and certain other insects, the spreaders of virus diseases, enjoy a natural advantage in production of healthy seed. Where the potato aphis occurs in large numbers it is almost impossible to maintain disease-free seed stocks even when some roguing and spraying is done. Irrigation. — No constant relation between irrigation and seed quality has been established, except that in some sections plants affected by certain diseases, especially spindle tuber, which would eliminate themselves under dry-farming conditions, are enabled to survive under irrigation, thus producing more diseased stock. Sub- jecting potatoes to high temperature and lack of water just prior to harvest may result in inferior stock (figs. 2 and 3). Maturity of Seed. — Growers generally believe that seed potatoes harvested in the immature condition produce more vigorous plants than potatoes that have fully matured in the ground before harvest; and there is a body of experimental data to justify this belief. Hence, crops intended for seed are often planted later than a market crop 1932] Potato Production in California 13 Fig. 2. — Knobby second growths produced on partly developed tubers. Irregu- lar moisture supply, especially lack of water during the middle of the season, causes this condition. It is not transmitted through the seed. Certain varieties are more subject to this trouble than others. (From Cir! 287.) Fig. 3. — Two rows on the right show perfect stand obtained from normal healthy seed. On the left the stand is poor, apparently the result of injury to the seed the previous year caused by high temperatures and lack of water. (From Cir. 287.) 14 California Agricultural Extension Service l ( ^ R - 61 would be planted in the same section, so that the vines will still be green when frosted in the late fall. There are probably several factors responsible for the advantages of immature seed. Storage Conditions. — Storage conditions have considerable effect on the quality of seed. Potatoes kept in a warm place and allowed to develop long sprouts before planting are usually unable to produce good stands of uniformly vigorous plants. Where cold storage is used, a temperature of 36° to 40° F is most favorable. Good ventilation is epecially necessary to carry off surplus moisture, thus inhibiting both decay and sprouting of the potatoes. Bins provided with false bottoms and with bottom and top ventilation have proved satisfactory for bulk storage. Disease Infection. — Potatoes that show a large amount of scab or of rhizoctonia infection on the surface should not be planted. These diseases in slight attacks are not serious if proper disinfection is practiced. Potatoes showing internal discolorations, such as black- heart, internal browning, heat necrosis, or the brown ring below the skin associated with wilt diseases, should not be used. These diseases, however, which present more or less evidence of their presence on or in the seed tubers, have less effect on the quality of potatoes for seed than the group of virus diseases. PREPARATION OF SEED FOR PLANTING Many comparisons have been made on the relative value of whole and cut seed-pieces. In general, small tubers (from 1 to 2 ounces in weight) may be safely used for seed purposes if they come from healthy plants. There is danger in selecting small tubers for seed purposes from the bin unless one is sure that all the plants in the field where the crop is grown were relatively free of virus diseases ; otherwise the chances are that most of these small tubers came from diseased plants and will produce diseased progeny. Whole seed or 'drop seed,' as it is usually called, seldom decays before or during sprouting, as cut seed sometimes does, and hence gives better stands. Also the slight commercial value of the small tubers makes them a cheap source of seed. Disadvantages of whole seed are the difficulty of planting with a machine, the danger of disease perpetuation, the slowness of germination if the seed is immature, and the excessive number of sprouts produced if the seed has been in storage a long time. The point should be made here that small potatoes — 1 ounce in weight or over — need not be discriminated against if the stock is 1932] Potato Production in California 15 from healthy fields ; hence seed stock need not be graded for size like market stock. As to the most desirable size of seed-piece, many experiments have been conducted, and many conflicting opinions are held. In deciding on the proper size to which the seed is to be cut, one must consider also the distance apart at which the seed is to be planted. If a wide spacing (14 to 18 inches) is used, the seed-pieces should be relatively large — averaging 2 ounces each. If relatively close planting distance (10 to 12 inches) is to be used, then smaller seed-pieces, average 1 to 1% ounces, will serve best, Experiments have generally shown that yields increase with the amount of seed planted to the acre, whether the greater amount is used by cutting to larger size or by using small pieces planted closer together. The larger the seed-piece, the more stalks there will be, and conse- quently the greater the number of tubers set in each hill. If, however, there are too many stalks in a hill and consequently too many tubers are set, few of them will reach marketable size. Hence the objection to seed-pieces that are too large, aside from the greater cost. The optimum size for any given planting distance depends on the variety, the fertility of the soil, and whether or not the seed is fully out of the rest period when planted. Seed-pieces from potatoes still in the dormant condition will usually produce only one or two stalks each; pieces from potatoes that have already begun to sprout will produce several; while pieces from potatoes that have been kept dormant in cold storage for several months beyond the end of their natural rest period will generally produce an excessive number. The practice of many growers in California is to use from 8 to 10 sacks to the acre. This amount of seed will generally give best results if cut to 1 or IVi-ounce pieces and planted 11 to 12 inches apart in the row. In a record-breaking yield of 57,000 pounds an acre grown at Stockton in 1924, 22 sacks of seed were planted. Both plant food and water must be supplied abundantly where such heavy seeding is practiced. PLANTING The date of planting depends upon the district where the crop is grown, the most profitable season of marketing, and the hazard from frost. It varies from November, in coastal districts like Colma and Carlsbad, to July, in interior locations where a late crop is grown. Generally speaking, the earlier the planting date (without undue risk of frost), the more satisfactory the development of the plants •and of the crop, and the less the danger of injury from heat and tuber 16 California Agricultural Extension Service [O**- 61 moth. The time of planting, although it affects both the production and the value of a crop, has to be determined largely by local conditions. For planting, a machine is generally used. Several types are on the market: namely, one-man machines, two-man machines, one-row planters, and two-row planters. The type of planter used depends on the preferences of the individual grower, the acreage he has to plant, and the amount of help available. More nearly perfect stands are likely to result where a two -man planter is used, but this is more expensive than the one-man or 'automatic' type. Some of the large growers obtain good results with three of the one-row automatic dropping type of machines, hitched abreast and drawn by a tractor. The soil should contain enough moisture at planting time to germinate the seed promptly and to support its development until the plant is several inches high. Most California potatoes are planted in soils moistened by winter rains. Occasionally, however, the land must be irrigated in preparation for the spring plantings; and irri- gation is always necessary before planting the fall crop. "When natural moisture is to be held in the ground for planting rather late potatoes, a shallow cultivation must be given occasionally to keep down weeds that would otherwise sap the moisture from the soil. If the weeds are kept down, most soils will hold their moisture near enough to the surface for late spring planting. IRRIGATION Almost every conceivable variation of irrigation practice is followed in the various districts of California, ranging from culture under winter rainfall conditions at Colma and dry-farming summer culture in Marin County, to irrigation every 5y 2 or 6 days in the Shafter district or almost continuous sub-irrigation in the Delta district. Overhead irrigation is practiced to some extent in certain sections, particularly in San Diego County. The amount of water required to grow a crop and the frequency of irrigation vary so much with local climatic and soil conditions that a general discussion is impossible. Growers as well as investigators are not in complete agreement regarding the time, method, frequency, and extent of irrigation. While considerable experimental work has been done in the western states, there are so many influencing factors affecting the results that general recommendations, applicable to all districts or soil types, cannot be made. It is safe to assume, however, that a, readily available 1932] Potato Production in California 17 supply of soil moisture should be maintained during the growing season. In the Delta district, the water level is maintained relatively close to the ground surface during most of the growing season. The ground water level usually fluctuates between one and two feet from the surface. Under conditions of surface irrigation it is advisable to make the furrows of such depth and width that the water never floods the ridge and that it flows at an even rate (fig. 4). Fig. 4. — Irrigating potatoes in Kiverside County. Note the pipes at right angles to the head ditch. This system provides a constant flow of water into the furrows and is particularly suited to light, sandy soils. Tuber formation begins when the plants are 5 or 6 inches high and is completed by the time the flower buds are formed. Conditions during this period determine the number of tubers formed. From the beginning of the flowering period until the plants begin to mature, the tubers grow at a regular rate. Hence, if the soil becomes too dry during this period, normal tuber development is retarded, the skin of the tuber 'sets' and subsequent irrigations may be responsible for growth cracks, rapid expansion in the region of the eyes and knobby second growth. Under conditions of high temperature and intense sunlight the rate at which potato plants exhaust moisture from the soil is propor- tional to their leaf area; hence the need for water is greater as the plants grow larger. Varieties having large vines with a large leaf area use more water than do those having a smaller foliage system. 18 California Agricultural Extension Service [ 0rB - 61 The use of water is further influenced by temperature and by humidity of the air : higher temperature and lower atmospheric humidity increase the transpiration rate. Large plants under midday condi- tions in summer may give off moisture faster than their roots take it up, even from a moist soil. Sometimes this excess loss leads to temporary wilting and is thought to be connected with several troubles to which potatoes are subject under irrigated conditions, such as knobby tubers (fig. 2), vegetative sprouts from half -grown tubers, heat necrosis, and internal browning. The time of the first irrigation is usually determined by the moisture content of the soil at planting time. If soil moisture is adequate for germination, the practice is to delay the first irrigation until the leaves of the plants show a decidedly dark blue-green color, but before any other evidence of wilting is apparent. Thereafter, irrigations should be frequent enough to keep the plants growing at a uniform rate. Frequent irrigations appear to be desirable to maintain a readily available supply of soil moisture for normal development of both plant and tubers. HARVESTING The yield of marketable potatoes increases rapidly as the plants approach maturity, even increasing somewhat after the vines have turned yellow. A considerable portion of the crop is nevertheless harvested when only partially matured, because early potatoes often bring higher prices and because heat and tuber moth may cause injury if the crop is allowed to grow until the vines die. Much of the crop in the extra-early districts, such as Colma and Carlsbad, is harvested when only partly grown, as 'new potatoes,' because of the favorable market conditions. Most of the spring crop in central and southern California is also harvested before fully ripened. The skin on immature potatoes is very thin and is easily rubbed off. As the potatoes approach maturity, the skin becomes thicker, tougher, and more firmly attached to the tuber. The advancement of this change in the coat of the tuber is important in determining when to start digging early potatoes. If they are harvested too immature, some of the skin is injured in handling, sacking, and loading. These 'skinned' areas quickly turn black and are often the starting places for decay. !932] Potato Production in California 19 FERTILIZERS During the past decade considerable attention has been devoted to soil fertility problems in potato production in California. Those practices which have been found applicable in one locality or on one soil type will not always prove economical when tried in other dis- tricts; but, because fertilizer trials have been conducted in most of the important potato-producing districts of the state, it seems advisable to present the results secured. In Los Angeles County continuous tests of the value and type of fertilizers for potatoes have been conducted since 1918, by the Extension Service of the University of California and the Potato Growers ' Department of the Los Angeles County Farm Bureau. In this county, it has been stated that there is, aside from the value of good seed, probably no factor more important than soil fertility. The soil types commonly used for potatoes are usually deficient in organic matter and nitrogen, but generally contain sufficient phosphorus and potash to mature a profitable crop. The beneficial effect of the application of barnyard manure was well demonstrated in a trial on the spring crop of 1924 at El Monte, where an application of 10 tons per acre increased the yield by 65 per cent (from 104 to 172 sacks per acre). Where barnyard manure was not available as a source of humus and essential elements, trials were conducted with certain leguminous covercrops. In a typical covercrop trial conducted at Van Nuys, the increased yield of potatoes was 70 and 114 per cent, respectively, for Melilotus indica and 5-year-old alfalfa. In an experiment conducted by W. M. Mertz at Riverside, a legume covercrop was responsible for an increased potato yield of 40 