CALIFORNIA AGRICULTURAL EXTENSION SERVICE CIRCULAR 20 JUNE, 1928 PEAR BLIGHT CONTROL IN CALIFORNIA LEONARD H. DAY PUBLISHED BY THE COLLEGE OF AGRICULTURE UNIVERSITY OF CALIFORNIA Cooperative Extension work in Agriculture and Home Economics, College of Agriculture, University of California, and United States Department of Agriculture cooperating. Dis- tributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. B. H. Crocheron, Director, California Agricultural Extension Service. UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1928 CONTENTS PAGE Introduction 3 Characteristics of the'disease 4 Structure and physiology of the pear tree 8 Seasonal history of pear blight in California 12 How blight is spread 14 Reducing the number of insects as a method of blight "control 15 Relation of cultural methods to blight control 16 Soil conditions 16 Pruning 16 Blight control by use of resistant roots, trunks, and 'scaffold branches 18 Surgical methods of treating blight cankers 21 Scarification 22 Zinc chloride for the control of blight cankers 25 Directions for using zinc chloride 30 Treatment of cases not cured by first application 38 Inspection of treated cankers 38 The zinc-chloride solution 40 Antidote 40 Directions for the control of pear blight in California 41 1. Use of blight-resistant roots and bodies 41 2. Cultural methods of blight control 41 3. Insect control 42 4. Calendar of blight-control operations 42 5. Disposal of blight cuttings 44 6. Treatment of badly blighted trees 44 7. The blight crew 47 8. Proprietary remedies 47 9. Community cooperation 48 Formulas 48 Formula No. 1 — 43 per cent zinc chloride 48 Formula No. 2 — 53 per cent zinc chloride 49 Formula No. 3 — Mercury-glycerine disinfectant 49 Formula No. 4 — Bordeaux-oil paint 50 Acknowledgments 50 PEAR BLIGHT CONTROL IN CALIFORNIA LEONAED H. DAYi Pear blight {Bacillus amylovoroics Burrill) first appeared in the San Joaquin Valley in epidemic form in the spring^ of 1900 and within two years had practically destroyed the pear industry in that section. The disease soon spread into practically all the pear-growing districts of the state. In the Sacramento Valley it became epidemic in the spring of 1904. An educational campaign among the growers was immediately begun by the Universitj^ of California Agricultural Experiment Station,^ in cooperation with the United States Depart- ment of Agriculture and the office of the State Commissioner of Horti- culture, in an effort to acquaint the growers with the nature of the disease and methods of control. As a result of this campaign the pear industry of the Sacramento Valley was saved. In the coast districts pear blight has seldom done extensive damage. Although apparently of American origin, pear blight has now spread to many parts of the world. Throughout the states of the East, South, and Middle West, the disease has been destructive from the beginning of the pear industry. It is probable that if it were not for the severity of the disease in the eastern part of the United States the Pacific Coast growers would not be able to compete in the Barlett pear markets east of the Rocky Mountains. Pear blight has often been called 'fire blight' because infected twigs and fruit-spurs wilt and turn black as though a fire had swept through the orchard. The term 'fire blight' has also been preferred -by some because the disease has host plants other than the pear. In California, the pear tree is the most commonly and seriously affected plant and the disease is, therefore, known to the fruit grower as ' pear blight. ' The term ' twig blight, ' ' blossom blight, ' ' root blight, ' ' crown blight,' 'collar blight,' 'body blight,' etc., have been used to designate infections in particular parts of the tree. Pear blight is caused by a species of bacteria {Bacillus amylovorous Burrill) working in the bark of the tree. It is spread mainly by insects which transfer the bacteria from infected to healthy trees. 1 Research Assistant in Pomology. 2 Smith, Ralph E. Report of the Plant Pathologist to July 1, 1906. Cali- fornia Agr. Exp. Sta. Bui. 184:222. 1907. 4 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 The disease is usually not active during the winter months. At blossoming time the bacteria in the diseased portions of infected trees become active. Gum, laden with the bacteria, exudes from these points of infection and is carried into the blossoms or new tender twigs by insects. Beginning thus in the blossoms or twigs in the early spring, the bacteria travel slowly downward in the bark to the larger branches, trunks, and roots, producing the characteristic gumming cankers in the bark. Insects continue to spread the disease throughout the spring and summer. The severity of the infection generally depends upon the vigor of the tree. Vigorous trees with much succulent growth are most susceptible. In these the disease travels rapidly in the twigs and bark, often advancing several inches a day. On this account pear blight at one time was thought to be caused by over-nourishment due to fertile soils, fertilization, or intensive cultivation. However, Pro- fessor T. J. Burrill, Botanist of the University of Illinois, about fifty years ago proved the disease to be infectious, isolated the bacteria, and grew them in artificial cultures. Control of the disease in California has in the past consisted in cutting off the diseased fruit spurs, twigs, and larger branches as fast as they appeared. Pruning tools and wounds have been disinfected after each cut. During the spring and early summer the orchard has been gone over every week or ten days in an effort to remove as much of the infection as possible before the whole branch or tree became involved, and before insects spread it to healthy twigs. Efforts at complete eradication have never been successful, possibly because the disease has been harbored by many host plants and carried over from season to season in small hidden infections in the rough bark or even in the bark of the roots of the pear trees. [ CHARACTERISTICS OF THE DISEASE Pear blight may be distinguished from other diseases by the sudden wilting of affected blossoms and twigs in the spring and early summer (fig. 1). Drops of a whitish-colored gum usually exude from such affected parts, and upon cutting into the bark of freshly wilted twigs a bright red color is evident. The leaves turn dark but do not drop. The diseased bark soon dies and turns brown inside and dark at the surface. The infection progresses largely downward in the bark, which becomes sappy (or red and sappy) inside, sometimes for six inches or a foot beyond any surface discoloration. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA Fig. 1. — Blighted pear blossoms and twigs. Note the dead leaves hanging on tightly and the drops of gum on the twigs and fruit stems. The term 'canker' is applied to an affected area on a large branch or trunk of the tree. The disease gains entrance to these older por- tions usually through infected fruit spurs or smaller branches. The canker may spread upward or downward in the bark from the point of infection (fig. 2, A). In the late summer and fall the cankers often do not exude gum. Dead or dormant cankers are not red inside but can usually be distinguished by beads of gum on the surface or by the appearance of the twigs which have conducted the disease into the CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 branch. Rains often wash off the gum but the dead leaves remaining attached to the twig are a characteristic of the disease. The inner layer of bark and cambium under the canker may or may not be injured. Fig. 2. — A, A dormant canker of pear blight. Note the twig which con- ducted the disease into the bark of the large branch. The leaves are dead but hang on tenaciously. B, A papery-barked, shallow canker. The only indication of blight in the branch is a raising or blistering of the epidermal tissues — indicated by the arrows. The disease is confined to the outer layers of bark at first, but after a week or two the bacteria penetrate into the fibrous inner layers and usually kill the cambium — the thin layer of cells between the bark and the wood (fig. 3). The canker may be confined to one side of the branch or it may extend all the way around it. In the latter case, if the cambium is killed, the parts above the injury slowly wither and die. