UNIVERSITY OF CALIFORNIA AGRICULTURAL EXPERIMENT STATION 
 
 BENJ. IDE WHEELER, PRESIDENT 
 
 COLLEGE OF AGRICULTURE 
 
 Vr « va " ,v ""' 1 -' v»nc THOMAS FORSYTH HUNT, Dean »nd Director 
 
 BERKELEY H. E. VAN NORMAN, Vice-director and Dean 
 
 University Farm School 
 
 CIRCULAR No. 161 
 March, 1917 
 
 POTATOES IN CALIFORNIA 
 
 By J. W. GILMOEE 
 
 The principal regions of California in which potatoes are grown 
 at the present time, are the Delta region, comprising parts of San 
 Joaquin, Sacramento, and Contra Costa counties; the Salinas Valley 
 in Monterey County; the Napa and Sonoma Valleys in their respec- 
 tive counties, and the regions comprising parts of Santa Barbara, 
 Los Angeles, and Riverside counties. The average yield per acre in 
 these several localities differs widely, ranging from 4500 to 12,000 
 pounds. In general where soil conditions are optimum the yields vary 
 according to the mean temperature of the growing season; the best 
 yields being obtained in localities with relatively cool summer tem- 
 peratures. Hence, the interior valleys are not so well adapted for 
 potatoes as the cooler coast sections. It should be noted, however, 
 that potatoes are grown in all counties of the state, and by improved 
 culture methods the potato acreage can be extended. 
 
 Soil Considerations. — In order to meet the requirements for profit- 
 able potato yields a soil must be superior in three respects — drainage, 
 organic matter content, and preparation. Of these three, granted 
 that there is reasonable fertility present, drainage is most important. 
 The good potato soils in all parts of the country are those that are 
 either naturally or artificially well drained. On the other hand, 
 there are good soils in potato-producing regions which are not fit 
 for potatoes because they are not well drained. Even though a soil 
 be heavy in texture, it will become friable and productive if it is 
 well drained and contains a fair amount of organic matter. 
 
 The problems of good drainage and organic matter content are 
 inter-related, because they both have to do with affording the growing 
 crop with a sufficient and constant supply of moisture. This is a 
 very important consideration, for tubers do not develop well under 
 
conditions of a fluctuating moisture supply. But in addition to aid- 
 ing in the maintenance of a sufficient and constant supply of moisture, 
 organic matter ameliorates temperature conditions to the end of 
 improving both yield and quality of the crop. A soil that is deficient 
 in organic matter will be of poor physical condition for potatoes, will 
 vary in moisture content during the growing season, and will become 
 warm and cool as the temperature of the air changes. 
 
 Preparation. — The importance of early and thorough preparation 
 cannot be overemphasized. It is estimated that this procedure gives 
 half the assurance of the crop. The land should be plowed as deeply 
 as possible in the fall and allowed to remain rough during the winter, 
 in order that it may be benefitted by the winter rains and weather. 
 As early in the spring as possible it should be worked with disc and 
 harrow as often as may be, in order to keep the surface in good con- 
 dition and to destroy early germinating weeds. It is desirable that 
 the interval between opening the furrows before planting and closing 
 them afterward shall be as short as possible. In case the land cannot 
 be plowed until spring it should be plowed deeply and worked fre- 
 quently, in order to give it sufficient compactness to facilitate the 
 distribution and retention of moisture. 
 
 The Seed Set. — The yield of potatoes on reasonably good soil will 
 increase with the size of the set. But there is a limit in size above 
 which the cost of the seed will be greater than the value of the extra 
 yield and below which the yield of marketable tubers will not pay 
 for the growing of the crop. The results of experiments, extended in 
 time and place, indicate that the seed set producing the greatest re- 
 turns over the cost of seed and of growing the crop should be between 
 two and three ounces. 
 
 Whole or Cut Sets. — The best results have followed the use of cut 
 sets. For optimum yields the size need not be so large for whole 
 tubers as for those cut. It has been found that whole tubers, when 
 size is equal to cut sets, often give somewhat better results than cut 
 sets. This is because the cut sets lose something by evaporation and 
 contact of the fresh surface with the soil, are more subject to attack 
 of diseases in the soil, and also because there are more eyes on the 
 whole tuber. Hence, the whole tuber has a better chance of setting 
 forth its strongest eyes as sprouts. 
 