per cent over a non-legume. Seasonal deficiency of irrigation water and high land values some- times prohibit the utilization of covercrops, and in such instances growers often depend upon commercial fertilizers. Extensive trials have demonstrated the kind and amount of fertilizer which may be economically applied to the soil. A nine-year summary of the results secured in Los Angeles County appears in table 4 and indicates that, in general, potato yields were appreciably increased by the application of nitrogenous fertilizers. General conclusions regarding potato fer- tilization practices in Los Angeles County are as follows : nitrogen and humus are usually deficient, and both may be added with profit ; phosphorus is not yet deficient; potash may be deficient in certain 20 California Agricultural Extension Service [Cm. 61 sandy soils but is usually present in sufficient quantity in the heavier soil types. In the Shafter-Wasco district of Kern County, potato fertilizer tests have been conducted continuously during the past five years. TABLE 4 Effect of Certain - Fertilizers on Potato Yields in Los Angeles County, 1918 to 1927, Inclusive* Fertilizer Pounds ferti- lizer per acre Number of trials Average yield, pounds potatoes per acre Per cent increase over corresponding check due to fertilizer None (check) 500 700 700 400 580 300 to 400 200 600 37 9 12 9 26 10 5 8 10 5,778 6,638 6,499 6,541 9,157 8,598 18,002 6,005 7,836 Fish meal 37 Cottonseed meal 35 Tankage 35 Fish meal (320) and ammonium sulfate (80). Cottonseed meal (500) and ammonium sulfate (80) 73 54 Ammonium sulfate 53 Potash 25 Fish meal (320), ammonium sulfate (80), and potash (200) 47 * Data furnished by F. H. Ernst, Assistant Farm Advisor of Los Angeles County. The effects of the various fertilizers cannot be compared with one another as separate checks where planted in each test. Only the per cent increase over the corresponding check can be considered. Fig. 5. — The response of potatoes to nitrogenous fertilizer in Kern County. The larger plants received 400 pounds of sulfate of ammonia per acre, while the smaller plants received no fertilizer. Results secured in 1926 indicated that only nitrogenous fertilizers increased the yield with profit. From 1927 to 1930 inclusive, tests were conducted with nitrogenous fertilizers of various kinds, applied in varying amounts per acre, and at different times with respect to 1932] Potato Production in California 21 the date of planting. The results of the comparative trials are pre- sented in table 5, showing that, under conditions of the experiments, the most economical source of nitrogen was either sodium nitrate or ammonium sulfate applied at the rate of 300 to 400 pounds per acre, and that the soil was not deficient in phosphorus. It is worthy of men- TABLE 5 Effect of Certain Fertilizers on Potato Yields in Kern County, 1926 to 1930, Inclusive* Trial No. Year Fertilizer Pounds fertilizer per acre Per cent increase over unfertilized 1 1926 1927 1927 1928 1929 1929 1930 1930 1930 600 900 300 571 685 250 40,250 300 450 600 228 685 685 300 400 500 250 400 600 350 300 Ammonium sulfate (300), muriate of potash (300), superphosphate (300) 31 2 136 3 3 4 Ammonium sulfate (114) and fish meal (571) 30 58 5 Ammonium sulfate (250) and manure (40,000) 99 27 41 50 6 122 Ammonium sulfate (114) and cottonseed meal (571) 136 7 Ammonium sulfate (114) and fish meal (571) 131 22 42 44 8 30 66 70 9 47 47 * Data furnished by L. W. Taylor, formerly Farm Advisor of Kern County. It should be pointed out that the respective yield increases for different trials are not comparable. In making comparisons it is only possible to compare increased yields within each separate trial. tion that 98 per cent of the commercial potato growers in Kern County used commercial fertilizer in 1930 and that the use of fertilizer increased from practically nil in 1926 to approximately 600,000 pounds in 1931 (fig. 5). Growers in the Marin-Sonoma section have demonstrated that from 200 to 300 pounds of a 2%-10-5 complete fertilizer is most economical, and that 500 pounds of a 4-10-10 or 4-10-0 fertilizer is as effective 22 California Agricultural Extension Service t 01 ^ 61 as 1,000 pounds of the same materials. The application of potash alone does not increase yields. In the Colma section barnyard manure is most commonly used, and comparatively little has been done to determine whether the appli- cation of commercial fertilizers is profitable. Green manure crops are not yet commonly used in rotations. In Butte County data are available from fertilizer trials conducted at the ranch of Mr. J. A. Ream, Paradise, California, in 1928 and 1930. TABLE 6 Effects of Commercial Fertilizers on Potato Yields at Paradise, Butte County, 1928 and 1930 Pounds fertilizer per acre Yield in 100-pound sacks per acre Year Fertilizer U. S. No. 1 U. S. No. 2 Per cent increase over unfertilized 1 None — check 98 184 132 139 63 115 370 380 325 317 22 25 28 40 37 17 24 30 35 46 38 1,000 1,000 2,000 3,000 459 1928 Ammophos 16-20-0 864 Complete 6-9-6 664 , Complete 6-9-6 700 Superphosphate 2,000 1,000 1,000 1,000 1,000 70 Ammophos 16-20-0 400 1930 Ammophos 13|-33-0 437 364 , Complete 12-24-12 339 The soil in the Paradise section, unlike most California soils, does not contain adequate phosphorus in available form. Data from his trials, presented in table 6, indicate that under the conditions of the experi- ment remarkable yield increases were obtained by the application of commercial fertilizers containing a relatively high percentage of nitro- gen and phosphorus. In 1930 a phosphorus fertilizer increased the yield by as much as 437 per cent. INSECTS3 Potato Tuber Moth (Phthorimaea operculella) . — This insect is considered one of the most serious pests of the potato in California. The adult of this insect is a rather inconspicuous gray moth % to% inch long with a silvery body and narrow fringed wings, the fore pair being marked with minute dark specks. The adults are of nocturnal habits and when disturbed fly with quick, darting move- 3 This discussion of potato insects was prepared by F. H. Wymore, Associate in Entomology. 1932] Potato Production in California 23 ments and immediately attempt to conceal themselves. They may appear throughout the year in storage and during the spring and summer in the fields. The minute, pearly-white eggs are deposited singly or in clusters of 2 to 25 or more on the under surface of the leaves and stems or on the exposed tubers. In storage the eggs are laid in the eyes or other depressions on the surface of the tubers. The eggs may be laid any time of day or night but are deposited most abundantly after sunset. One female may deposit from 1 to 300 eggs. Fig. 6. — Tuber-moth injury to potatoes. On the tuber at the left, the sunken area and the piles of black frass at the eyes, are typical symptoms of infestation by the tuber moth. On the right, are shown tunnels of the larvae. The larvae are yellowish white, pinkish, or greenish, with a dark brown head. They mine the leaves and stems of the plant and gradually work down into the tuber (fig. 6) as the plant matures, making small shallow tunnels under the skin, rendering them unattractive as food or unfit for seed. They continue their work on the tubers in storage as long as anything suitable for food remains. The mature larvae emerge from the host and pupate in a delicate white silken cocoon in the soil or debris near the base of the plant, in depressions of the tubers, in burlap sacks, or in almost any secluded place. There are several overlapping generations annually. In midsummer the life cycle may be completed in about 30 days but during the winter 90 days may be required. Besides the loss due to actual destruction of potatoes by the caterpillars of this pest, ship- 24 California Agricultural Extension Service t ClR - 61 ments from districts infested with the tuber moth into certain other states is prohibited unless certified by the California State Depart- ment of Agriculture to be free of tuber moth. Before this certificate is granted, all visibly infected tubers must be culled out and the remainder subjected to a 75-minute carbon disulfide fumigation under vacuum and placed immediately into clean refrigerator cars to avoid reinfestation. This, of course, means an added expense in handling the crop and may cause some delay in shipment. Control. — Effort should be made by the grower to prevent infesta- tion of the tubers. The following suggestions for a control have been adapted from Special Publication 24 of the California State Department of Agriculture : 1. Plant only seed known to be free of tuber moth infestation. 2. A crop planted 5 to 6 inches deep will be less subject to attack than one planted nearer the soil surface. 3. Clean culture at all times is desirable. The tuber moth hiber- nates in volunteer potato vines and certain weeds. Deep plowing and thorough cultivation will destroy these and protect the future crop. 4. Soil cracks about the hill readily admit the caterpillars to the tubers. These cracks can largely be prevented by high ridging and careful irrigation. If they occur after the crop is mature, they can sometimes be closed by running a light roller over the field. 5. The crop should be harvested early since the longer the crop is allowed to remain in the ground after maturity, the greater will be the danger of tuber infestation. 6. Never leave potatoes in containers or loose in the field exposed overnight. The moths fly and oviposit most of their eggs at night so that any potatoes left exposed are liable to be covered with eggs which may hatch during transit or storage. 7. Never cover containers with potato vines because the vines are usually infested with the larvae and they will move onto the tubers if placed near them. 8. Storage at temperatures of 37° to 40° F will prevent further development of the worms if already present. 9. The fumigation of infested potatoes with carbon disulfide in a 20-inch mercury vacuum for 75 minutes will destroy all stages of the tuber moth. In order to gain the greatest assurance against injury from fumigation, the potatoes should not be irrigated within, at least, one week before treatment. Instances of fumigation injury to potatoes have been traced directly to the fact that the crop had been irrigated only a few clays before being dug and immediately fumigated. 1932] Potato Production in California 25 Wireworms. — Wireworms, of which there are several species (fig. 7), are larvae or young of certain insects and as the name implies, they have wiry, smooth, cylindrical bodies with well denned segments. Those most commonly seen in the fields and gardens are about a /2 to 1 Fig. 7. — Cross-section of a potato showing wireworms and their burrows. Fig. 8. — Click beetles (Limonius calif ornicus) , the adults of wireworms. inch in length and range in color from pale yellow in the smaller worms to dark amber brown in the larger ones. They live in the soil and often do great damage by burrowing into the roots and tubers. The burrows, although small in diameter, may often completely per- forate the tuber or extend several inches up the center of the potato stalk. Frequently the worms may be found in their burrows. The larval stages last from one to three years. The adults (fig. 8) of 26 California Agricultural Extension Service [Ora. 61 wireworms are commonly known as click beetles because of the click- ing noise which they produce when they jump into the air or are held between the fingers. Those most commonly seen in the potato fields are V2 to % inch in length and are pale brown to black in color. Although usually rather inconspicuous, these beetles become active in early spring and may be seen flying about in great numbers in certain areas the latter part of April and the first part of May. Control. — Because of their underground habits, the larvae are very difficult to control. Clean culture and rotation of crops seem to be the most effective practices. Clean summer fallow will greatly reduce infestation. Large numbers of the adults may be trapped and burned in small piles of straw placed about the field during the flight Fig. 9. — The green peach aphis {Myzus persicae) , a very common pest of potatoes throughout the country. season in the spring or fall. The best success may be obtained from this method by keeping the field free from all trash and debris. Aphis or Plant Lice. — Potatoes are frequently attacked by various species of aphis. These are delicate, soft-bodied, pear-shaped insects (fig. 9) with sucking mouth-parts. The species most commonly found on potatoes are generally green in color. They may occur in such numbers as to completely cover the under surface of the leaves, Feeding consists in extracting the juices from the plant tissues and results in weakening and often drying up of the whole plant. While aphis may not always cause obvious damage they often are responsible for the dissemination of virus diseases. Aphis feed on diseased plants and then migrate to healthy ones, carrying the virus with them. The control of these pests is of especial importance to the seed grower. Control. — A 2 per cent nicotine dust is effective as a control for aphis commonly found on potatoes, A spray of Black Leaf 40 (nico- 1932] Potato Production in California 27 tine sulfate 40 per cent), 1 part to 800 parts water, is effective in controlling aphis, but must be applied directly on the insects in order to kill them. In some seasons, aphis are especially numerous on potatoes in the early part of the season, at which time the infestation may occur in spots in the field. One should, therefore, be on the lookout for such early infestations and treat them immediately with nicotine dust or Black Leaf 40 spray, which may be put on with a hand applicator at considerable less expense than when the field is Fig. 10. — The tobacco flea beetle (Epitrix parvula), a very common pest of potatoes. generally infested. Should the infestation be extensive enough to warrant the use of a power duster, the effectiveness of the nicotine dust may be considerably increased when applied under a trailing canvas 30 to 50 feet in length. Flea Beetles. — Several species of flea beetles attack potato plants in California. They are small beetles (fig. 10) 1.5 to 2 mm in length, oval-oblong in shape, and reddish to dark brown, or shining black in color. They have the ability to jump quickly, like a flea, when dis- turbed; hence, the name 'flea beetle.' The beetles eat small holes in the leaves, giving them a distinctly shot-hole appearance (fig. 11). In addition to the direct injury from feeding on the leaves, the beetles may spread certain potato diseases. The larvae are small, white grubs which live in the soil and feed on the roots of various plants as well as on the developing potato tubers, causing what is known as ' pimply ' 28 California Agricultural Extension Service [ Cik * 61 potatoes. There are usually two broods a year. The winter is passed in the adult stage and any trash or debris left about the field and along fence rows will serve as hibernation quarters for the beetles. Control. — The 4-5-50 formula of bordeaux mixture as applied for fungus diseases acts as a very effective repellent for the flea beetles. Fig. 11. — The potato flea beetle (Epitrix cucumeris) , and characteristic work on a potato leaf. Fig. 12. — Adults of the potato stalk borer (Tricliobaris trinotata). A 70 per cent sodium fluosilicate dust thoroughly applied to the under surface of the leaves will effectively control this pest. Good control may also be secured with a dust composed of one part of lead arsenate and ten parts of nicotine dust (85 parts hydrated lime to 15 parts nicotine sulfate). Repeat the applications of the above materials as needed. In the presence of moisture sodium fluosilicate is liable to cause injury to the foliage ; therefore, in the fog belt or in localities where dew occurs, it would be safer to use a 70 per cent barium fluosilicate. 