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 7 The diease may run its course and stop without making a large canker, or it may continue to progress downward and kill the whole tree. In some cases the canker stops without penetrating to the cam- bium or even into the fibrous inner layer of bark. In case the cambium is not killed it soon produces a new layer of inner bark. The edges of a dormant canker usually crack away from the healthy tissue, in which case the dead shriveled area is distinctly set apart from healthy bark (fig. 2, A). The disease may or may not renew activity again later in the season. The term 'hold-over canker' is used to designate those cankers which remain on the trees over winter. Hold-over cankers may be on the roots or in the rough bark of the trunk or scaffold branches of old trees. Small cankers in rough bark are often overlooked even with the most careful inspection. This makes it a most difficult matter to eradicate the disease completely from an orchard. However, the fewer the hold-over cankers, the fewer are the infections the following spring. Almost invariably where a large number of infections occur on a small group of trees in an orchard the responsible hold-over canker may be found nearby. The terms 'dormant,' 'active,' 'slow,' and 'half-dormant' cankers have been used to describe the various conditions of activity of cankers. In active cankers the disease spreads rapidly in the bark and the bright red, juicy condition of the bark may extend from several inches to a foot or more beyond the blackened, older part of the canker. It is in this sappy, red portion that the bacteria are most abundant. In the case of slow or half -dormant cankers the progress is not rapid and the red, juicy condition may be found only an inch or two beyond the blackened portion. With dormant cankers there is often no visible sign of bacterial activity at the margins, and deep cankers usually have both the bark and cambium killed. In some orchards and in whole districts in some seasons, a majority of the cankers are shallow and the cambium layer is not injured, and many of the branches recover even though the disease extends completely around the branch. Cankers are sometimes so shallow in the bark of large branches that they can be detected only with considerable diffi- culty. They may consist of mere blisters or there may be swollen areas lifting the paperlike outer bark (fig. 2, B). In some seasons rain or insects carry the disease into the half -grown fruits. A milky gum exudes and the fruit gradually becomes blackened. At first the bacteria generally invade only the more or less granular outer layers of the bark (fig. 3), where they move in a jelly-like mass 8 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 between the living: cells at a relatively sIoav rate. The cells of the outer bark contain stored foods and from this the bacteria derive their nourishment and multiply rapidly, and finally cause so much pressure as to break small openings through the outer bark layer with the resulting jelly-like ooze. This gum may also ooze out through the lenticels or so-called breathing pores in the bark (fig. 13) . Occasionally the bacteria travel up or down the branch in the inner bark near the cambium layer, usually in a streak rather than in a wide band. In the outer bark the infection may progress upward as rapidly as down- ward but it progresses much more slowly sidewise around the branch. The disease travels in the bark of rapidly growing trees more readily than it does in older, slow-growing trees. On entering an older branch from a vigorous twig the infection often stops completely. In gnarly fruit-spurs and old twigs it usually progresses a short distance and dies out. In fact a majority of infections die without progressing down to the main branch. In the large branches many of the cankers often stop development (spontaneously to all appearances) and the bacteria may never resume activity. The reason for this behavior is not definitely known. In succulent twigs the disease apparently some- times travels in the sap-wood, especially upwards. Pear-blight bacteria do not live long when exposed to sunlight and dry conditions. In hot dry weather the disease does not spread as rapidly as during warm, moist weather. Diseased branches cut off during the summer months in California soon dry up, if not piled too deeply or in damp situations, and are not apt to be serious sources of new infections. During damp weather in spring, fall, and winter, blight cuttings may be the source of new infections if not burned. Since the first infections in an orchard usually take place in the blossoms, pear blight does not become epidemic in young orchards until the trees reach the blossoming age. Also the disease is not found in nursery stock in California because the trees are sold when only one year old. In the eastern states the trees are held two years in the nursery and often sufficient bloom appears the second year to make infection possible if older susceptible trees are nearby. STRUCTURE AND PHYSIOLOGY OF THE PEAR TREE In order that the pear grower may more fully understand the art of controlling blight and treating cankers and diseased trees, the following short account is given of the structure of the pear tree, together with some of the principles of plant growth. The foods used by the tree are manufactured in the leaves, by the aid of energy derived from sunlight, from the water and minerals 1928" PEAR BLIGHT CONTROL IN CALIFORNIA taken up from the soil, and from the carbon-dioxide gas taken in from the air. The soil minerals cannot be used directly for food by the roots or other parts of the tree. MhxY Cj/fc/c Oarer BarH Fig. 3. — Diagram of 6-year-old pear branch. A short block split leugtliwise through the pith. The bark is shown thicker than natural. The medullary rays are strands or bundles of cells extending from the outer bark into the wood — only a few of these strands are shown. The new wood next to the bark is commonly called 'sapwood, ' and the older wood toward the center, 'heart- wood.' Possibly pear branches as young as this have no true heartwood. Note particularly the thin cambium layer between the wood and inner layer of bark. Water from the soil moves upward in the sap wood, and according to widely accepted opinion all of the food manufactured in the leaves moves downward toward the roots in the inner layers of bark (fig. 3). From this point of view it is evident that the roots will be starved if the bark becomes girdled by blight and killed to the wood. 10 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Fig. 4. — Natural shedding of bark of a pear tree. The upper patches on the right-hand scaffold branch were moistened before photographing to increase the contrast. This is the beginning of the natural scaly-barked condition which is shown at the base of the middle scaffold. For this reason a tree girdled to the wood in the upper part of the trunk must be cut off as soon as discovered, if a new tree is to be grown from the roots or from the lower part of the trunk, for if the roots become starved the new sprouts will not be vigorous. The wood and bark of a tree is made up of countless millions of microscopic cells. These cells are like small boxes, for they have an 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 11 outer wall and an inner caAaty. The interior (or cavity) of living" cells is filled with a jelly-like living matter (protoplasm), together with water and food materials. The walls of all cells have minute openings into the adjoining cells. Between the walls of the cells of the outer bark are many spaces (intercellular spaces). It is in these intercellular spaces that the pear-blight bacteria live. However, they finally dissolve the cell walls and enter the interior of the cells. The sap movement from cell to cell in the bark of a tree is relatively slow. This is probably one of the reasons why blight bacteria are not carried rapidly throughout the tree. The cambium is the very thin, slippery layer of cells between the inner bark and sap wood. By division the cambium cells produce new wood cells and water-conducting tubes, which are added to the outside of the wood layers. Thus the tree grows in diameter. The cambium cells also produce, outwardly, new food-conducting tubes and bark cells to take the place of those which are broken apart and weathered away as the tree grows in diameter. When blight kills the inner bark the cambium can soon produce new bark cells. But if the cambium cells also are killed then the branch soon dies above the injury because there will be no new bark cells formed. The pear tree begins shedding (sloughing) its outer layers of bark when about six or seven years old. Patches of the outer layers of bark die (fig. 4), become dark in color and shrivel, leaving sunken areas varying from a fraction of an inch to several inches wide. These patches are usually much longer than wide. These dark, sunken areas, look like dormant blight cankers and are especially confus- ing to the operator when a blight canker lies beside these patches. Often the blight canker will not run far beneath these sloughing patches because the layer of outer bark, which the bacteria would normally live in is dead, but as these patches become older and new bark is produced (by the live cambium cells beneath), the blight readily runs beneath them. The operator can usually distinguish these sloughing patches from cankers by the fact that there is always normal, healthy bark beneath and healthy bark at the sides. The presence of a blight canker can also be distinguished by the presence of gum or of dead leaves, dead blossoms, or a small dead shoot through which the bacteria entered the bark. The natural dark patches finally begin to crack and peel off and then we have the scaly-barked condi- tion shown in the tree in figure 4 at the base of the middle scaffold branch. 