 On the other hand, the planting of whole tubers is more often 
 questionable because the small whole tuber is likely to represent the 
 progeny of a weak producing hill. For example, in a crop of potatoes 
 where 18 per cent of the total crop were unmarketable or culls, it was 
 found that 76 per cent of these culls were from weak or unproductive 
 
hills. These culls were planted in comparison with cut sets of the 
 same size from large tubers. The yield of the culls was only 56 per 
 cent of the yield from the cut sets. It follows, therefore, that the 
 safe practice is to plant cut sets from large tubers rather than 
 small whole tubers, unless they represent productive hills. If, how- 
 ever, the whole tubers, though small, are from productive hills, these 
 may be better for an additional reason : that small tubers are likely 
 to be less mature than large tubers ; and after the tubers have passed 
 through the required rest period, stronger sprouts emanate from 
 immature than from mature tubers. This is the reason why stronger 
 sprouts come from the bud end than from the stem end. This also 
 explains why second crop potatoes are often better for seed than those 
 of the main crop. 
 
 Cutting. — Since the strongest buds are near the bud end of the 
 tuber the seed set should be cut in such a manner as to include one 
 or two of these strong buds on each piece. With this in mind the 
 tubers should be cut so as to expose as little cut surface as possible. 
 Usually a diagonal cut rather than a straight lengthwise or crosswise 
 cut gives best results. Skill in this work is gained only by experience 
 and when one is expert a decision as to how to cut can be made while 
 the tuber is brought by the one hand to the knife in the other. 
 
 There are mechanical cutters on the market, but these do not 
 perform good service, for there is much waste from small pieces which 
 often possess strong eyes. 
 
 Seed potatoes are often infected with diseases. The presence of 
 these can generally be detected by cutting off a small portion of the 
 stem end. If there appears either in the center or around the edge a 
 spot or ring of brown, the tuber should be cast out, for this is likely 
 to be the fusarium wilt and will be transmitted to the growing crop. 
 
 It is a widespread practice to sprinkle some dry, dusty material 
 over the tubers when cut. The substances usually used for this pur- 
 pose are gypsum, lime, ashes, and road dust. Neither experience nor 
 experiment has shown any decided advantage in this practice except 
 under those conditions when the addition of one of these substances 
 materially benefits the soil ; or when the cut tubers may not be planted 
 for several days after they are cut. Gypsum shows a benefit more 
 often than the other substances, but this is because nearly all soils are 
 benefitted by gypsum. Likewise ashes give good results when the 
 soil requires potash. One must use lime with care, however, for lime 
 promotes conditions in the soil favorable to the growth of disease 
 fungi, hence the crop may be injured more by disease than benefitted 
 by the lime. Seed sets should be planted as soon after cutting as 
 possible, within twenty-four hours at most. 
 
Sprouting. — It is a good practice when possible to sprout seed 
 tubers before cutting and planting. This is usually done by placing 
 the dormant tubers in diffused or subdued light for about three weeks 
 before planting. Sometimes it is well to cover the tubers with a light 
 layer of moist straw, especially if they are placed out of doors. By 
 this process short, stubby and somewhat green sprouts about one- 
 fourth to three-eighths inch long grow out from several eyes. They 
 are not so easily broken off as if sprouted in the dark. There are 
 several advantages in sprouting as suggested. In the first place only 
 those tubers showing vigorous sprouts need be selected for planting, 
 thus a more perfect stand is assured, and a more uniform growth is 
 also secured. In the second place, recognizing that rhizomes come 
 out only at the nodes or joints of the sprout underground, by sprout- 
 ing, more nodes will form beneath the surface than if the tubers 
 begin their sprouting in the ground. Thus, the yield of tubers is 
 likely to be increased. Another advantage is that there may be 
 additional time to finish preparing the land if that has not been 
 completed for an early planting. 
 
 Planting. — If the soil is rich, deep, well drained and well pre- 
 pared, potatoes may be planted as close as 12 inches in the row, 
 with rows 20 inches apart. But under usual soil conditions in Cali- 
 fornia it seems best to plant from 12 to 14 inches in the row with 
 rows 36 to 42 inches apart. In this manner the crop may be cultivated 
 by horse labor without seriously disturbing the vines in their position 
 and growth. The wider distance is essential under conditions where 
 soil is poor and the moisture supply inadequate. 
 
 The best depth for potatoes under normal conditions is four to 
 five inches. If planted too deep the tubers will be small and of 
 poor quality. If planted too shallow they will be of variable size, 
 unshapely and often greened by exposure to the light. In heavier 
 soils the shallower depth may not be exceeded, but in lighter soils the 
 greater depth may be followed. There are several makes of machines 
 on the market for planting. The main advantage in their use is in 
 their enabling one to plant the largest possible area in good season. 
 Their main defect is that they do not work the soil thoroughly and 
 deeply, and in poorly prepared soils the tubers are likely to be planted 
 too shallow. 
 
 Time of Planting. — The optimum time of planting potatoes in 
 California extends over a considerable period. In the coast regions 
 of southern California the planting may begin in the early winter 
 months, especially for crops that are designed for the early market. 
 In most regions, however, the planting may begin in early March and 
 
continue until June, the prospects for largest yields diminishing as 
 the season advances. For the second crop, the planting is usually 
 begun in late July or early August. 
 