1932 J Potato Production in California 29 Potato Stalk Borer (Trichobaris trinotata). — This is a small bluish-gray weevil (fig. 12) about ^-inch long, which occasionally becomes a serious pest on potatoes in California. A premature yellow- ing of the plants during late summer is an indication of the presence of this insect. 4 The adults spend the winter in the stems of the host plants, emerge in the spring and attack the growing plant. The female eats a small hole in the stalk in which she deposits a small Fig. 13. — Larvae of the potato stalk borer (Trichobaris trinotata) in stalks of potato. white egg. In a few days a tiny, yellowish, legless grub with a brown head hatches and makes its way into the center of the stalk, where it continues to work. By the time the grub is full-grown it may com- pletely hollow out the stem (fig. 13). Pupation occurs within the burrow and in a short time the adult develops and remains in the old stalk during the winter. Control. — The most effective method of controlling this insect is to rake up and burn all the old vines as soon as they are dry. On peat soil it may become necessary to remove the dry stalks to one side of the field and burn them where special arrangements can be made for extinguishing the fire. 4Wireworms may cause a similar effect on the plants earlier in the season. 30 California Agricultural. Extension Service i Gm - 61 Gutworms. — The adults of the cutworms are practically all night- flying, rather somber colored moths. Several species of these insects attack potatoes. The eggs are laid in large masses on the host, usually in the early spring. The caterpillars have the habit of hiding away in the daytime and usually are found singly or in groups of two or three just beneath the surface of the soil. They cut or gnaw off the young plants near the ground level; hence, the name 'cutworms.' They are stout, soft-bodied, smooth, cylindrical, varying in color from pale gray to dark brown or nearly black and are often marked with spots or stripes. They normally feed on native plants but migrate to cultivated crops when the former source of food supply becomes exhausted. The cutworms are most active at night but in the early spring they often emerge and feed during cloudy days. When full- grown, the caterpillars burrow beneath the surface of the ground or under clods or other objects and pupate in a small earthen cell. There may be several broods a year. The winter is usually passed in the pupal stage, although a few caterpillars and adults may hibernate. Control. — Clean culture in the fall and plowing of infested fields to destroy the pupae will greatly reduce next year's broods. Early cultivation in the spring keeps down the food supply and destroys a great many over-wintering caterpillars by starvation and exposure. If the cutworms are found to occur in large numbers, a poison- bran bait sown thinly over the ground will poison most of the worms before the plants push through the soil. A poison bait made up according to the following formula has been effective : Bran (red bran preferred) 25 pounds Liquid sodium arsenite y 2 pint Water approximately 4 gallons One pound of white arsenic or one pound of sodium fluosilicate could be substituted for the % pint of liquid sodium arsenite. White arsenic has in the past been most commonly recommended as a killing agent in poison bran mash but is often unsatisfactory, due to imper- fect mixing. Sodium arsenite is fully as satisfactory a killing agent as white arsenic and has the advantage of being easier to mix and is cheaper in cost. The liquid sodium arsenite can sometimes be pro- cured on the market under the name 'weed-killer' but it should not be confused with other chemicals used as weed killers. Prepare the above material as follows: Mix the sodium arsenite with about 3 gallons of water. Slowly pour this liquid over the bran and stir thoroughly until an even mixture is obtained. Add only enough water to make a crumbly mass. The bait should be sown broad- cast over the infested field at the rate of 5 to 10 pounds per acre. The 1932] Potato Production in California 31 bait is most attractive to the insects when fresh so the application should be made in late afternoon or early in the evening, as the caterpillars do most of their feeding at night. Caution. — This insect poison is also poisonous to other animals and man; therefore, it is not advisable to use this bait in the vicinity of buildings where chickens or other fowls will have access to it. It may, however, be sown in pastures if it is scattered thinly and not put out in lumps or lines. The poison and poison bran mash should be kept out of reach of irresponsible persons. Armyworms. — These pests belong to the moth family (Noctuidae) and have a life cycle similar to cutworms ; but because of the migrat- Fig. 14. — The yellow-striped armyworm {Prodenia praefica). ing habits of the caterpillars of certain species they are commonly known as armyworms. The caterpillars (fig. 14) of this group nor- mally feed on native plants but if tfhese have been devoured before the caterpillars are full-grown they will move in great hordes or armies over to cultivated crops, devastating vegetation as they go. Various species of armyworms may attack potato vines. Control. — Clean culture, summer fallow, or poison-bran bait serve to control the armyworms. A poison bait, made according to the formula recommended for cutworms, and sown about in fields, pas- tures, orchards, or gardens when the caterpillars begin to migrate, will attract and poison countless numbers of them and may completely check their invasion. If the poison bait is not available, a trench plowed across their line of march, with a perpendicular wall next to the field to be protected, will afford a great amount of protection. 32 California Agricultural Extension Service [Gib. 61 If this barrier is properly constructed, the caterpillars will not cross it and will gather in great masses in the trenches where they can be easily killed by spraying with crude oil or kerosene. The trenches require a certain amount of attention and repair to be effective. Grasshoppers. — While grasshoppers may feed to a slight extent every year on potatoes and other cultivated crops, they frequently prove a serious pest particularly to crops grown adjacent to grasslands or alfalfa fields. There are two general types of grasshoppers, nonmigratory and migratory. Those most commonly seen in cultivated areas usually belong to the nonmigratory group and live and breed almost exclu- sively in the same locality. Under favorable conditions the migratory species often leave the breeding grounds, normally in the grassy foothills, and migrate in great hordes to cultivated crops. Egg laying occurs in late summer and fall. The female grass- hoppers usually seek out a rather dry, compact soil, such as found along fence rows, ditch banks, cow paths, etc., in which to oviposit. The eggs are laid, in regular order, in a hole in the ground in a matrix of glutinous fluid which hardens into a moisture-proof mass and protects them from the winter weather. The number of eggs in a mass may vary from only a few to as many as 60, each female depositing two clusters. The eggs hatch in early spring and the small wingless hoppers (nymphs) begin at once to feed on green plants. Several moults occur at various intervals and maturity is reached in midsummer or fall. At cold temperatures the winter is passed in the egg stage, but in the warmer valleys a few of both the nymphs and adults may be found through the winter. Control. — Thorough cultivation of breeding grounds destroys a large percentage of the eggs and thereby effects a very practical con- trol. A poison-bran bait, made according to the formula recommended for cutworms, and scattered thinly (about 5 pounds per acre) over a field is effective in the control of grasshoppers. A flock of -turkeys herded in an infested field will often provide control. Nematodes (Eelworms). — The garden nematode, or eelworm, Het- erodera radicicola, is not an insect, but is a true roundworm, micro- scopic in size. This pest is a general feeder and is the cause of a. very common disease known as root knot on the roots of many plants. While this pest sometimes affects the fibrous feeding roots of potatoes, the tubers are chiefly affected. The worms produce a pimply or warty surface on the potato (fig. 15) and small brown channels just beneath the skin. Potatoes so affected are worthless for market or for seed. Nematodes prefer sandy to the heavier soil types. 1932] Potato Production in California 33 Control. — No satisfactory method has yet been developed for the control of this pest in the field ; however, the rotation of crops is very helpful. Small grain and the Iron and Brabham varieties of cowpeas are seldom attacked by this nematode. These immune crops should be grown at least two years before again planting the fields to potatoes. Plant only nematode-free seed. Fig. 15. — Tuber infested with nematodes. The galls are especially prominent at the eyes of the tuber. (From Cir. 287.) DISEASES General Considerations Regarding Diseases. — Potato diseases may be classified as follows: (1) parasitic, (2) virus, and (3) nonparasitic. Parasitic diseases result from living organisms, whether animal or plant, but as potato insects are discussed elsewhere in this circular, the discussion of parasitic diseases is herein limited to those caused by plant and not by animal organisms. Successful potato growers are familiar with the habits and response of the potato plant, and they should be just as well informed regarding the nature of the diseases which attack the crop. Thus it is considered advisable to describe briefly the more important features of a disease-producing microorganism. Essentially the potato plant has much in common with the organ- ism responsible for black scurf (commonly known as the rhizoctonia disease) for the latter is likewise a plant. Khizoctonia does not, of course, have visible structures such as leaves, roots, stems, tubers, flowers, and seeds ; but it does have microscopic structures which func- tion in a manner similar to those of potato roots, leaves, etc. The 34 California Agricultural Extension Service [Cm. 61 root-like structures of rhizoctonia obtain nourishment from the potato plant much as the latter obtains nourishment from the soil. Being without chlorophyll (the green coloring matter found in the potato and other green plants), rhizoctonia is unable to manufacture the food necessary for its existence and thus must rely on the potato plant for manufactured food. This is why rhizoctonia is a parasite ; it steals the manufactured food which nature intended for the use of the potato plant. Whereas the latter reproduces vegetatively by means of buds on the tubers, rhizoctonia is perpetuated by means of interwoven root- like structures known as sclerotia, or by means of spores (seed-like structures) which form on the stem of the potato plant. These spores germinate, or the sclerotia send out threads ; in either case, the young potato plant is infected. The other microorganisms which produce disease in the potato differ from rhizoctonia in some characters but essentially resemble it. This limited description will make clearer the discussion of parasitic diseases of the potato. The cause of virus or degeneration diseases of plants is unknown. It has been demonstrated, however, that many potato diseases are in- duced by a virus which will pass through a fine filter whose pores are so small that all forms of known living microorganisms such as cause parasitic diseases are excluded. This virus (infectious principle) is systemic; that is, it passes through the cells into all the vegetative structures of the infected plant. A potato virus disease might be likened to hog cholera and human scarlet fever. The virus may be transmitted mechanically or by certain insects from infected to healthy plants. Nonparasitic diseases may result from (1) lack of proper inherent qualities, (2) improper environmental conditions of soil or air, or (3) injurious mechanical influences. They are noninfectious and are not always perpetuated in asexual propagation. Rhizoctonia or Black Scurf (Corticium vagum). — Rhizoctonia is one of the most widespread diseases of the potato and causes some annual loss wherever the crop is grown. In California the annual loss varies from 2 to possibly 5 per cent. The disease is manifested on the roots, stolons, stems, leaves, and tubers. Black sclerotia or resting bodies (fig. 16) varying in size from those just visible to those % inch in diameter may be found on the tubers at or after digging. These black bodies resemble dirt particles but are not soluble in water, so that the disease is often designated as "the dirt that won't wash off." The disease manifests itself in one or more of several ways. Pri- mary injury is confined to that portion of the plant at or below the 1932] Potato Production in California 35 soil line. Secondary symptoms are manifested on the leaves and on the aboveground portion of the stem. Probably the most severe symptom on growing" plants is the "burn- ing" of the young sprouts just before or as they emerge from the soil. This injury may lead to the production of additional sprouts, which in turn may be burned off, with the result that a rosette or cluster of sprouts with dead tips is formed, none of which reach the surface of the ground or are able to produce plants. Obviously such a condition may often be the cause of poor stands. V Fig. 16. — "Black scurf," the sclerotia or resting stage of rhizoctonia on the surface of the tuber. Note that the black bodies resemble dirt particles. These sclerotia are superficial, never penetrating through the skin of the tuber. Seed disinfection kills these sclerotia. (From Cir. 287.) If the young sprouts are attacked later (that is, after they have reached the soil surface), the severe burning is not so evident; but the stems become covered with brown lesions which often show a corroded condition. Similar spots may also develop on the stolons during tuber formation. These lesions interfere with normal growth of the plant, are responsible for the production of small tubers, and induce a sec- ondary effect on the aboveground parts of the plant. Burning is usually most severe under cool, moist conditions. Root blight, caused by rhizoctonia, is a symptom not commonly observed by growers. This symptom results when the stems become severely corroded ; the main roots are largely prevented from develop- ing, or die during the progress of the disease. When the feeding roots become infected the assimilation of food materials from the soil is inhibited. 