12 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 SEASONAL HISTORY OF BLIGHT IN CALIFORNIA Usually only a few of the first blossoms are infected in California, but just enough infections are scattered about to make a severe epi- demic in the orchard if conditions are favorable during the latter part of the regular blossoming period or the scattering second bloom. Late- blooming varieties are apt to have a severe epidemic of blossom blight if adjacent earlier blooming varieties have had a few blossoms affected. The disease next appears in the new succulent growth which is pro- duced immediately after the blossoming period. Insects carry the disease from infected blossoms to these tender growths and later from one tender growth to another. The disease then runs down these twigs and forms cankers in the larger branches and trunks. If the infection is in a new shoot (sucker) from near the surface of the ground, the disease may quickly extend into the roots. Not all of the conditions which are necessary to produce these spring epidemics are definitely known. Apparently weather condi- tions must be favorable either to the bacteria or to the particular insect carriers, or to both. In some years but few of the hold-over cankers revive in the spring, or else they revive too late to infect the earlier blossoms. In some seasons there is a long blooming period and this is favorable to blight epidemics, if other conditions are favorable. Continued rain or foggy weather after the disease has made a good start is favorable to the spread of the disease. Not only are the bacteria washed down from the upper parts of the tree but they will live longer in the open when the air is damp and the sun is not shining brightly. Rains during May and early June (especially if the weather is damp for a day or two following) will cause infec- tions in the scaffold branches, crotches, trunks, and root crowns. Such a condition occurred in the Sacramento Valley region during the second week of June in 1927 and was followed by the most severe epidemic of body blight that has been experienced in that region for many years. The infections became noticeable in ten days to two weeks after the rains. As a usual thing most of the spring cankers begin to go dormant during June as the flush of spring growth slows down, the wood hardens, and the fruit begins to grow rapidly. In the case of the Bartlett and other early varieties many of the dormant cankers revive and begin active development after the fruit is harvested. After harvest the new shoots begin to grow again at the tips, new buds push 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 13 out about the root crown and trunks and often some out-of -season blossoms open. In some years these new fall growths and out-of- season blossoms become infected by blight. At that time of year the cankers either do not exude gum or do not do so until after the blight has reached the cambium layer, and for this reason the cankers are often not discovered until after the aifected branch or tree is killed. In some years the loss of trees is much greater in late summer and fall than in spring because of the difficulty in finding cankers during the later season. Cankers which cease development before harvest time and then do not revive in the fall seldom if ever renew activity thereafter. On the other hand, many of the fall cankers which cease development on the approach of winter will renew activity the following spring if conditions are favorable. In some years only 1 or 2 per cent of the fall cankers renew activity the following spring, but in other years as high as 20 or 25 per cent on large branches may revive. In some years practically all fall cankers become dormant by winter time, but in other years some of them continue to develop slowly or even actively through January and then may continue slowly at the advancing margins of the canker until the warm weather of spring again induces rapid progress. At this latter date the move- ment of sap again becomes active and much oozing of gum occurs. Infections in smaller twigs which become dormant in the fall dry up and do not usually awaken the next spring. Besides the seasonal irregularities in the prevalence of blight in a given district there are also yearly irregularities. The disease is seldom epidemic for more than one or two years at a time. For instance, in the Sacramento Valley the disease, though present every year, has appeared in virulent epidemics in cycles of about five to seven years since its first appearance in 1904. After a year or two of little blight the disease becomes more active and gradually becomes virulent after two or three years. Then it gradually becomes less active, usually over a year or two, though sometimes it suddenly fails to reappear in abundance the season following a severe epidemic. The reason for this more or less cyclic or periodic behavior is not known. It may be due to some natural check to the bacteria them- selves, such as parasitism or a weakening of the bacteria, or the insects particularly active in the spread of the blight may occur in cycles (thus corresponding to the cycles of the disease), or possibly the trees may become more resistant. 14 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 In many districts pear blight has not appeared at as regular intervals as in the Sacramento Valley. In certain coastal districts the disease has been present since 1905 but has appeared only in a mild form, and in only a few seasons has it become prevalent enough to attract notice. In the foothill sections of the Sacramento Valley it occasionally becomes epidemic at the same time as on the floor of the valley, but it usually disappears suddenly. The same has been true in some of the pear districts of southern California. Apparently it is more destructive along the southern coast than along the northern and central coasts of California. In seasons when blight is not epidemic it has often been found difficult to make artificial infections. For instance, in an experimental orchard 85 per cent of the inoculations were successful and resulted in active cankers one year; whereas, on the same date of the following year and in the same trees, only 6 per cent of the inoculations were successful ; and the cankers resulting therefrom soon ceased develop- ment, and all natural infections in the orchard behaved in the same way. It is also common for a few orchards to escape severe injury one year in a district where blight is in epidemic form, and then the next year to have a severe infestation. Or the disease may have been epi- demic in an orchard one year but not the second year, even though it is epidemic in the district the second year. The reason for this behavior is not definitely known. HOW BLIGHT IS SPREAD All evidence points to hold-over cankers as the source from which new blight infections start in the spring. Ants or other insects carry on their mouth-parts the bacteria from the gum which exudes from the reviving cankers, to blossoms, succulent twigs, fresh wounds, growth cracks, and soft callus tissue of healing wounds. Each new point of infection soon becomes a source of bacteria for further infection. The disease may be carried from diseased branches to healthy ones by pruning shears, hoes, cultivators, tractors, harness hames, single- trees, tools used by blight operators, etc. Rains may also carry the disease down into fresh wounds on the roots, the soft callus tissue of healing wounds, growth cracks in trunks, and crotches and scaly branches. The disease sometimes gains entrance through the lenticels on branches and twigs during wet weather and into the stomata 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 15 (so-called breathing pores) of leaves. Fresh wind brnises are also susceptible to infection in wet weather. A very common point of entry of the bacteria into the bark is shallow wounds in the forks of branches made by the shoes of workmen who climb into the trees instead of using ladders. There is also evidence that birds, such as sap-suckers, woodpeckers, and flickers (yellow-hammers) may at times spread the disease. Flickers, especially, are commonly seen pecking about the crowns of pear trees and are sometimes responsible for new infections. REDUCING THE NUMBER OF INSECTS AS A METHOD OF BLIGHT CONTROL Any insect capable of biting into or penetrating tender twigs and the soft callus tissue of healing wounds is a possible blight carrier, as well as the many kinds of flower- visiting insects which may carry the bacteria from flower to flower, or from oozing cankers to flowers. A number of growlers have reported that blight infections are more numerous in their orchards near hedges and other mindbreaks — con- ditions which probably aid insect activity. The flower-visiting insects especially active in carrying the disease may be the larger kinds (ants, bees, wasps, blow-flies, etc.) in search of the nectar excreted by the flowers, or they may be insects, large or small, which bite or puncture the flower parts. Many minute species such as aphids (plant lice) and thrips live in the flowers as well as in the opening leaves at the growing points of tender twigs. Other insects commonly found about pear trees and which perhaps often carry the disease are click beetles, diabrotica (the so-called green lady bugs or beetles) , tarnished plant bugs, squash bugs, and leaf hoppers. Ants, blow-flies, plant lice, thrips, and leaflioppers are known to carry blight from disease to healthy parts of trees and it is probable that some effective method of dealing with these insects would prevent many infections. However, it may be that there are so many kinds of insects con- cerned that not a great deal can be done in the way of destroying them to prevent the spread of blight. The same kind of insect may not be responsible each year. A more careful investigation of this matter might yield results. A number of experimenters have worked on the problem but the results to date are not conclusive. Reimer^ succeeded in reducing the number of infections in the blossoms and 3 Eeimer, F. C. Pear blight control. Blue Anchor Magazine (California Fruit Exchange, Sacramento, California), Vol. 5, No. 3, p. 2, March, 1928. 