 There is a general correlation between the yield and the period 
 of maturation. The longer the growing season the greater the yield. 
 Early potatoes require from 70 to 90 days to bear a crop, while the 
 late varieties require from 130 to 150 days. It is recognized that 
 potatoes are good food at any stage of growth, but immature potatoes 
 do not possess good keeping or shipping qualities. 
 
 Culture. — If the land is deep, rich, and well prepared, and if 
 the seed sets have been well chosen and prepared and seasonably 
 planted, little more can be done to insure a crop. Cultivation may 
 be given as the conservation of moisture or the destruction of weeds 
 may require. Usually three or four cultivations are as many as the 
 crop will pay for. The first cultivations may be deep, but the later 
 treatments should be shallow, because of the danger of disrupting 
 the shallow feeding rootlets. It is best to cultivate potatoes level, 
 but under conditions where water is excessive they may be ridged 
 in order to secure drainage. Under conditions where the crop has 
 to be grown entirely by irrigation they may also be ridged and 
 especially if the rows are close together, for it is inimical for free 
 water to come in contact with the growing tuber for any length of 
 time. 
 
 Irrigation. — Under most conditions where irrigation is necessary 
 it has been found that about 1% acre feet of water are necessary to 
 produce a good yield. This amount should be put on in three or 
 four applications, beginning when the plants are approaching full 
 growth and repeated at intervals of fourteen to eighteen days until 
 the vines show signs of maturity. A light cultivation should be given 
 after each irrigation to break the crust in the furrow and conserve the 
 water applied. The water should be applied by furrows rather than 
 by flooding. Sometimes in light soils a furrow between every alter- 
 nate pair of rows is sufficient, but a furrow between every two rows 
 is best. The potato does not thrive under a fluctuating moisture 
 supply, hence, every attempt should be made by irrigation and culti- 
 vation to keep the moisture supply sufficient and constant. 
 
 Diseases. — There are many diseases attacking the potato in Cali- 
 fornia, and because of our mild climate and other conditions favorable 
 to development they are often difficult to control. 
 
 Perhaps the most destructive disease is the Fusarium wilt. This 
 pest may be detected by a brown spot or ring appearing on the fresh 
 surface of the stem and of the tuber when cut. Such tubers for seed 
 
should be cast out. If, however, they are planted, or if the disease 
 is already in the soil it manifests itself by a sudden wilting of the 
 plant as it comes into vigorous growth. The plant soon turns brown 
 and dies. When this disease becomes established in the soil six 
 or seven years are required to eradicate it, during which time no 
 host plants should be grown upon the land. The remedial measures 
 against this disease are, therefore, rotation of crops and the choice 
 of disease-free seed. 
 
 The disease second in importance is Rhizoctonia. The presence of 
 this disease may be detected by small wart-like and dirty specks, or 
 excrescences, on the surface of the tuber. It manifests itself by a 
 yellowing and wilting of the growing plants as they approach full 
 size. The ravages of this disease may be lessened to a considerable 
 extent by treating the seed tubers in a solution of formalin, one pound 
 to thirty gallons of water for one and one-half to two hours according 
 to the prevalence or intensity of the disease. If the tubers have 
 sprouted before they can be treated it would be well to treat them 
 in a solution of corrosive sublimate (mercuric chlorid), four ounces 
 to thirty gallons of water, for one and one-half hours. This treatment 
 does not injure the sprouts as formalin does. It must be remembered, 
 however, that corrosive sublimate is an active poison and every pre- 
 caution should be taken in the interests of plants and animals. 
 
 Scab. — Oospora: This disease manifests itself by large dried or 
 scabby spots occurring on the tuber, both before and after they are 
 dug. The appearance on the plant is not so marked as with the 
 diseases previously mentioned; but the yield from infested fields is 
 materially reduced. The treatment for seed tubers supposed to be 
 infested is the same as for Rhizoctonia mentioned above. But it does 
 not seem feasible by the evidence of experiments to treat tubers for 
 planting in soil that is already infested with either disease. One 
 pound of formalin will treat from three to five sacks of potatoes, 
 according to the intensity of the diseases, or the prevalence of dirt 
 on the tubers. 
 
 Another disease that is very prevalent and destructive to potatoes 
 in the Delta region is that commonly called Leaks (Rhizopus nigri- 
 cans). This disease manifests itself after the tubers are dug and 
 usually occurs while the tubers are piled in sacks on the levees await- 
 ing shipment. The effect is to cause the potatoes to collapse, the water 
 running away from a large pile in a stream. Sometimes the disease 
 does not manifest itself until the tubers are in transit or in storage. 
 