36 California Agricultural Extension Service [ ctR * 61 Near the base of attacked plants and extending up varying dis- tances from the ground surface, a gray or white mold-like growth on the surface of the stalks may frequently be found during the growing season. This mold-like growth is the sexual stage of the rhizoctonia fungus and often resembles salts collected on the stalks from the soil. This symptom is more often found on irrigated than on dry-land potatoes, except under conditions of excessive precipitation. Leaf rolling is commonly observed on plants which are severely attacked underground. This probably results from injury to the feed- ing roots, for the appearance resembles that occasioned by water short- age. Usually the youngest leaves show the leaf -rolling symptom and are rolled upward. Prominent veins may be associated with leaf rolling. Yellowing of the foliage of large and fully developed plants has been found to be caused by rhizoctonia; but, in certain soils de- ficient in nitrogen, yellowing may result from this deficiency rather than from the attack of an organism. Aerial tubers often develop on severely infected plants. These tubers form in the axils of the leaves, and in such instances the stem near the soil line is commonly found to be corroded. The lesions on the stem are thought to prevent the translocation of manufactured starch from the leaves into the tubers, a condition which would account for tuber formation aboveground. Under irrigated conditions, these aerial tubers may form in the axil of nearly every leaf, sometimes developing leafy shoots. On the tubers, black scurf is manifested as (1) sclerotia, (2) pit- ting, (3) rotting, (4) tuber irregularity, and (5) indirectly, as small potatoes. Sclerotia are the black resting bodies of the rhizoctonia fungus and consist of interwoven strands of the mycelium (root-like structures) of the organism. They vary in size according to soil type and growing conditions, and may be few or many in number on a tuber. Under most California conditions sclerotia are either absent, inconspicuous or small. Tuber pits are said to develop as a result of rhizoctonia infection. They vary in size and shape, being sometimes very small but deep and cylindrical in shape, or varying from this size to large irregular cavities that extend as far as % inch or more into the flesh. These pits are perhaps not the result of rhizoctonia action alone but may result from dual infection by other organisms, whether plant or animal. Tuber rotting because of rhizoctonia infection is said to occur only in certain irrigated sections of the West. It was first described on elongated pointed-end tubers of the Netted Gem and Burbank varie- 1932] Potato Production in California 37 ties in 1922 in California and Idaho. In early stages of infection the diseased tissue is nearly white ; but as the rot progresses the color changes to yellow, then light brown, and finally brown. Such dis- coloration has been observed only on the pointed stem-end and does not advance far into the tuber. A rather sharp division between healthy and diseased tissue may be seen. The rot is jelly-like in con- sistency and is thought to be limited in its attack on potato tissue to that portion of the tuber which is deficient in starch. Rhizoctonia has been reported to cause irregularity in tuber shape. Trials conducted in Washington in 1925 indicated that more irregu- larly shaped tubers were produced by severely infected seed than by seed only mildly infected with rhizoctonia. Small potatoes may form either when the stolons are severely infected or when the feeding roots are partially destroyed. In the former case, the little potatoes form in their normal position in the soil ; but in the latter, they develop near the soil surface in a cluster with very short stolons. The soil temperature is known to have a direct bearing on the severity of attack by rhizoctonia. Studies made in "Wisconsin showed that lesions were produced on the lower portion of the stem at soil temperatures between 48° and 81° F; and although severe damage occurred between 59° and 70°, the most favorable soil temperature for infection was 64°. Approximately the same soil temperatures were found favorable for growing-point injury as for lesion production on the lower portion of the stem. It was also possible to correlate injury with the rate at which the young shoots grow through the soil : at soil temperatures above 70° F the rapid growth of the potato, together with decreased pathogenic power of the fungus, was found to permit emergence from infested soil without marked injury to the young shoots. These findings help to explain the reason for the degree of severity induced by rhizoctonia when potatoes are planted at different soil temperatures, and indicate what might happen to the crop planted in early spring in the interior valleys or to the winter crop of the coastal counties. In some sections of the country the loss from rhiz- octonia has been decreased by shallow planting. It is a common observation in the Delta region of California that rhizoctonia injury to young sprouts is much less severe when seed-pieces are planted shallow instead of deep. Factors other than soil temperature may have direct or indirect influence on the severity of rhizoctonia injury. Some of these are soil reaction, soil fertility, and soil moisture. The organism seems to thrive best in acid soils, a condition just the reverse with respect to 38 California Agricultural Extension Service [Cir. 61 common scab. Liming the soil for the correction of acidity has occasionally afforded some relief, but has not been uniformly suc- cessful. Applications of sulfur or acid phosphate, which increase soil acidity, have been reported to increase the percentage of infected tubers. Barnyard manure in liberal applications has in general given a reduction of severity of attack. Many plants other than potato are attacked by rhizoctonia; hence effort should be made to avoid them, if possible, in rotations. Beans, beets, carrots, lettuce, onions, turnips, parsnips, celery, spinach, rhu- barb, eggplant, peas, radishes, sweet potatoes, alfalfa, red clover, and strawberries are known to be susceptible. Obviously not all these crops can usually be avoided in potato rotations, but their susceptibility should be more generally recognized. In general, cereal and grain crops are not infected. In this connection it should be remembered that rhizoctonia is able to live for several years on decayed plant refuse in the soil even though nonsusceptible crops are grown. Rhizoctonia has, furthermore, been reported as found in soil which has not produced a cultivated crop for many years. Thus it is that rhizoctonia is considered as a soil organism. Control of Rhizoctonia. — Control measures for rhizoctonia vary in different localities, with the reaction and fertility of the soil, with the extent of seed infection, with the time of planting and harvesting the crop, and with various methods of culture and irrigation. Usually not all the control practices are necessary or practicable for each grower ; but the following suggestions should be considered : ( 1 ) seed selection; (2) seed disinfection as discussed elsewhere in this circular; (3) crop rotation; (4) use of lime or manure; (5) late planting; and (6) early harvesting. Whether one or several of these practices can be followed advan- tageously remains for the grower to decide. Obviously he who plants a relatively large acreage will not be able to select seed free of sclerotia. The grower who plants only a few acres or who maintains a seed plot would profit by seed selection. Seed disinfection should be practiced by every potato grower in California, regardless of the acreage planted. Obviously seed must sometimes, of necessity, be planted in infested soil ; but even in such cases seed disinfection will more than repay the grower for the expense involved, because many organisms other than rhizoctonia are often seed-borne. The average cost of seed disinfection, inclusive of machinery and materials, is approximately $1.50 per acre. The value of crop rotation should be recognized, and the practice should be followed whenever possible, 1932] Potato Production in California 39 for it will prove beneficial both from the standpoint of disease con- trol and from that of maintenance of balanced soil fertility. The extent of loss through soil infestation with rhizoctonia will vary in direct proportion to the degree of soil infestation ; hence any practice which will tend to prevent accumulation of the organism in the soil will tend to lower loss through infection. The use of lime and manure is limited to those rhizoetonia-infested soils which are decidedly acid and deficient in organic matter. The application of either lime or manure makes conditions more favorable for common scab — an un- fortunate fact which must be thoughtfully considered before either lime or manure is added. The value of late planting is limited to the spring crop and probably to those growers who plant a relatively small acreage. The practice is of additional value, however, to the grower who has an established seed plot. If planting can be delayed until the soil temperature reaches 70° F, the loss from rhizoctonia will be materially reduced. Early harvesting will benefit the seed grower in that the tubers can be dug before maximum formation and development of sclerotia is reached. Seed relatively free of rhizoctonia sclerotia is more readily salable than heavily infected seed. Blackleg {Bacillus atrosepticus) . — Potato blackleg is so named because of the black lesion (fig. 17) which forms on the stem at or near the soil line. This lesion is usually initiated at or near the seed-piece, the organism having originated in the seed or having been spread by the seed-corn maggot burrowing in the diseased seed-piece. After infection, the discolored stem begins to rot, being charcoal-black in color, appearing slimy, and having a particularly disagreeable odor. As an indirect result of this stem lesion, the leaves near the top of the vine become stunted, reddish-yellow, and curled upward. Later the stem becomes so weakened that the heavy top falls over. In some cases aerial tubers form in the leaf axils. Tubers produced by infected plants become infected either through lenticels or at -the stem end through the stolon. Usually the infected tissue softens but remains normal in color until the epidermis is removed, when the soft pulp turns first reddish and then almost completely black. Under irrigated conditions the infected tuber tissue may not soften, but may lose its moisture and become leathery in texture. Sometimes the tissues are not softened at once, but only a few cells are discolored, forming a darkened ring. Tubers may be infected without manifesting any visible symptom; but if used for seed they may, under certain conditions, produce diseased plants. Such infected tubers serve as centers of infection because the seed- cutting knife spreads the organism to healthy tubers. Recent investi- 40 California Agricultural Extension Service t 015 - 61 gations in Minnesota have shown that the blackleg organism may survive in the soil during the severe winter weather prevalent in the Fig. 17. — Potato blackleg. Note the inky black lesion extending upward from the seed-piece, and the rolled condition of the leaves. (Courtesy, Oregon Agr. Exp. Sta.) Middle West. Under California climatic conditions, obviously, the organism may persist in the soil from one growing season to another. The disease is said to be favored by relatively low soil temperature and high precipitation. 1932 ] Potato Production in California 41 The seed-corn maggot serves as a disseminating and inoculating agent for the blackleg organism. If the adult fly lays eggs on the cut seed-piece, the maggots which hatch may burrow into the seed- piece while in the soil, carrying the organism with them. Infected seed-pieces may produce healthy plants because there is deposited Fig. 18. — Potato scab. Note that the skin of the tuber is ruptured and the injured portion is more or less regular in outline. Often certain insects burrow into the spots resulting from scab infection. between the new stem and the old seed-piece a cork layer, through which the organism is unable to penetrate unless aided by the maggot. Not only does the organism often depend upon the maggot for dissemination, but the maggot depends partially upon the organism for its food. Control of Blackleg. — (1) Diseased plants should be rogued from the field, care being taken to remove and destroy any tubers which may have formed; (2) cut seed-pieces should be protected from the adult flies of the seed-corn maggot, not left exposed to their feeding; (3) seed should be disinfected before being planted; and (4) potato crops should be rotated. 42 California Agricultural Extension Service [ Cn *, 61 Potato Scab (Actinomyces scabies). — Common scab affects only tubers and is manifested during any stage of their development from the time when they are mere buds until they cease to increase in size. The lesions (fig. 18) which form on the tubers may show as slightly raised or bulging, roughened, corky areas; these may be nearly on a level with the remainder of the potato skin, or a corky depression may occur. The lesions may be small and few in number; or they may be numerous and larger, producing a general infection with little or no normal surface remaining. A single scab spot is usually less than % inch in diameter, although a whole potato may be covered with scab because of the coalescence of individual scab spots. The normal symptoms may be modified by the work of wireworms, white grubs, mites, or millipeds, which frequently extend and deepen the lesions. If infected when small, tubers may be disfigured and thus rendered unfit for market. Later infections may not cause appre- ciable disfiguration but will render the tubers unsightly, lowering their market and seed value. Careful experiments have shown that untreated scab-infected seed, as contrasted with healthy, may cause a reduction equal to one-fifth or one-sixth of the crop yield. Investigations in Wisconsin have shown that the amount of scab that develops is partially dependent on the soil temperature. The optimum soil temperature was found to be about 73° F for the highest percentage of scabby tubers and about 69° for the greatest percentage of total surface scabbed. The disease may, however, develop at soil temperatures between 52° and 77°, with slight infections at either extreme. Field observations seem in general to accord with the results obtained by experiments. They indicate that potato scab is comparatively more prevalent in regions having high summer tem- peratures than in those of lower temperature. The reaction of infested soil is known to bear a direct relation to the amount and severity of infection. Scabbing is light in decidedly acid soils, and the organisms develop best in alkaline or very slightly acid media. Soil acidity of the intensity of pH 5.2 exerts an injurious action on the scab organism to the extent that only slight scab develops. The addition of lime has long been known to increase the amount of scab, probably because of neutralization of soil acidity. The application of barnyard manure to scab-infested soil favors the disease in at least two possible ways : by the introduction of the organism in increased numbers, and by augmenting the organic matter content, which affords more favorable conditions for the development of the scab organism. 