16 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 twigs by the use of Bordeaux mixture (3-6-50) as a repellent, sprayed on the trees just before the buds opened (in the pink stage). Ants and thrips in particular seemed to be repelled by the Bordeaux spray. A number of growers in Sacramento County, California, have tried this method at the suggestion of Mr. A. E. Morrison, Horticultural Commissioner of that county. A few of these experiments yielded encouraging results but most of them did not reduce the number of infections. Possibly the spray was not applied at the right time for the particular insect concerned in that district. It perhaps will well repay the grower to use nicotine sulfate (1 pint to 200 gallons of spray) with the codling-moth sprays to kill plant lice, thrips, and leafhoppers. Summer oils also may be added to the codling-moth spray for these insects^ — 2i/4 gallons of summer oil to a 200-gallon tank. Or both nicotine sulfate and summer oils may be used. When spraying with oils, the basic rather than the standard arsenate of lead should be used. Also, in using summer oil be careful to remove all residues of sulfur sprays (lime-sulfur solution, sulfur pastes, etc.) sticking to the inside walls and lid of the tank, for oil and sulfur combinations are injurious to leaves. The relation of honey bees to the spread of blight is not sufficiently understood and should be carefully investigated. Studies by Gossard and Walton^ in Ohio seem to indicate that after blossoms have been pollinated for over three days they are not apt to become infected. They suggest, therefore, that there should be many bees present in orchards during the first days of the blooming period in order to pollinate the blossoms quickly before blight has much chance to spread. Whether this holds true for conditions in California has not been determined. If it does, then it would indicate that the hives should be taken away from the orchard before the late blossoms open. RELATION OF CULTURAL METHODS TO BLIGHT CONTROL Soil Conditions. — It has already been stated that the disease is more virulent in rapidly growing trees than in trees that are grow- ing more slowly and thus harden their new growth early in the summer. In fertile soils and in soils supplied with adequate moisture the vegetative vigor of the tree is usually greater than in less fertile soils. In the eastern states it has been found very helpful to keep the orchard in sod (planted to grass) rather than giving clean cultivation. The grass apparently uses up excess nitrates or other mineral salts and hence prevents succulent growth. 4 Gossard, H. A., and E. C. Walton. Dissemination of fir© blight. Ohio Agr. Exp. Sta. Bui. 357:108, 123. 1922. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 17 Under certain condition.s omitting one irrigation may be sufficient to bring about the results desired in making use of sucli methods. The planting of summer or permanent cover crops, as has been prac- ticed to some extent in southern Oregon, in order to keep down excessive vegetative growth, may be advisable under especially favor- able conditions. The fact, however, that it takes large amounts of irrigation water to keep both cover crop and trees growing must not be overlooked. Good judgment must be exercised so that neither crop nor trees suffer. A number of California growers have had considerable success in reducing the virulence of blight by using less irrigation water or by growing summer cover crops of weeds, grass, clover, alfalfa, and the like, to take up the excess water in the soil. If this practice is fol- lowed it may be necessary to hoe around the trees to protect them from field mice which may live in the cracks of the dry soil about the trees. Pruning. — A moderately light method of pruning^ may often be used to advantage to discourage excessive succulent growth. This requires some experience and careful observation on the part of the grower, for what may work in one condition of soil (in a neighbor's orchard for instance) may not work in his own orchard. In some dis- tricts cutting and thinning out of fruiting twigs seems to be necessary to secure fruit setting. The cutting out of water sprouts about the scaffold branches and of suckers from the root crown during the growing season has often been observed to induce more blight. In this case the blight may be either washed down from above by rain or by wet fogs or carried by insects into the fresh wounds or into the soft callus tissue after heal- ing begins. The cutting off of these sprouts in the early summer also encourages new ones to start as soon as the crop is harvested in the case of the Bartlett and other early varieties. These pushing buds are readily attacked by blight, and trunk infections are thus started at a time of year when the cankers run deep before gumming occurs — if it occurs at all. The advisability of cutting off all fruit spurs about the body of the tree is still an unsettled question, but some growers in the Sacra- mento Valley believe that cutting these off only encourages water sprouts to take their places, and these are as readily infected as are the fruit spurs and blossoms. It is perhaps best to encourage the 5 Tufts, W. P. Pruning bearing deciduous fruit trees. California Agr. Exp. Sta. Bui. 386:38. 1925. 18 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 development of fruit spurs, especially on hanging laterals or twigs about the lov^er parts of the scaffold branches. Blight is not usually serious in the regular blossoms of the Bartlett and off-season blossoms do not often occur on these spurs low down on the scaffold branches. If the fruit spurs become infected they are readily found and the parts treated to stop the blight in ways described later in this publication. It is a common practice to convert into fruit spurs some of the smaller water-shoots located on the scaffold branches on the inside of the tree. This is done by leaving the smaller shoots uncut. The follow- ing summer these develop fruit-spurs and become fruiting 'hangers.' They may then be shortened, if desired, by pruning. If gnarly fruit- ing twigs of this type become infected the blight travels very slowly in them, while if water shoots become infected the disease travels very rapidly down to the scaffold branch. BLIGHT CONTROL BY USE OF RESISTANT ROOTS, TRUNKS, AND SCAFFOLD BRANCHES Most varieties of the pear, apple, and quince are susceptible to pear blight, although some varieties and species of these are prac- tically immune. The disease has been found also on other species of the Rosaceae or rose family. In California, varieties of loquat, coton- easter, pyracantha, hawthorne, and California holly (Toyon or Christmas berry) are commonly infected. The following plants are also susceptible to a greater or less extent : rose, plum, cherry, peach, service berry, mountain ash, red raspberry, blackberry, and straw- berry. Whether some of these and other ornamental and native plants of the rose family assist in spreading the disease in years of epidemics has never been determined. The most important commercial varieties of pears are susceptible to blight, but the degree of susceptibility varies considerably. The compartive resistance among the commercial varieties varies from dis- trict to district. Apparently, if the district is favorable to rapid tree growth of a given variety that variety will be more susceptible to the disease than it is in a district where it grows more slowly. At the University Farm at Davis, California, the following order of increas- ing resistance among the leading commercial varieties has been deter- mined : Eastern Beurre, Glou Morceau, Bartlett, Bosc, Comice, Hardy, Anjou, Clairgeau, and Winter Nelis. The following varieties have also been severely blighted at Davis: Madeleine, Fox, Dana Hovey, Flemish Beauty, Forelle, and Clapp Favorite. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 19 As a usual thing, the more susceptible the variety the more deeply the blight penetrates into the bark in the early stages of the disease. In the case of some of the more resistant varieties the cankers in large branches usually run their course and die out without killing the cambium and innermost layer of bark. There is some difference in degree of susceptibility of blossoms and young twigs even in the same variety. Thus with the Winter Nelis, which is generally considered as semiresistant to blight, the blossoms are often more severely attacked than is the case with the Bartlett, which is regarded as very susceptible. The natural resistance of certain pears, such as the oriental species (Pyriis ussuriensis and Pyrus calleryana) , has led to the use of seed- lings of these species as rootstocks and trunks on which to propagate the Bartlett and other commercial varieties which contract the disease readily. A few varieties of Pyrus communis (the common species to which all California commercial varieties belong, usually called French pear) are resistant to the disease, but the seedlings of these are not usually resistant. The following are some of the resistant French varieties: Old Home, Surprise, Estella, Longworth, Douglas. These do not produce fruit of high commercial quality. Since some of these resistant French varieties make better unions with our commercial varieties than do the oriental varieties, they are being used as trunks and scaffold branches on the tops of which commercial varieties are grafted, the roots in these cases being either seedlings of blight- resistant species or of the non-resistant French stock. Since the disease usually kills the tree when it gets into the trunk and roots of non-resistant pear trees, it is desirable that new plantings of non-resistant commercial varieties be top-worked on trunks and scaffold branches of blight-resistant species or varieties. It would also be very desirable to have resistant roots, but as yet it is not certain that a suitable one has been developed. Several combinations are being tried by growers but it requires fifteen or twenty years to demonstrate the value of these combinations ; for, aside from blight resistance, there may be some undesirable quality of the rootstocks such as susceptibility to woolly aphis, oak root fungus, black-end disease, overgrow^th of root, and trunk sensitiveness to excess water, drought, marly subsoils, or alkali. A combination that has been planted by a number of growers in the past two or three years consists of Pyrus calleryana rootstock. Old Home trunk and scaffold branches, and on top of this the Bartlett or other commercial variety. This combination seems very promising in 20 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Oregon*^ and may prove to be successful under some conditions in California but in several instances there is evidence that the Cal- leryana will not do as well as the French root in soils tending; to be wet or in those containing small amounts of alkali. The Calleryana is resistant to woolly aphis, but it may not be resistant to oak root fungus, since the other oriental species that have been used are not resistant to this disease. It is also possible that black-end disease may develop on trees grown on this rootstock, since the Bartlett often shows tliis trouble when grown on other oriental stocks. A few growers are trying a combination consisting of French seedling roots, the Old Home variety for trunk and scaffold branches, and Bartlett (or other commercial variety) for the top. This com- bination would seem to be safe, because in this case all three varieties are French pears and they all unite well with each other. The French root is not resistant to blight, but if planted with the graft union several inches below the surface, so that very little suckering occurs, it will not contract the disease so readil}^, especially with a non- blighting body. The French root was the standard root for pear trees in California until recently and except for the blight has been very successful. Reimer^ has found a few French seedlings which are resistant to blight, and whose seeds produce seedlings a large percentage of which are resistant to blight. He believes it possible to develop extensive orchards of these resistant French stocks solely for the production of seeds from which nurserymen can grow blight-resistant French roots. The use of resistant roots and body does not render the Bartlett top immune to blight, but if the Bartlett top becomes infected, only the top is lost. A new Bartlett graft or bud can then be placed on the Old Home scaffold, and the tree can be brought back into bearing. This combination is probably best secured by planting, in the orchard, nursery stock consisting of one-year-old Old Home on French seedling roots. The tree is then grown in the orchard for two or three seasons until it has considerable size and has been trained to three scaffold branches. Buds or scions of the commercial variety are then placed on the scaffold branches a foot or two out from the trunk. Some growers prefer to bud or graft the commercial variety onto the second set of branches instead of the first set, thus putting in five to seven G Eeimer, P. C. Blight resistance in pears and characteristics of pear species and stocks. Oreg-on Agr. Exp. Sta. Bui. 214:66-74. 1925. 7 Eeimer, F. 0. Blight resistance in pears and characteristics of pear species and stocks. Oregon Agr. Exp. Sta. Bui. 214:66-74. 1925. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 21 buds per tree. Budding" is done late in July or August after an irri- gation. The following spring just before growth starts the branch is cut off about one-half inch above the bud. The Old Home makes a very good body, because it has a fine spreading habit that is easily trained into a good form of tree. Some prefer to graft rather than bud as this is done in the late winter when work is more slack than in the summer time. Grafting gives a more uniform tree because all the branches are cut back to the same height. About sixteen years ago the so-called Japanese pear (Pyrus sero- tina) was generally advocated as a rootstock for commercial pears in California. This root was supposed to be much more resistant to blight than the root of French varieties. It had long been used with success in the eastern states and in Oregon. Approximately 50 per cent of the pear orchards planted in this state since that time have been on this root. It is now known that the Japanese root is not immune to blight and that it does well only under certain soil con( tions. It cannot stand either wet soil or dry soil as well as the French root. Also the fruit of Bartlett and some other varieties on the Japanese root is subject to the so-called black-end trouble in Cali- fornia and is, therefore, not a desirable rootstock for our pears. SURGICAL METHODS OF TREATING CANKERS In the earlier blight w^ork in California all infected branches were cut off. If the disease had girdled the trunk the tree was either destroyed or sawed off well below the infected area, especially where the canker was only in the upper part of the trunk. It was soon dis- covered, however, that in case of cankers confined to one side of a scaffold, trnnk, or root-crown the canker could often be arrested by shaving off the bark and scraping the wood for a long distance beyond the evidence of disease in the bark (fig. 5) and applying disinfectants. However, even the most careful operators often failed to stop a large percentage of cankers by this method. About the year 1916, Mr. Hayward Reed^ in his orchard near Sacramento (following the suggestion of W. P. Womack, of Sacra- mento) began trying a new method of bark surgery which is now known as 'scarification.' He had noted that the blight works in the outer layers of bark first and had found that by shaving off this outer layer and applying disinfectants that the disease was often arrested and the inner bark and cambium layer saved. A number of growers 8 Weldon, Geo. P. Pear growing in California. California State Comm. of Ilort., Mo. Bui. 7:364. 1918. 22 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 then adopted this method and used for a disinfectant Ileimer's formula, which consisted of cyanide of mercury and bichloride of mercury, 500 parts of each in water. This solution served fairly well in years when blight was not very virulent but in the severe outbreak beofinning in 1920 it failed to stop a large percentage of the cases treated. Fig. 5. — Old method of canker treatment, used when the canker was confined to one side of the trunk or branch. Now advised to follow zinc chloride treat- ment in spots where cambium is killed and healing of edges has begun. The dead bark should be scraped off and the exposed wood covered with Bordeaux- oil paint. In the spring of 1921 the Division of Pomology of the University of California undertook a study of methods of treating blight cankers. The two principal lines of attack were the use of various disinfectants with and without surgery. Scarification. — Various depths of shaving oif (scarifying) the outer layers of bark were tried, and a number of disinfectants were 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 23 Fig. G. — Bartlett pear tree treated by the scarilic-atioii method. The blight had spread from a water-sprout, just below the crotches, up into the scaffolds and half way down the trunk and all the way around. This was a fresh canker. The white lines indicate the ends of the scarified area. This photograph was taken the year after the operation. Note the vigorous new growth. If the operation had been delayed a week or two the inner bark would have been injured and the tree left in a weakened condition. 