 The disease attacks only those tubers that have been broken, 
 chipped, or abraded in digging and handling. The trouble cannot be 
 
absolutely prevented or cured, once it has made its attack, but 
 precaution and care can be exercised in the digging and handling to 
 avoid bruising or breaking the skin. 
 
 Of the several insects that attack potatoes the most noteworthy are 
 the flee beetle, which punctures the leaves, thus inviting and favoring 
 the attack of fungi ; the eel worm, which occurs in rich soils and finds 
 in the growing potato a rich supply of food ; and the tuber moth, the 
 larva of which eats holes and cavities in the tuber. 
 
 The flee beetle cannot be entirely prevented from attacking the 
 potato, but it can be discouraged by sprays of bordeaux mixture. To 
 combat the eel worm it is necessary to plant clean seed sets, or if the 
 soil is already infested to plant other plants non-attractive to the pest 
 until the soil is rid of it. 
 
 Mulching. — In some parts of the state, especially in the interior 
 valleys, where the summers are warm, potatoes do not grow to 
 desirable size. Encouragement and advantage may be offered by 
 mulching the ground with broken or partly decayed straw, or other 
 light waste material. This mulch should be applied after the vines 
 are well along in their growth. If the ground has been well prepared 
 and early cultivations are given this mulch may serve in lieu of late 
 cultivations. The effect of the mulch is to keep the ground cooler 
 and more constantly moist than it would otherwise be, thus simu- 
 lating soil conditions in more northern latitudes. 
 
 Fertilizing Potatoes. — There are few experimental data for Cali- 
 fornia bearing on the question of the use of fertilizers. It should 
 be noted, however, that the benefits from fertilizers depend upon 
 several factors, prominent among which may be mentioned the type 
 and character of the soil and the amount of moisture available. In 
 general where fertilizers are desirable a mixture containing 2 to 3 
 per cent of nitrogen, 6 to 8 per cent of phosphoric acid, and 8 to 10 
 per cent of potash, will probably give good results. A mixture of 
 forty pounds of sulfate of ammonia, 120 pounds of acid phosphate 
 and eighty pounds of sulfate of potash per acre will be sufficient for 
 all but very light soils. The mixture should be applied in the furrow 
 and stirred in the soil before planting. 
 
 Varieties. — While many varieties of potatoes have been tried in 
 California, but few have been found well adapted. The following 
 have been found of some merit in various parts of the state : 
 
 Early. — Chili : A pink, smooth-skin potato, adapted to the north- 
 ern Sacramento Valley and coast sections. 
 
 Early Ohio: A pink-skin tuber, grown largely in the southern 
 part of the state. 
 
8 
 
 Early Rose : White and red. Adapted to a wide range of con- 
 ditions and used both for a first and second crop. 
 
 Medium. — American Wonder: A white, shapely potato of good 
 yielding and cooking quality. 
 
 British Queen : A white potato which may be used also for late. 
 
 Late. — Burbank: The most widely grown potato in the state. It 
 has a tendency to grow too large and long, and if not carefully grown 
 its eyes will become rather prominent and deep. 
 
 There are a number of other varieties which may be good for 
 certain localities and conditions, such as Triumph, Pearl, and Uncle 
 Sam, but those mentioned have been tried extensively and perhaps 
 are more certain one year with another. 
 
 Cost of Production. — The following figures represent as nearly as 
 can be presented, average costs of the various stages in potato pro- 
 duction. In some instances the range is wide. The figures under 
 market value represent average for several years past and do not refer 
 to prices at present prevailing. 
 
 Preparing land and planting — 
 
 Plowing and preparing land ($2.50-$8.00) (Coast sections) $ 5.00 per acre 
 
 Plowing for planting ($0.00-$3.00) (Delta sections) 1.50 
 
 Seed ($2.00 per sack) 12.00 
 
 Preparing seed — cutting 1.00 
 
 Preparing see — disinfecting .35 
 
 Planting (six acres per day) ... 90 
 
 Growing tire crop — 
 
 Cultivating ($0.00-$2.00) 1.00 
 
 Hoeing ($0.00-$2.00) 1.00 
 
 Irrigation ($0.00-$10.00) 6.00 
 
 Harvest — 
 
 Digging (machine) ($1.00-$200) ! 1.50 
 
 Picking up and sacking (after machine) 06 per cwt. 
 
 Digging, picking up and sacking (by hand) (10c-20c) 15 per sack 
 
 Sacks 10 
 
 Hauling off (five miles) 1.00 per ton 
 
 Covering sacks (on levee or in field) 10 
 
 Hauling to levee (Delta) 50 
 
 Market Value of Yield — 
 
 High $2.50 (lsts) per cwt. 
 
 1.75 (2nds) 
 
 Low .65 (lsts) 
 
 .50 (2nds) 
 
 Average 1.00 (lsts) 
 
 .65 (2nds) 
 Culls 15