1932] Potato Production in California 43 While common scab is primarily a disease of potato tubers, the organism is also pathogenic to other economic crops. Turnips, garden beets, sugar beets, and radishes are susceptible ; and an effort should be made to avoid growing these crops on infested soil or in rotation with potatoes. Just what role is played by these crops in perpetuat- ing the scab organism is not known ; but certainly the pathogen may persist for years in the soil, independent of any of the commonly recognized host plants. The organism also retains its infectiousness after passage through the alimentary tract of domestic animals. With the exception of those potato tubers having russet skin, standard American varieties of potatoes seem to have no appreciable resistance to scab. Such varieties as Russet Burbank, Idaho Russet, and Carmen No. 3 have proved relatively resistant. The thickness of the skin probably determines the resistance of tubers to scab. Control of Scab. — Preventive measures necessary to produce a scab-free crop vary with the amount of seed infection, the degree of soil infestation, and certain environmental factors. The control measures listed below have been found effective in the reduction of scab and should be considered when planting : ( 1 ) use scab-free tubers for seed if possible, at least discard those tubers which are severely infected; (2) practice seed disinfection whether scab is visible on the seed or not (see "Seed Disinfection") ; and (3) avoid, if possible, badly contaminated soils. In the event that severely in- fested soil must be used, take advantage of those cultural practices which make soil conditions unfavorable for scab: that is, avoid the use of alkaline fertilizers or barnyard manure — particularly manure from animals which have eaten scabby tubers; use such fertilizers as sulfate of ammonia, which tend to leave the soil in an acid condi- tion ; plow under a green covercrop ; and experiment with sulfur applied to the soil before planting. During the past decade considerable work has been done with various forms of sulfur or other acid-producing chemicals applied to infested soil in attempts to make conditions unfavorable for scab. Such investigations have been conducted in practically every state where scab has been serious, and similar studies have been made in Canada and Europe. The profitable application of such chemicals to scab-infested soils is limited to those districts where long rotation of potato crops is not possible. Sulfur, when applied to scab-infested soils in California, has not given uniformly good results. Significant control was obtained when sulfur was broadcast at the rate of 300 pounds per acre ; but similar trials conducted in Southern California 44 California Agricultural Extension Service [Cir. 61 had no effect on the prevalance of scab. The control of scab by means of soil treatments is a problem of local significance in California, and general recommendations cannot be advocated for the state. The above discussion will guide growers whose soil is severely infested. Fusarium Wilt (Fusarium oxysporum and F. eumartii). — Fusa- rium wilt is commonly found wherever the crop is grown, the severity of infection varying with seed and soil infestation and with environ- mental conditions. To the untrained observer the disease symptoms caused by F. oxysporum and F. eumartii are identical, but some work conducted in Nebraska has shown that the symptoms and nature of these two wilt diseases differ. It is suspected that both species of Fusarium may live in the soil for some time and that F. oxysporum is able to infect tomatoes, cow- peas, peas, and sweet potatoes. The similarity in nature of behavior of the two diseases indicates that preventive measures would be practically the same. Preventive measures are : ( 1 ) rotation, wherein potatoes are grown not oftener than once in three or four years; this precaution is con- sidered necessary only when soil infestation is such that severe damage has previously resulted; (2) careful roguing of diseased plants from the seed plot, or purchase of certified seed; and (3) seed disinfection to destroy any organisms that may be present on the tuber surface. Verticillium Wilt (Verticillium albo-atrum). — The symptoms of verticillium wilt are similar to those of fusarium wilt and may be described as follows : affected plants lose their bright green color, and the leaves tend to curl upward along the midrib, which exposes the lower sides. The lower leaves turn yellow at first and dry up as the disease slowly progresses up the stem. The internodes become shortened, and the plant remains stunted. Inside the stems, the woody portions are discolored, and in the final stages of the disease they are often a pronounced brown. All stems of a plant may not be affected at one time, but all are usually affected before the plant dies. Spots of irregular shape and size may develop on the portions of the main stem underground and on the larger fibrous roots. By means of the char- acteristic discolorations of the vascular system the fungus may be traced through the stolons into the tubers. Ordinarily a brown dis- coloration appears in the tissue of the tuber immediately under the point of stolon attachment. From this point the discoloration may spread out in the vascular system to the apical end of the tuber. Externally the tubers may appear normal, although usually they are smaller and fewer in number than those produced by healthy plants. 1932] Potato Production in California 45 Verticillium wilt is more often found in the coastal sections than in the interior valleys of California. The probable explanation of this distribution is that verticillium is favored by relatively low soil tem- perature, as compared with the optimum soil temperature for infection with F. oxysporum or F. eumartii. It was found in Oregon that the disease might be spread from plant to plant in the row during one growing season, a fact indicating that under irrigation considerable field spread probably takes place. None of the important commercial varieties are resistant. Prevention of loss from verticillium wilt may be accomplished by the measures which apply to fusarium wilt. Leak (Pythium debaryamim) . — Potato leak is so named because of the extremely watery nature of the affected tuber tissues. The water is usually held by the disintegrated tissues, but when pressure is applied to the tuber a yellowish to brown liquid is readily given off. Another characteristic symptom is the granular nature of the diseased tissues. Externally the affected tissues appear turgid and may show discoloration, ranging from a metallic gray in red-skinned varieties to brown shades in white and dark-skinned varieties, Internally they are creamy in color at first; but upon exposure to the air they first become tan or slightly reddish, then change to brown, and finally turn black. The diseased areas are generally rather sharply set off from the healthy tissue. Rarely is any fungus growth visible either inside or outside the tuber. Tubers become infected in the field where Pythium debaryanum lives as a soil fungus. Infection takes place during hot weather and apparently only through wounds, though these need not be visible. When infection has once taken place the disease progresses very rapidly and develops in contaminated tubers even in refrigerator cars. Freezing temperatures arrest the development of the disease so that the brown or black lesions frequently dry out, forming cavities. At temperatures between 32° and 50° F leak is followed by fusarium tuber rot. Under warm conditions (50° F and higher) leak lesions are usually invaded by bacteria, which check the development of the leak organism, leading to foul-smelling, sticky, or slimy decays. Control of Leak. — Prevention of damage due to potato leak in- volves careful digging of mature tubers so as to prevent injury, and keeping the tubers as cool and dry as possible during harvesting and loading. With the increasing prevalence of machine diggers in the Delta district, potato leak is much less serious there than when the crop was dug by hand. It is the opinion of some growers that washing of tubers before sacking for the market tends to check leak damage. 46 California Agricultural Extension Service [Oir. 61 Late Blight (Phytophthora infest cms). — Potato late blight, a dis- ease of the foliage and tuber, is not widespread in the state. Most of the California crop matures during that season of the year when high temperature and drought prevail, two conditions unfavorable for spread and infection. The disease occurs annually in the coastal sec- tions and occasionally in the interior mountain regions. Considerable loss is often suffered by growers in San Diego County and occasionally by those in Marin, Humboldt, and San Francisco counties. Much of the crop in San Diego County matures from February to April, where overhead irrigation practices combined with low air temperatures make conditions favorable for infection. In the San Diego and Colma sections late blight may cause damage during March or April, while in Humboldt County the disease may be prevalent in the fall of the year. Potato late blight may attack tubers, leaves, petioles, stems, and blossom pedicels. Lesions are evident on the upper side of the leaflet, appearing as brown spots. If the weather conditions are favorable, these spots rapidly increase in size, and soon the whole vine may become infected. As the lesion enlarges, wefts of white mold appear adjacent to the lesion on the lower surface of the leaflet and other aboveground parts of the plant. Infected tubers manifest slightly sunken, dark-colored, rotted areas of irregular size, varying from very small spots to large ones involving the whole tuber surface. At harvest time the rot does not, as a rule, extend very far into the tuber ; but in storage the rot may become complicated with other rot-produc- ing organisms and the entire tuber may decay, particularly if the temperature remains near 40° F and if ventilation is poor. Control of Late Blight. — To control late blight one must use seed free of infection ; and, if the disease appears on the foliage one must spray or dust with a copper fungicide. When the disease develops on the crop growing in hilly sections, spraying is practically impossible. Under such conditions the best plan is to dust with a mixture consist- ing of 15 pounds of partly dehydrated copper sulfate and 85 pounds of hydrated lime. Approximately 50 pounds of this dust mixture is required per acre for each application on full-grown plants. Propor- tionately less than this amount would suffice when the plants are smaller. The first application should be made at the first evidence of the disease, but in localities where late blight occurs every year the first application should be made before any signs of the disease are evident and should be continued at intervals so that new growth is kept covered. The dust will be much more effective if applied when the foliage is moist. 1932] Potato Production in California 47 In localities where spraying* with liquid material is possible, bor- deaux mixture 5-5-50 may be applied. This fungicide should be made as follows : dissolve 5 pounds of copper sulfate in 25 gallons of water, either by suspending the former in a sack in water overnight or by dissolving it first in a small quantity of hot water. Then slake 5 pounds of lime in a small amount of water and dilute to 25 gallons. Pour the two solutions together into a third barrel or simultaneously through a strainer into the spray tank with the agitator moving. The mixture is then ready for use and should be applied at once. If it is impossible to spray at once, add a heaping tablespoon of sugar dis- solved in a little water, to each 50 gallons of the bordeaux. This pre- serves the mixture for as long as 2 or 3 months. Follow the same spray schedule as if dusting. Early Blight (Alternaria solani). — This disease is of rather gen- eral occurrence but causes relatively little loss in California. The disease is manifested on the leaves, where brown spots, circular and concentrical in appearance, develop ; and on the tubers, where sunken, irregular, darkened lesions, surrounded by a slightly raised border, occur. Preventive measures are ordinarily unnecessary in California, but the disease may be checked by spraying with bordeaux mixture or by dusting with copper-lime dust as for late blight. Seed-Piece Decay. — The decay of the seed-piece before sprouting or before the young plant has established itself independent of the 'mother tuber' is often responsible for uneven stands in the summer or fall-planted crop in California, Similar conditions prevail in other regions, particularly when the crop is planted during hot weather. Various organisms are known to be capable of rotting seed-pieces, even at low temperatures; and growers on Long Island, New York, report that improper mixing of commercial fertilizers with the soil often causes injury to the cut surface of the seed-piece, thus providing favorable conditions for decay by certain microorganisms. The black- leg organism {Bacillus atrosepticus) , Fusarium oxysporum, and certain species of Sclerotium may destroy the seed-pieces. Under California conditions, decay of sets may be partial or com- plete, particularly in the late-planted crop when soil temperatures are relatively high. If partial decay occurs soon after planting, a weak- ened plant results, which produces a relatively low yield or dies before marketable tubers are formed. Observation shows that while the seed- piece contributes to the growth of the potato plant in the early stages of development, it may nevertheless become a source of injury if it undergoes rotting in the later stages. Perhaps, as has been suggested, 48 California Agricultural