24 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Fig. 7. — Home-made scarifier. The ruler and inch squares will aid the blacksmith in constructing this tool. The blade is made from quarter-inch, square steel stock. The point, lower edge and bend are sharpened. used. One of the most successful disinfectants tried consisted of 500 parts each of cyanide of mercury and bichloride of mercury in a solvent consisting of three parts glycerine and one part water (see formula No. 3, p. 49). The outer layer of bark had to be shaved practically to the depth to which the disease had penetrated inwardly. This method was widely adopted by the growers and was very success- ful iu the hands of careful operators, especially in treating new 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 25 cankers, but in the case of older, deep cankers the percentage of cured cases was often as low as 50 per cent or less. Root cases were especially difficult to treat by scarification because it was difficult to prevent accidental cuts and bruises, made while digging- around the tree from becoming infected. This method was also too laborious and slow to be very practical during severe epidemics. Also in the case of deep cankers there was such a thin layer of inner bark left that the branch was very slow in recovering. The scarification method may have its place under certain condi- tions, mentioned later; therefore, the following directions are given regarding the method of performing the operation : In older parts of the canker the dead or diseased outer bark is shaved inwardly until wide streaks of healthy tissue appear and nearly all of the discolored streaks are removed. At the active margins of the canker the disease is usually not so deep but nevertheless the discolored bark should be practically all shaved off. Thick bark such as occurs in crotches and the collar tissue on the opposite side of the branch from the crotch, old bud-scars, etc., should be pared away to the fibrous inner layer next to the cambium. Old, dry cankers in which the cambium is entirely killed over a fairly large area may be treated in a similar way, provided the cambium is not injured all the way around the branch. After disinfecting the scarification wound and allowing the disinfectant to dry, all exposed wood (where no live cambium remains) should be painted with Bordeaux-oil paint (see formula No. 4, p. 50) to prevent the entrance of wood-decaying fungi. Figure 6 shows a scarified Bartlett pear tree in which there was a new canker at the base of the scaffold branches and upper part of the trunk that had passed all the way around those parts. A heavy, long-bladed jackknife is very good for performing the scarification work, but if there is much to be done a box scraper is better. The home-made scarifier shown in figure 7 does the work better and more rapidly than either of the above tools. ZINC CHLORIDE FOR THE CONTROL OF PEAR-BLIGHT CANKERS A great many chemicals have been tried during the past six years in an attempt to discover one which, without ])revious surgery, will penetrate the bark of the pear tree, kill the pear-blight bacteria, and yet not kill the branch or tree. The most promising of the materials found was a solution of zinc chloride powder in denatured alcohol, acidified with hydrochloric acid. 26 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 The chief difficulty experienced with zinc chloride, as well as the other chemicals for the treatment of blight cankers, was in finding a concentration (or strength) of solution that could be used on branches of all sizes, and on cankers of all ages without 'over-penetration' and consequent death of the branch. It was found that the absorption of chemicals not only varied with the size and age of the branch but also that trees in different orchards acted differently in this respect. For instance, in trials on healthy branches, in some orchards a 53 per cent solution of zinc chloride did not injure the cambium layer of branches as small as 11/4 inches in diameter, while in other orchards this strength penetrated too far into the bark and could not be used with safety on branches less than 21/2 or 3 inches in diameter. The older part of the canker was more readily penetrated than either the advancing margins of the disease or the normal bark beyond the cankers where the disease was not evi- dent. Trials over a period of several years were made in an effort to overcome these difficulties in the use of zinc chloride. Various concen- trations of the zinc chloride in the alcoholic solution were tried exten- sively (especially with concentrations of 33, 43, 53, and 63 per cent) with the hope of finding a solution which would cure the largest num- ber of cases with the least amount of over-penetration of the bark and killing of the branch. It was feared that the use of more than one strength of solution might make the process too complicated. How- ever, during the blight epidemic of 1927 several growers had their operators use two solutions of zinc chloride, 43 and 53 per cent (formula on p. 48), without confusion, so that we now believe it to be entirely feasible to use two concentrations. The zinc chloride solution has been used extensively by one orchardist over a period of three years and by two others for two years. During the past year (1927) a large number of growers in various parts of California used the material. This extensive trial of the method has revealed many important points regarding its prac- tical application. Besides checking up the work of many growers we have, during 1927, treated about five hundred cankers in orchards where the blight was severe. The results of these trials by growers and of our own experiments may be summed up as follows : 1. Orchards vary widely in the size of branches that may safely be treated with the 53 per cent zinc chloride solution. In some orchards branches as small as 1% inches in diameter were successfully treated, while in others larger branches were over-penetrated and killed. Branches were not injured below the treated area nor below a point 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 27 where it would be necessary to cut off the branch to stop the blight if canker treatments had not been used. Smooth-barked branches up to 1% inches in diameter were successfully treated with a 43 per cent solution, but cankers on scaly-barked branches often did not yield to this treatment. This weaker concentration often saved branches as small as % inch in diameter. 2. The cambium layer is often killed by zinc chloride solution if the canker is over ten days or two weeks old, especially if the cankers are much darkened before treatment. Older cankers on trunks and scaffold branches are less liable to be over-penetrated than on smaller branches. Commonly a large proportion of both young and old cankers are very shallow and in that case they are not so readily over-penetrated. 3. During the spring and early summer the zinc chloride solution seldom fails to stop the blight. Growers reported that as high as 98 per cent of new cankers (or shallow old ones) treated at this time of year were permanently arrested and the branches saved. 4. Trees usually absorb zinc chloride solutions more freely after the new growth begins to harden. This hardening occurs about the third week in June in the case of the Bartlett pear in the interior districts of the state. However, since the hardening period differs from orchard to orchard and from season to season, it is not best to change to the 43 per cent solution on branches larger than 1% inches in diameter without first trying the penetration of the 53 per cent solution on healthy branches of various sizes, as directed on page 36. Less gum is exuded during the summer and fall months, hence the cankers are not so readily discovered. The percentage of cases stopped by zinc chloride is less in the fall because the cankers are often not discovered until they are comparatively old and deep. But usually the blight was stopped in over 80 per cent of the fall-treated cases and the branches saved in case of new cankers or shallow older ones. During the late fall and winter and up to the time the trees are in full leaf in the spring, the bark is less easily penetrated by zinc- chloride solutions. However, dormant cankers are readily cured dur- ing this period because zinc chloride penetrates the injured, darkened bark more readily. Active cankers are not as readily cured during the late fall and winter as during the summer because the bacteria under the healthy bark beyond the margins of the discolored area are not as easily reached by the zinc chloride solution. During foggy weather or damp nights the zinc chloride is often washed off* before it has a chance to penetrate into bark beyond the margins of the canker. 28 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 It requires from one to three days for a sufficient quantity of the zinc chloride to penetrate into the healthy bark to be effective. These results show the importance of cleaning up the blight before late fall and winter. 5. Zinc chloride treatment has been very effective in cases where it is impossible to tell whether or not the disease has yet passed from the infected twig or spur into the larger branch. It is with cases of this kind that blight cutters take chances, hoping to save the larger branch and the fruit growing thereon. The zinc chloride solution apj^lierl to the larger branch (after cutting off the infected twig or spur) has seldom failed to stop the infection. Figure 12C shows such case before and after treatment. 