Extension Service [Cm. 61 a rotting seed-piece is the source of toxic materials which weaken the plant after gaining entrance. Authorities do not know at present whether seed-piece decay results primarily from attack by microorganisms or whether decayed seed-pieces are of such dormancy that their slowness in germinating enables a rot-producing organism to gain entrance more easily. The fall crop in southern California is usually planted with seed which was produced in the summer immediately preceding. Such seed is more dormant than seed which matured the previous fall and usually germinates more slowly. On the other hand, growers have planted their fall crop with seed dug only a week before and obtained satis- factory stands. All of the contributing causes of seed-piece decay under California conditions are yet too imperfectly understood to warrant inclusive recommendations for prevention. Partial prevention of seed-piece decay in California was accom- plished by the late Dr. J. T. Rosa in 1926 and 1927. These experiments were concerned with seed disinfection to protect the cut surface, plant- ing of small uncut seed, and storage of cut sets under different environ- mental conditions for several days before planting. The results were not strikingly uniform, nor did they indicate that the problem was solved ; but they showed that ( 1 ) the longer the cut sets were kept in storage at temperatures from 35° to 72° F before planting, the more severe was seed-piece decay; (2) that whole, uncut tubers did not decay so rapidly as cut seed-pieces, but the former germinated slowly ; (3) some evidence was obtained to show that certain seed disinfectants partially prevented seed-piece decay; and (4) less decay resulted when 2-ounce tubers were cut transversely than when larger tubers were cut into several pieces. Blackheart. — Potato blackheart is not caused by a disease-producing microorganism, but results when tubers are exposed to air tempera- tures of 100° to 118° F or when tubers are stored under conditions of deficient oxygen. The symptoms of blackheart vary, depending upon whether the uninjured tubers were exposed to high temperatures and a normal air supply or to high, low, or normal temperatures with an insufficient air supply. In the former case no external symptoms develop; in the latter, both external and internal symptoms appear. The external symptoms of blackheart are manifested as moist areas on the surface, which may be purplish at first but become brown or black within a short time. The internal symptoms are dark-gray to purplish or inky black discoloration. Injured tissues, when freshly cut soon after injury, are of normal color; but after contact with air 1932] Potato Production in California 49 for a short time they turn pink, then gray or purplish, and finally jet black. Sometimes all the above colors except pink are found in the same tuber, at other times only gray or brown colors are found, as is the case when tubers are heated above 130° F or when they are deprived of all oxygen for considerable periods, as in waterlogged soils. The discolored areas are usually sharply set off from the healthy tissues. The affected tissues are firm and even leathery if they have been dried a little, quite unlike those affected with leak, which fre- quently show colors similar to those of blackheart. Generally the dis- coloration is restricted to the heart of the tuber, but frequently it radiates to the exterior as well. It may also appear on the side of a tuber which has been exposed to a stove in a car or in storage, or to the sun in the field while growing or after digging. The discolored regions may appear in zones in the outer parts of the tuber and be absent or less evident in the center. In advanced stages the affected tissues dry out, and cavities result. Control of Blackheart. — Blackheart injury may be prevented by keeping the tubers at temperatures below 95° F and in an environ- ment not deficient in oxygen. Tubers should not be allowed to remain in hot, light soils after the vines are dead or to lie long exposed to the heat of the sun after digging. It is also unsafe to store tubers in bulk in piles more than 6 feet deep, for such a practice may result in oxygen deficiency and initiate blackheart. Hollowheart. — Potato hollowheart is an abnormality of the tuber only, characterized by a lens-shaped hole in the flesh at the center of the tuber, surrounded by brownish discoloration consisting of layers of dead cells. It is most often found in the larger tubers. The disease is believed to be a physiological trouble brought about by environ- mental conditions which induce excessively rapid tuber development. Within recent years much attention has been given this disease, especi- ally in the northern states and Canada. Hollowheart is seldom observed in those southern states where tubers are harvested before they are fully mature. It is generally believed to result when periods of slow tuber growth are followed by conditions of moisture, chiefly rainfall, which encourage rapid second growth. This disease has received most attention in Michigan and Nebraska. Although it has been observed in California, it is not considered a major trouble here. The disease is not transmitted by the seed tubers. Reports from Nebraska indicate that hollowheart may be prevented by those cultural practices which provide for a heavy set of tubers and favor uniformly vigorous growth. Observations in Michigan show 50 California Agricultural Extension Service [ Cir - 61 that several cultural practices tend to reduce the amount of hollow- heart, The recommendations are as follows: (1) apply stable manure and high analysis commercial fertilizer; (2) plant the best seed avail- able, insuring a good stand; and (3) use large seed-pieces (1% to 2 ounces) and plant as close as practicable. Heat and Drought Necrosis. — This type of tuber discoloration is commonly found in tubers grown in light, hot soils and is most likely to occur in tubers allowed to remain in hot soils after the vines begin to die. It is manifested by a golden yellow to brown discoloration of the water-conducting vessels of infected tubers. The necrosis is usu- ally more evident in the ring tissues at either end of the tuber, but is also found in the tissues between the ring and the tuber surface. Sometimes in light-skinned varieties the discolored tissues give a darkened appearance to restricted areas of the surface. When cut open, the discolorations are found to result not from a solid, dark mass of tissue, but rather from discolored strands which impart a dark hue to the tissue under which they lie. More frequent light irrigations should be helpful in preventing the trouble, and the disease may be checked somewhat by digging the tubers as soon as practicable, when they mature under conditions of extreme heat. Although the disease is not tuber-transmitted, infected tubers should not be used for seed purposes, because they may produce weak plants. Freezing Injury. — The symptoms of freezing injury to potato tubers vary with the temperature to which the tubers are exposed. One, the ring type, is limited to the vascular ring and immediately adjoining tissues. Another, the net type, is marked by more or less blackening of the water vessels and the finer strands which extend from them into the interior pith and outer tissues. Both types are frequently restricted to the stem end. Finally, there is the blotch type, marked by irregular patches ranging in color from an opaque gray or blue to sooty black, which may occur anywhere in the tuber, though they are found generally in the water vessels and in the tissues outside them. These blotches, when in the outer tissues, may be apparent externally in clean tubers with white skins. This is the only type of freezing necrosis which may be visible externally. Tubers affected with any one or all of these types of freezing necrosis gener- ally shrivel or wilt more than non-affected tubers. Excessive shriveling alone, however, cannot be relied upon as a sign of freezing necrosis. Some confusion exists as to the proper use of the terms frost in- jury, freezing injury, and chilling injury, which are customarily used in connection with low-temperature injury to potatoes. Frost injury 1932] Potato Production in California 51 is properly caused by freezing temperatures to potatoes in the field, the results being identical with those following freezing injury. Freez- ing injury is the term usually applied to injury caused by freezing after the potatoes have been dug and removed from the field. It occurs while they are in storage or in transit. Chilling injury is a term brought into general use by an apparent misconception of the effects of low temperatures. Investigators agree that injury to potatoes is not caused by low temperatures alone unless actual ice formation has taken place in the tissues. Potato tubers will not freeze at 32° F. The critical temperature — that is, the temperature at which ice begins to form — lies between 26.6° and 29.5° F. The critical temperature for any individual tuber cannot be forecast, because there is variation in the individual susceptibility of tubers to freezing. While some investigations have indicated that tubers known to have been more or less frozen can be used for seed if properly handled, it is considered unwise to use for seed purposes those which have been exposed to temperatures below 32° F. Sunburn and Sun Scald. — These troubles are induced by the ex- posure of tubers to the sun during growth or after digging, either in the field or in transit or storage. Sunburn is a greening in response to exposure to light and does not involve the killing of the affected tissues, and frequently occurs in growing tubers which are not com- pletely covered with soil. It is accompanied by a bitter taste, renders tubers unpalatable, and in cases of long exposure, the outer tissues turn deep green and the underlying ones a greenish or deep yellow. Such tubers usually wilt and shrivel abnormally. Frequently the exposure to sunlight and accompanying high temperatures leads to the killing of the cells, a condition known as sun scald. Often the affected tubers become watery and turn brown throughout, or at least to a considerable depth. In other cases, freshly scalded areas exter- nally appear blister-like and have a metallic color, the underlying tis- sues being watery. Such areas may dry out and appear chalky and granular or hard and leathery. Most frequently, however, they are attacked by bacteria or fungi which cause foul-smelling rots. Obvi- ously, control is accomplished by prevention of exposure of tubers to the sun for prolonged periods of time. Growth Cracks and Second Growth. — These abnormalities are commonly found among tubers grown on irrigated land, but also develop under abnormal conditions in localities where artificial irriga- tion is not practiced. Such deformities appear to result when the 52 California Agricultural Extension Service [Cir. 61 moisture supply during the growing season is uneven. Drought induces ripening of the tubers and premature hardening of the skin while the tops are still in full vigor. An abundance of moisture fol- lowing drought results in growth renewal ; and, as the hardened skin is incapable of uniform expansion, cracks or knobs result (fig. 2). Growth cracks are usually covered with the normal cork layer and are not readily subject to decay. Knobs are mostly immature and are easily broken off; occasionally they begin to decay in the ground. These abnormalities may be partly prevented by proper applications of irrigation water. Potato Virus Diseases. — While parasitic diseases are caused by known microorganisms and nonparasitic diseases by certain environ- mental conditions, the cause of virus diseases is unknown. In this circular, potato virus diseases are classified as (1) mosaic (manifested by mottling in the foliage), and (2) nonmosaic (mani- fested by symptoms other than mottling). The old idea that potatoes 'run out' when the same stock is grown continually in one locality, necessitating importation of 'new' seed 'from the north' has been largely replaced by the idea, that the so-called 'running out' results from virus infection. The virus theory of degeneration stands in opposition to the older theory in which degeneration was attributed to physiological disturbances in the potato plant. This physiological deterioration was explained in two ways: (1) the potato, being native to cool climates, responds abnormally when grown in warmer regions, and thus the idea developed that seed from cool regions was superior to that grown in the warmer localities; and (2) seed grown in warm regions lost its vitality when it matured in warm soil. Recent work in England, Nebraska, Ohio, and elsewhere has shown that these two theories are untenable ; and investigators are now generally agreed that degeneration is the result of virus infection and not simply the result of continued propagation of a particular lot of seed in a given locality. During the past 15 years much information regarding symptoms, transmission, hosts, yield reduction, and control of potato virus diseases has been published by workers in practically every state in this country, by workers in the United States Department of Agricul- ture, and by investigators in many foreign countries. New and, in some cases, rather striking measures have been found of value in eliminating virus diseases from potato seed stocks. The following discussion of the various virus or degeneration diseases will be of particular interest to the seed grower, for he must understand control measures if he wishes to maintain a ready market 1932] Potato Production in California 53 for his seed. Much of the California potato acreage is planted with seed grown in other states, particularly Minnesota, Wisconsin, Oregon, Idaho, Montana, and Washington. In each of these states potato seed certification is in practice, and usually high quality seed may be obtained, providing the California grower specifies that he will accept only seed which has passed inspection and has been certified by the proper authorities. He should understand, however, that certified seed is in no sense resistant to diseases. Certified seed growers pro- duce seed which has been carefully inspected by trained officials, and only after several field and bin inspections will the various certifying officers issue tags for attachment to the sacked seed stock. Prevalence of Virus Diseases in California. — The virus diseases found to date in this state are mild mosaic, crinkle mosaic, leaf rolling mosaic, rugose mosaic, leafroll, streak, spindle tuber, unmottled curly dwarf, witch's broom, and calico. As the symptoms of some of these diseases are masked under weather conditions which prevail in the interior valleys, some of them probably cause more loss than is now realized. Transmission of Virus Diseases. — Some of the agencies which serve in transmission of potato virus diseases are insects, the seed-cutting knife, planting machinery, and actual contact of diseased with healthy foliage. Experiments have proved that seven of the potato virus diseases are carried from diseased to healthy plants by aphis. By sucking the juice from the sick plants these insects at the same time obtain the "virus" which they inject into healthy plants upon which they feed. Under favorable conditions aphis multiply very rapidly, so that large numbers will disperse from one plant to another, thereby spreading these diseases. Consequently, the larger the number of diseased plants among the healthy ones, the more favorable will be the conditions for spreading these diseases. Besides aphis, several other insects have been found to spread virus diseases. Among these are grasshoppers, leaf hoppers, plant bugs, flea beetles, leather jackets, and white flies. Aphis are generally believed to be the most important insect vectors of potato virus diseases. It is known that spindle tuber may be transmitted by cutting healthy tubers with the same knife used for cutting those infected with this disease. These results suggest the probability that spindle tuber may be transmitted by contact between diseased and healthy seed-pieces in the planter box or by the pickers of the planting machine. 54 California Agricultural Extension Service [ Cte - 61 Tuber Perpetuation of Virus Diseases. — The infectious principle of virus diseases of potatoes passes from infected foliage into the tubers during the growing season. If healthy plants have been located near diseased plants in the field, the former may become infected without manifesting current season symptoms; but tubers from such plants will produce diseased plants in the next generation. Sometimes not all the tubers from a diseased hill produce diseased plants. This condition may be attributed to the fact that a seed-piece may have produced two or more stalks, only one of which may have become infected. Experiments have likewise demonstrated that not all seed-pieces of a tuber will produce diseased plants, probably because of incomplete diffusion of the virus in the infected tuber. With the exception of leaf roll, spindle tuber, and witch's broom, infected tubers do not manifest symptoms of the virus diseases. Not all tubers infected with these three diseases manifest symptoms, so that one can never be sure that a healthy appearing tuber is free of virus infection. Field Spread of Virus Disease. — Transmission from diseased to healthy plants in the field, except by actual foliage contact, is accom- plished almost entirely by insects. Hence the rate and distance of transmission is directly proportional to the kind and number of potential insect vectors. Definite knowledge of the extent of field spread becomes of prime importance to the seed grower, and for this reason the rate and distance of spread of the more common virus diseases have been studied in many of the important seed-producing districts of the United States. No general conclusion can be drawn from such experiments, because the insect vector population is prob- ably never the same from year to year even in one locality. The grower must remember to isolate the seed plot from the main field by at least 300 feet, or, better still, a quarter of a mile. Potato Mosaic. — Four types of mosaic are known to occur in Cali- fornia. These are mild mosaic, crinkle mosaic, leaf rolling mosaic, and rugose mosaic. Mild mosaic symptoms are manifested by a mottling in the green leaf, in which yellowish or light-colored areas alternate with the normal green. Infected plants are slightly dwarfed, and the leaves are tender in texture. Symptoms of mild mosaic are often masked because of weather conditions common to the interior valleys, and many diseased plants escape notice. Tubers known to be infected with mild mosaic have been quartered, and the seed-pieces planted at Davis, Stockton, San Jose, and Riverside. No mosaic symptoms could be detected at Stockton or Riverside, symptoms were very faint at Davis, but at San Jose characteristic mottling developed. 1932] Potato Production in California 55 At higher altitudes, where the air temperature is relatively low, mild mosaic symptoms develop well. Symptoms are evident in Humboldt County and in the winter crop as grown in San Diego and San Fran- cisco counties. Mild mosaic is easily transmissible; and although progressive deterioration does not seem to occur, yields are said to be reduced as much as 20 per cent as compared with healthy plants. Symptoms and effects of crinkle mosaic are similar to those of mild mosaic, but in addition the leaves of infected plants are dis- tinctly ruffled and wrinkled. Mottling caused by crinkle mosaic is often masked by high temperatures ; and when crinkle mosaic infected tubers were quartered and planted at the four places, as indicated for mild mosaic, much the same situation prevailed. One exception, however, was that while mottling was not observed on the plants at Stockton, yet decided wrinkling of the foliage was noted. Yield reduction is approximately equal to that caused by mild mosaic. The symptoms of leaf rolling mosaic resemble those of mild mosaic in so far as mottling of the foliage is concerned, but in addition the younger leaves tend to roll upward. The leaf texture of infected plants is tender, in contrast to brittleness of plants infected with leafroll. Mottling is usually less distinct than with mild mosaic, and is accompanied by slight wrinkling and ruffling. Symptoms were somewhat obscured by high temperatures, but the leafrolling tend- ency was evident at Stockton and Riverside. Yield reduction varies ; it is seldom in excess of 20 per cent. The most severe type of the mosaic diseases and one of the most easily recognized is rugose mosaic. The disease is manifested by distinct dwarfing and slight spindliness of infected plants; by distinct chlorosis, mottling, rolling, curling, streaking, spotting, and burning of the foliage ; and by premature death of the plant and marked rugosity of the leaflets. Unlike the first three types of mosaic, the symptoms of rugose mosaic are somewhat masked by low temperatures. The disease is the most severe of the four mosaic types and may cause as much as 50 per cent yield reduction. Because of the serious yield reduction, rugose mosaic might be considered particularly serious; but the fact that diseased plants are easily recognized makes early roguing possible and effective as a means of control. The disease is readily transmissible; and, if infection takes place before the plant is half grown, current season symptoms manifest themselves before the plant dies. If, however, infection occurs late, symptoms are not evident until the next generation of plants is produced. Thus it is essential that infected plants be roguecl from the seed plot as soon as they are recognized. 56 California Agricultural Extension Service [ Gm - 61 Non-Mosaic Virus Diseases. — This group of diseases may be dis- tinguished from the preceding group by the absence of mottling or mosaic appearance in the leaves. The non-mosaic virus diseases of potatoes which are known to occur in California are leafroll, spindle tuber, streak, unmottled curly dwarf, and witch's broom. Leafroll is manifested by the upward rolling of the leaves so that the midrib remains at the middle of the trough thus formed. Plants becoming infected with leafroll when young or plants from tubers infected with leafroll show rolling at first on the lower leaves, followed by rolling of progressively higher leaves, until all leaves may be rolled later in the season. Plants infected late in their development may show rolling only in the upper leaves. Other symptoms of leafroll include dwarfing, rigidity, leathery texture, uprightness, chlorosis, shortness of stolons, reduction in number and size of tubers, and yellowish, reddish, or purplish discoloration of the affected leaves. A netted discoloration of the inside of the tubers is a symptom of leafroll in some varieties; this symptom is designated as 'net-necrosis.' The flesh of infected tubers is usually more brittle than normal, and such tubers often produce spindling sprouts which lack sufficient vitality to produce a crop of marketable tubers. Leafroll is transmissible by means of aphis as well as by tuber contact. The disease commonly designated by growers as "running out" is in reality the disease known as spindle tuber and is so named because of the spindle-shaped tubers produced by infected plants. In addition to spindliness, infected tubers exhibit more numerous and more conspicuous eyes than normal tubers. In comparison with healthy plants, symptoms on the growing plant are spindling and upright stalks, somewhat smaller and slightly darker green leaves and usually a sharper angle between the leaf blade and stem. In- fected plants are believed to be slightly frost-resistant and longer- lived than healthy plants. The disease becomes progressively severe with continued propagation of tubers from infected plants and may cause significant reduction in yield. It is transmissible by aphis and grasshoppers, leaf mutilation, tuber grafts, and seed-cutting knife. Not all infected tubers show symptoms of the disease; hence the practice of discarding visibly infected tubers will not eliminate spindle tuber from seed stock. A particularly malignant disease of potatoes is streak and is char- acterized by streaking, spotting, burning, brittleness, leaf dropping, and premature death of infected plants. Infected stalks and petioles are exceedingly brittle, so that the leaves drop and frequently remain 1932] Potato Production in California 57 suspended at the base of the petiole. The tubers from plants infected early in the season remain small. Second generation plants remain severely dwarfed, curled, and streaked, and frequently die before tuber formation. Streak is transmitted by aphis, leaf mutilation, and tuber grafts. Certain conditions known as unmottled curly dwarf and witch's broom have been found in California, but not to the extent that much damage results, The former is believed to be associated with spindle- tuber symptoms, while the latter is manifested by a rosette effect. Control of Potato Virus Diseases. — To the potato seed grower the following facts are pertinent to the elimination of virus diseases from his stock: (1) the virus diseases are not soil borne, (2) all known virus diseases are tuber-perpetuated, (3) insects are the most per- sistent offenders in field transmission, (4) certain weeds such as nightshade are hosts, (5) spraying or dusting the growing crop pre- vents the spread of virus diseases only in so far as insect vectors are killed or repelled, and (6) seed disinfection is of no avail. To the grower who does not wish to make a practice of producing his own seed it is suggested that, whenever possible, certified seed should be obtained ; or, in the event that it is not available, seed should be secured from a reliable grower who is making honest effort to improve his seed stock. One of the most serious handicaps with which the California potato seed grower has to contend is the masking of symptoms of mild, crinkle, and leafrolling mosaic under conditions of relatively high air temperature. A close second is the lack of knowledge of the insect vectors, their prevalence, distribution, and role in trans- mission of all virus diseases, Because symptoms are masked in the interior valleys where much of the crop is produced, the attempt to produce good seed under such conditions might seem hopeless. In the coastal sections and at higher altitudes there is reason to believe that isolated spots might be found where, with proper precautions, high quality seed could be produced. Recently, however, some evidence has been obtained which indicates that, even in the interior valleys, seed of reasonably high quality may be produced as a fall crop. Late planting cannot yet be recommended as a. general practice in seed production in California, but investigations dealing with this aspect are now under way. A second factor of considerable economic importance concerns prevention of field transmission of virus diseases by the use of insecticides for the control of the insect vectors. Such investigations are likewise under way. 58 California Agricultural Extension Service [ Cir - 61 The limiting factor in potato seed production is probably the masking of the symptoms of such virus diseases as mild and crinkle mosaic, particularly in the spring crop as grown in the interior valleys. This masking makes it physically impossible to detect plants infected with these two diseases ; therefore the maintenance of a seed plot in the interior valleys is not generally recommended if the seed stock is known to be infected with either mild or crinkle mosaic. If the stock is infected with such virus diseases as rugose mosaic, leafroll, or spindle tuber, much improvement is possible in a seed plot even in the interior valleys. Growers who annually plant a large acreage sometimes feel that they do not have time to devote to the seed plot, but they should remember that 1 pound of ' tubers planted in the seed plot should produce at least 10 or even 20 pounds of seed for the next planting. In the seed plot one can also remove an'd destroy plants infected with such diseases as blackleg, fusarium wilt, and verticillium wilt, in addition to those which show symptoms of virus diseases. Such diseases as scab, rhizoctonia, and late blight can also be more easily checked in the seed plot than in the main field. The seed plot should be started with either certified or tuber- indexed seed stock, although progress can be made with uncertified stock which is not heavily infected with virus diseases. All tubers should be planted in the seed plot by the tuber-unit method ; that is, all the sets of one tuber should be planted in succession in the row and about four feet left vacant before the sets of another tuber are planted. Some growers prefer to mark each tuber unit with a wooden stake set in the row. The object of tuber-unit planting in the seed plot is to facilitate roguing during the growing season. If one plant of a tuber unit is diseased, the entire unit should be removed, even though some plants of this unit may appear healthy. Evidently, then, tuber-unit planting makes roguing easier and much more accurate than promiscuous planting of sets in the seed plot. Roguing of diseased plants should begin as soon as any disease is evident and should continue at intervals depending on growing conditions. As insects spread virus diseases from plant to plant, the necessity of prompt removal of diseased plants is evident. The prevalence of insect vectors of virus diseases likewise magnifies the necessity of isolation of the seed plot from the main field by at least 300 feet. A quarter of a mile is preferable. The value of tuber-indexing of stock before planting the seed plot has been demonstrated not only in California but in many other states. This practice is nearly ideal, but requires facilities not gen- 1932] Potato Production in California 59 erally available to the average grower. Tuber-indexing involves the growing of one eye from each tuber well in advance of the planting season. The eye which is planted is numbered to correspond with the mother tuber; those which index as diseased are discarded, so that only healthy-appearing tubers are planted. Even with the precaution of tuber-indexing, some diseased plants may appear in the seed plot, and one must usually rogue a few units. Growers may have access to a greenhouse where seed stock may be tuber indexed during the winter months, and growers who live in frost-free districts may be able to index in the open field or in coldframes which may be covered in case of frost danger. Growers in Oregon have accom- plished nearly as good results by ordinary tuber-unit planting as by tuber-indexing. Either method, if intelligently used, will tend to rid seed stock of virus infection. SEED DISINFECTION Rhizoetonia and common scab infection may be evident on the surface of seed potatoes, and the spores of other disease-producing microorganisms may be lodged there without being visible to the naked eye, hence the necessity of seed disinfection. Two methods of disinfection are recommended, both being equally effective. Corrosive sublimate (mercuric chloride) is dissolved in water at the rate of 1 ounce in 8 gallons. In the resulting solution the tubers are allowed to soak for 90 minutes. The necessary quantity of corro- sive sublimate can best be dissolved in a gallon of warm water and later diluted to the proper strength. This disinfectant should never be used in metal containers for it is corrosive in nature. Satisfactory containers are wooden barrels, troughs, or concrete tanks, Corrosive sublimate solution is a deadly poison, and care should be exercised' in its use. The strength of the solution weakens during continued use ; and more of the corrosive sublimate must be added from time to time when the same solution is used. Recent work has shown the difficulty of maintaining the correct concentration of corrosive sublimate solution even by the addition of more of the chemical to the used solution. The grower who prefers corrosive sublimate to other chemicals, is advised, after the solution has been used four times, to add a /2 ounce of corrosive sublimate for each 8 gallons of water and to restore the solution to its original volume. This advice is, at best, only a guess; and the problem of concentration is one of 60 California Agricultural Extension Service [Cir. 61 the limitations of corrosive sublimate as a seed disinfectant for potatoes. Hot formaldehyde lacks many of the disadvantages of corrosive sublimate. It does not lose strength during continued use, it may be Fig. 19. — Machine used for treating potato seed-tubers with hot formaldehyde. Heat is applied from two gas burners below the tank. Gasoline or kerosene is used in the pressure tank. This machine will handle about 15 sacks per hour. used in metal containers, it is not highly poisonous, and tubers treated with it may be safely used as food. Hot formaldehyde is used as follows : add commercial formalde- hyde (37 to 40 per cent solution) to water at the rate of 1 pint in 15 gallons. Heat this solution to 124°-126° F and dip the tubers (in sacks) for 3 or 4 minutes. The solution may be heated by means of an open fire, steam coils, or gas burners (fig. 19). Commercial machines for doing this work are now on the market. 1932] Potato Production in California 61 THE POTATO DISEASE CONTROL PROGRAM This circular emphasizes the fact that profitable production of potatoes is partly limited by the intelligent application of disease control measures. The crop is reproduced entirely by tubers which are easily injured during digging, handling, and storage and are subject to nonparasitic diseases and to infection with parasitic and virus diseases. As most of these diseases are carried in the tuber, it is essential that effort be made to secure or propagate relatively dis- ease-free seed stock. Adoption of the practices listed below will enable the grower to reduce losses due to diseases and thereby to increase his profit. 1. Secure the best seed obtainable. If certified stock cannot be obtained, either buy from a reliable seed grower or maintain a seed plot. 2. Disinfect the seed tubers with either hot formaldehyde or corrosive sublimate. 3. Rotate so that potatoes are not grown on the same land oftener than once in two or three years. 4. Handle the tubers as carefully as possible in order to avoid unnecessary injury. ACKNOWLEDGMENTS The writer wishes to thank Messrs. F. H. Ernst, L. W. Taylor, and J. A. Ream for data on fertilizers and to credit Prof. E. 0. Essig for the photographs used in figures 7, 8, 9, 10, 11, 12, and 13. AGRICULTURAL EXTENSION SERVICE CIRCULARS No. 3. Feeding Beef Cattle in California. 5. Lettuce. (Series on California Crops and Prices.) 7. Suggestions on Grapefruit Culture in Imperial Valley. 9. Rabbit Raising. 10. The Home Preparation of Fruit Candy. 11. Cauliflower Production. 12. Wool Production in California. 13. The Manufacture of Monterey Cheese. 14. Selection and Care of Electrical Equip- ment Used in Dairy Manufacturing. 15. Pork Production in California. 16. Irrigation of Orchards by Contour Fur- rows. 17. Liver Fluke and Stomach "Worm of Sheep. 21. Bovine Tuberculosis. 22. Thinning Sugar Beets. 23. Strawberry Culture in California. 25. Bush Fruit Culture in California. 26. The Home Vegetable Garden. 29. Control of Pocket Gophers and Moles in California. 30. Elements of Grape Growing in Cali- fornia. 31. Powdery Mildew of the Grape and Its Control in California. 32. What to do About Bovine Tuberculosis. 33. Rearing Dairy Heifers Free from Tuber- culosis and Abortion Disease. No. 34. Plum Growing in California. 35. Alfalfa Production. 36. Beekeeping for the Beginner in Cali- fornia. 37. Home and Farm Preparation of Pickles. 38. Alfalfa Varieties and Seed Supply. 40. Frost Protection in California Orchards. 41. Prune Culture in California. 42. Peach Culture in California. 43. The California Avocado Industry. 44. Bang's Disease (Infectious Abortion). 45. Zinc Chloride Treatment for Pear Blight Cankers. 46. Cherry Culture in California. 47. Equipment for the Bulk Handling of Grain. 48. The Manufacture of Cottage Cheese. 49. Sheep Production in California. 51- Apricot Growing in California. 53. Home Floriculture in California. 54. The Control of Weeds. 55. Growing and Handling Sweet Potatoes in California. 56. Girdling Grape Vines. 57. Commercial Fertilizers and Soil Fertility in California. 58. Turkey Raising in California. 59. The 1932 Agricultural Outlook for Cali- fornia. STATION PUBLICATIONS AVAILABLE FOE FKEE DISTRIBUTION BULLETINS No. No. 253. Irrigation and Soil Conditions in the 433. Sierra Nevada Foothills, California. 263. Size Grades for Ripe Olives. 279. Irrigation of Rice in California. 435. 283. The Olive Insects of California. 310. Plum Pollination. 331. Phylloxera-Resistant Stocks. 439. 343. Cheese Pests and Their Control. 348. Pruning Young Olive Trees. 349. A Study of Sidedraft and Tractor Hitches. 357. A Self-Mixing Dusting Machine for 440. Applying Dry Insecticides and Fun- gicides. 361. Preliminary Yield Tables for Second- 445. Growth Redwood. 364. Fungicidal Dusts for the Control of 446. Bunt. 447. 369. Comparison of Woods for Butter Boxes. 370. Factors Influencing the Development 448. of Internal Browning of the Yellow Newtown Apple. 449. 371. The Relative Cost of Yarding Small and Large Timber. 450. 373. Pear Pollination. 374. A Survey of Orchard Practices in the Citrus Industry of Southern Cali- 452. fornia. 454. 3 79. Walnut Culture in California. 386. Pruning Bearing Deciduous Fruit 455. Trees. 389. Berseem or Egyptian Clover. 456. 392. Fruit Juice Concentrates. 393. Crop Sequences at Davis. 458. 394. I. Cereal Hay Production in California. II. Feeding Trials with Cereal Hays. 459. 395. Bark Diseases of Citrus Trees in Cali- fornia. 462. 396. The Mat Bean. Phaseolus Aconitifolius. 464. 404. The Dehydration of Prunes. 406. Stationarv Sprav Plants in California. 465. 407. Yield, Stand, and Volume Tables for 466. White Fir in the California Pine Region. 467. 408. Alternaria Rot of Lemons. 468. 409. The Digestibility of Certain Fruit By- products as Determined for Rumi- 469. nants. Part I. Dried Orange Pulp 470. and Raisin Pulp. 410. Factors Influencing the Quality of Fresh 471. Asparagus After It is Harvested. 416. Culture of the Oriental Persimmon in 472. California. 473. 417. Poultry Feeding: Principles and Prac- tice. 474. 418. A Study of Various Rations for Fin- ishing Range Calves as Baby Beeves. 419. Economic Aspects of the Cantaloupe 475. Industry. 476. 420. Rice and Rice By-Products as Feeds 477. for Fattening Swine. 421. Beef Cattle Feeding Trials, 1921-24. 479. 423. Apricots (Series on California Crops and Prices). 425. Apple Growing in California. 480. 426. Apple Pollination Studies in California. 427. The Value of Orange Pulp for Milk 481. Production. 428. The Relation of Maturity of California 482. Plums to Shipping and Dessert 483. Quality. 484. 431. Raisin By-Products and Bean Screen- ings as Feeds for Fattening Lambs. 485. 432. Some Economic Problems Involved in 487. the Pooling of Fruit. Power Requirements of Electrically Driven Dairy Manufacturing Equip- ment. The Problem of Securing Closer Rela- tionship between Agricultural Devel- opment and Irrigation Construction. The Digestibility of Certain Fruit By- products as Determined for Rumi- nants. Part II. Dried Pineapple Pulp, Dried Lemon Pulp, and Dried Olive Pulp. The Feeding Value of Raisins and Dairy By-Products for Growing and Fattening Swine. Economic Aspects of the Apple In- dustry. The Asparagus Industry in California. A Method of Determining the Clean Weights of Individual Fleeces of Wool. Farmers' Purchase Agreement for Deep Well Pumps. Economic Aspects of the Watermelon Industry. Irrigation Investigations with Field Crops at Davis, and at Delhi, Cali- fornia, 1909-1925. Economic Aspects of the Pear Industry. Rice Experiments in Sacramento Val- ley, 1922-1927. Reclamation of the Fresno Type of Black-Alkali Soil. Yield. Stand and Volume Tables for Red Fir in California. Factors Influencing Percentage Calf Crop in Range Herds. Economic Aspects of the Fresh Plum Industry. Prune Supply and Price Situation. Drainage in the Sacramento Valley Rice Fields. Curly Top Symptoms of the Sugar Beet. The Continuous Can Washer for Dairy Plants. Oat Varieties in California. Sterilization of Dairy Utensils with Humidified Hot Air. The Solar Heater. Maturity Standards for Harvesting Bartlett Pears for Eastern Shipment. The Use of Sulfur Dioxide in Shipping Grapes. Adobe Construction. Economic Aspects of the Sheep In- dustry. Factors Affecting the Cost of Tractor Logging in the California Pine Region. Walnut Supply and Price Situation. Poultry Houses and Equipment. Improved Methods of Harvesting Grain Sorghum. I. Irrigation Experiments with Peaches in California. II. Canning Quality of Irrigated Peaches. The Use. Value, and Cost of Credit in Agriculture. Utilization of Wild Oat Hay for Fat- tening Yearling Steers. Substitutes for Wooden Breakpins. Utilization of Surplus Prunes. The Effects of Desiccating Winds on Citrus Trees. Drying Cut Fruits. Asparagus (Series on California Crops and Prices). BULLETINS— (Continued) No. .488. Cherries (Series on California Crops and Prices). 489. Irrigation Water Requirement Studies of Citrus and Avocado Trees in San Diego County, California, 1926 and 1927. 490. Olive Thinning and Other Means of Increasing Size of Olives. 491. Yield, Stand, and Volume Tables for Douglas Fir in California. 492. Berrv Thinning of Grapes. 493. Fruit Markets in Eastern Asia. 494. Infectious Bronchitis in Fowls. 495. Milk Cooling on California Dairy Farms. 496. Precooling of Fresh Fruits and Tem- peratures of Refrigerator Cars and Warehouse Rooms. 497. A Study of the Shipment of Fresh Fruits and Vegetables to the Far East. 498. Pickling Green Olives. 499. Air Cleaners for Motor Vehicles. 500. Dehydration of Grapes. 501. Marketing: California Apples. 502. Wheat (Series on California Crops and Prices). 503. St. Johnswort on Range Lands of California. 504. Economic Problems of California Agri- culture. (A Report to the Governor of California.) No. 505 The Snowy Tree Cricket and Other Insects Injurious to Raspberries. Fruit Spoilage Disease of Figs. 507. Cantaloupe Powdery Mildew in the Imperial Valley. The Swelling of Canned Prunes. The Biological Control of Mealybugs Attacking Citrus. 510. Olives (Series on California Crops and Prices). 511. Diseases of Grain and Their Control. 512. Barley (Series on California Crops and Prices). 513. An Economic Survey of the Los Angeles Milk Market. Dairy Products (Series on California Crops and Prices). The European Brown Snail in Cali- fornia. Operations of the Poultry Producers of Southern California, Inc. Nectar and Pollen Plants of California. The Garden Centipede. 519. Pruning and Thinning Experiments with Grapes. A Survey of Infectious Laryngotrache- itis of Fowls. Alfalfa (Series on California Crops and Prices). 506. 508 509 514. 515. 516. 517 518 520. 521. CIRCULARS No. 115, Grafting Vinifera Vineyards. 178. The Packing of Apples in California. 212. Salvaging Rain-Damaged Prunes. 230. Testing Milk", Cream, and Skim Milk for Butterfat. 232. Harvesting and Handling California Cherries for Eastern Shipment. 239. Harvesting and Handling Apricots and Plums for Eastern Shipment. 240. Harvesting and Handling California Pears for Eastern Shipment. 241. Harvesting and Handling: California Peaches for Eastern Shipment. 244. Central Wire Bracing for Fruit Trees. 245. Vine Pruning Systems. 248. Some Common Errors in Vine Pruning and Their Remedies. 249. Replacing Missing Vines. 253. Vineyard Plans. 257. The Small-Seeded Horse Bean (Vicia faba var. minor). 258.. Thinning Deciduous Fruits. 259. Pear By-Products. 261. Sewing Grain Sacks. 262. Cabbagre Production in California. 265. Plant Disease and Pest Control. 269. An Orchard Brush Burner. 270. A Farm Septic Tank. No. 279. The Preparation and Refining of Olive Oil in Southern Europe. 282. Prevention of Insect Attack on Stored Grain. 288. Phylloxera Resistant Vineyards. 290. The Tangier Pea. 292. Alkali Soils. 294. Propagation of Deciduous Fruits. 296. Control of the California Ground Squirrel. 301. Buckeye Poisoning of the Honey Bee. 304. Drainage on the Farm. 305. Liming: the Soil. 307. American Foulbrood and Its Control. 308. Cantaloupe Production in California. 310. The Operation of the Bacteriological Laboratory for Dairy Plants. 316. Electrical Statistics for California Farms. 317. Fertilizer Problems and Analysis of Soils in California. 318. Termites and Termite Damage. 319. Pasteurizing Milk for Calf Feeding. 320. Preservation of Fruits and Vegetables by Freezing Storage. 321. Treatment of Lime-induced Chlorosis with Iron Salts. 322. An Infectious Brain Disease of Horses and Mules (Encephalomyelitis). 16m-4,'32