6. Long cankers and cases in which two or more cankers are so close together (figs. lOB, IIB, and 12A and B) that a continuous treatment of 3% to 5 feet in length is necessary, are often not success- fully treated. In these cases the zinc chloride solution creeps down- ward and gives an overdose to the lower parts of the canker. For the same reasons roots and graft unions of young trees (as mentioned below) are often over-penetrated if the treatment extends from the scaffold crotches down to those parts. 7. The graft or bud union of young trees is more readily injured by zinc chloride than are other parts of the trunk and root-crown. Whether the blight penetrates more deeply at that point has not been determined. This injury is especially common when the solution is applied from the root-crown and up the trunk to, or above, the crotches (fig. lOB). In these long cankers the zinc chloride solution creeps downward and thus increases the concentration at the root- crown. Treatments at the root crown which extend only a short dis- tance up the trunk (fig. IIA) seldom over-penetrate the graft union unless the blight canker was old at the time of treatment. 8. Zinc chloride treatment is of great value in treating cankers on large branches shortly before the fruit ripens, because, even if the branch is eventually killed by the treatment, the fruit usually matures. If the diseased branch were removed as soon as the blight appeared, as formerly was the common method of attempting to control blight, both branch and fruit were lost. With the zinc chloride treatment the blight is stopped and the fruit is usually saved. Even with cases in which the cambium is killed four or five weeks before harvest time the branch usually does not wither till after the fruit matures. 9. The rains in early June of 1927 caused many cases of trunk and root blight and in some orchards many of these were not discovered 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 29 Fig. 8. — A, A new pear blight canker before treatment. The arrows indi- cate the extent of the canker. The infection began in the small blackened shoot near the center of the canker. The bark in the older parts of the canker is beginning to turn dark. A week later this canker would have been too old to treat without danger of killing the cambium layer. B, The same as A after stubbing the twigs and applying zinc chloride solu- tion. The solution darkens the older parts of the canker the instant it is applied. It is very important that the stubs be thoroughly coated with the solution. until they were too old to be treated successfully. Many orchardists stopped their blight work during picking operations without attend- ing to these cases and as a result many trees were past recovery by the end of the harvest season. Blight cases treated a few hours before a heavy shower of rain in June were successful except on outside branches exposed directly to the rain. A hard rain, how^ever, would undoubtedly have washed off 30 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 most of the solution. It is likely that in the fog belt along the coast, treatments in foggy weather or when the nights are damp will not be very successful in stopping blight cankers. Scarification may have to be used instead of zinc chloride during long-continued, damp weather. 10. Compared with surgical methods of treating cankers (new or dormant) or with complete amputation of affected branches, the zinc chloride method is more certain in arresting blight, except possibly during the late fall and winter. Scarification (page 22) may have to be substituted for the zinc chloride treatment on active cankers in some districts during the late fall and winter because of the slow rate of penetration and of the prevalence of rain or heavy dews at night. The zinc chloride treatment has not been tried in the coast districts. It may be possible that even in the spring and early summer the fogs and damp nights may cause poor results along the coast. The scarification method will usually cure older and deeper cankers than can be cured with the zinc chloride treatment; however, with these older scarifed cases the branch does not recover its full vigor so readily as does the same sort of case when successfully treated with zinc chloride. 11. The above results were almost entirely on the Bartlett variety. A few trees of Hardy and Bosc were treated with good results. It is doubtful whether the zinc chloride will penetrate sufficiently in the bark of such varieties^ as the Madeleine (in which the blight runs rather deeply) to be effective. Directions for Using Zinc Chloride. — The solution is usually car- ried in a fruit jar or wide-mouthed bottle and applied to the cankers with a paint brush about 1% inches wide. Round and oval brushes are especially good because they hold a large amount of solution. Larger brushes may be used for large scaffold branches and trunks. If an open jar is used, only enough of the solution to last about an hour should be taken at one time as the alcohol evaporates rapidly. A heavy wire can be wrapped about the neck of the jar to serve as a handle (fig. 9). In applying the solution, begin about 16 inches above the signs of the disease in the bark. Ijift the brush carefully from the solution, so that it will be dripping-full of the liquid, and then paint downward, allowing the solution to run freely over the surface of the bark, and finish about 12 inches below the last signs of the disease at the lower end of the canker. In this way the solution will creep into all the cracks and lenticels (fig. 13). Do not try to apply less material to a 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 31 Fig. 9. — Tools for tre.ating pear-blight cankers. Note the folding saw which can be conveniently carried in the bucket. The drain or tiling spade is veiy good for removing soil from between roots. The towel is kept damp and used to wipe zinc chloride from the hands. The sponge tied to the shear handle is especially useful in applying disinfectant to cuts when clipping twig blight in the spring and early summer. 32 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Fig. 10. — A, Trees four years old or younger. Use only 43 per cent solution on trees four years old or younger. However, if trees are exceptionally large for this age 53 per cent solution may have to be used as indicated in B. B, Trees five to ten years old with long cankers. Use 43 per cent solution on smooth-barked branches up to 1% inches in diameter and on the lower half of long cankers on trunks or roots where 53 per cent is used on the upper part of the canker. Use 53 per cent on all parts above ground over 1% inches in diameter except on the lower half of long cankers. canker on a small branch than on a large one as this practice is certain to give poor results. Use the weaker solution (43 per cent) in treating- small branches, but apply it freely. The solution should be applied fully around the branch even though the disease does not extend all the v^ay around — except in case of very small side-cankers on large branches or trunks. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 33 Avoid spattering the zinc chloride solution because it will spot the fruit and is painful to the eyes and more or less harmful to clothing and shoes. Very few inspection cuts should be made in the bark in determin- ing the extent of the canker because the solution is very penetrating and will kill a large spot of bark to the wood wherever a wound has been made. All twigs and spurs within the cankered spot, and for 5 or 6 inches beyond, must be cut back to stubs about two inches long (figs. 8, 12 A, and 12B). If the twigs are cut off short, the solution will kill a large spot of the cambium layer around the cut. Be careful to apply the solution well beyond the last stub, for the pruning shears may have infected the cuts with blight even though the disease had not pro- gressed that far. All except the cut ends of the stubs must be covered with the solution. Leaves remaining on the stubs should be removed for they interfere with the proper application of the solution. It is not safe to leave twigs and spurs in the treated area, because blight may have already progressed into them, even though they may appear healthy. In case the disease extends down a twig and comes within 8 inches of the larger branch, cut off the twig and apply zinc chloride to the larger branch (fig. 12, C). Scales of bark should not be removed in order to treat scaly-barked trees unless these are already loose and ready to fall away naturally. On the underside of horizontal branches it is sometimes necessary to remove the scales in order to apply the solution. Scraping or cutting off old dead bark and scales, unless very carefully done, causes wounds into which the zinc chloride penetrates and kills large spots of the bark and cambium layer. Rough, scaly and corky bark should be very thoroughly saturated with the solution. Figures 10, 11, and 12 give the strength of solution to use on cankers on the roots, root-crowns, trunks, and branches for various sizes of trees and conditions of disease. These recommendations are for the average orchard. There are orchards in which the zinc chloride penetrates more freely than in others so that the smallest size of branches to be treated with the stronger solution (53 per cent) may have to be 2^2 or 3 inches instead of 1% inches. But this can only be determined by the experience of each individual grower in his own orchard. Young rapidly growing trees are often the ones which absorb the solutions most freely and the branches of old trees less freely. • 34 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Fig. 11. — A, Trees five to ten years old with short cankers. Use 53 per cent solution on short cankers, on large branches, trunks or roots. Use 43 per cent on small branches (up to 1% inches in diameter) and on small roots (up to 2l^ inches in diameter). B, Trees over ten years old. Use 53 per cent all the way down on long cankers aifecting trunks, and root crowns. Use 43 per cent on roots up to six inches in diameter when these are at the lower ends of long cankers being treated with 53 per cent. Use 53 per cent on a scaly-barked branch, no matter how small the branch. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 35 f 3//f/)/ S5% ■S5% f -3/^qhf Si% 43% \f f -B//^/>/ ■43%, A B Fig. 12. — A, Long canker (requiring a treatment extending S^/^ to 4 feet in length) on a branch over 1% inches in diameter. If the branch is more or less vertical use 53 per cent above and 43 per cent below. Note that all twigs within the diseased area are cut to stubs about 1% to 2 inches long. B, Two cankers, one immediately above the other on a branch over 1% inches in diameter. If the branch is more or less vertical use 53 per cent on the upper canker and 43 per cent on the lower one. C, Cases in which there is doubt as to whether or not the blight has yet passed from the small twig into the main branch. Cut the twig off and apply either 53 per cent or 43 per cent solution, according to size of the branch. Apply it a distance of 8 or 10 inches above and below the twig and sidewise for about two inches. In this type of case it is probably best to cut off the branch closely rather than to a stub as shown in A and B. A large spot of bark may be killed where the stub is cut closely, but this will soon heal over. 36 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Fig. 13. — Healthy branch treated with zinc chloride solution and the bark shaved off three days later to show the reddish spots caused by the penetration of the solution into the lenticels (so-called 'breathing- pores'). Note also other lenticels that are now the center of sunken spots to the right of tlie scarified patch and normal lenticels above. Since trees differ from season to season and from year to year in the readiness with which they absorb zinc chloride, we advise that each grower apply the 53 per cent solution to healthy branches of several sizes in his own orchard several times during- the blight season — preferably May 1, June 1, and July 1. After a week or ten days he can tell how small a branch can be treated without injury. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 37 The lenticels will be penetrated but the inner layer of bark not severely injured (fig. 13). A narrow strip of bark is often killed to the wood on the underside of the branch, but this is of no conse- quence and should not be considered in making' these tests. When in doubt as to which solution to use on a branch, use the stronger (53 per cent) solution, for it is better to kill the branch than not to stop the disease. The 43 per cent solution is intended for use on young trees, small branches, small roots, and on the lower parts of long cankers on large branches, trunks, or roots. It will aid the grower in studying the effect of zinc chloride solu- tions on blight if he will use some system of marking so that he will know what percentage of zinc chloride solution was used, the date, whether the canker was new or old, active or dormant. One grower used large-headed roofing nails^ — one nail for new cankers and two nails for older cankers. Others have used cloth of different colors, paints, shipping tags, etc. It is important to know which cankers were old and hence likely to be overpenetrated. In order to prevent possible mistakes in using the two solutions in the orchard it is desirable that one of them be colored. A few drops of laundry blueing added to the 43 per cent solution will give it a blue color which will distinguish it from the 53 per cent solution. This color, however, is not sufficiently intense to stain the bark. Growers should be careful about adding coloring matters intended to stain the bark, for some kinds of coloring agents might reduce the efficiency of the solution or else cause injury to the bark. Large areas in which the bark has been killed to the wood, on one side of the branch only, should (after healing at the edges has begun) be scraped to the wood (fig. 5) and the wood covered with the Bordeaux-oil paint (formula No. 4, p. 50) to prevent wood rots. In cases where the canker was too old when treated and the branch was killed, the amputation cut should not be made very far below the killed area because new sprouts often groAV out from the area of healthy bark treated with zinc chloride. If in doubt as to how far the branch was injured cut rather high or else leave the branch until the following summer. It is highly important for the grower to understand that the zinc chloride solution must be properly applied, and that the treatment has its limitations. Without a proper appreciation and understanding of these matters, the grower will be disappointed with results. 38 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 Treatment of Cases Not Cured hy First Application. — Except on trunks and very large scaffold branches it is usually not possible to successfully re-treat cankers that were not stopped by the first appli- cation. The second application usually kills the cambium layer, especially of smaller branches. It is sometimes better to re-treat by the scarification process (p. 22) than with zinc chloride solutions. If a case has to be re-treated with zinc chloride, be careful to treat only the area affected and only as much as necessary of the older parts of the canker where the previous treatment had already penetrated deeply. In case the solution runs over these older parts, then the most of it should be sponged up with a large damp cloth. Usually an unsuccessful treatment will be too far advanced before discovered to make further treatment possible ; amputation is then necessary. Inspection of Treated Cankers. — Four weeks after treatment and two or three times during the summer and fall all treated cankers should be inspected in order to locate any cases in which the blight has not been stopped. After a little experience the blight operator can tell whether or not a treatment has been successful without cutting into them. Even if there is no gumming there will usually be areas with a wettish or oily appearance in the older parts of the treated canker if it has not been cured. In rough bark it may be necessary to gouge into these areas. Again just before the blossoms open in the spring all of the cankers should be inspected and any suspicious spots explored with a gouge to find if the disease is awakening. Figure 14 shows the best gouge we have seen for this work, for it removes the chip and makes a smooth cut which is easily disinfected. During the blossoming period the treated areas should be looked over again. The gouging tools should be kept disinfected as well as the inspection cuts in the bark, because the disease is readily carried from place to place should the inspector accidentally cut into a tiny live infection without detecting it (see disinfectant formula No. 3, p. 49. The inspector must become acquainted with the appearance of the bark (both inside and outside) in treated cases. The red color and oily or watery appearance inside the bark often does not disappear for several weeks after the application of zinc chloride solution. The zinc chloride itself often causes a reddish color in the bark, but this is not as bright a red as that caused by blight. Zinc chloride also occasionally causes a gummy exudation from injured lenticels or from 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 39 u Fig. 14. — Tools for gouging bark when inspecting for pear-blight cankers on large branches, trunks, and roots. The U-gouge is made from a half-round chisel, as in the middle figure. The gouge to the reader's left is made by- grinding a half-inch, flat chisel to a point. the older parts of old cankers where the wood may be injured. A pinkish cast to healing tissue and to regenerating cambium, and red- dish streaks in the bark and wood are rather confusing until the operator learns to distinguish these conditions from the color of these tissues in the case of active blight. Figure 13 shows dead and reddish spots in healthy bark to which zinc chloride was applied. The epi- 40 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 dermal layer of the bark was shaved off several days after the appli- cation to show how the material had penetrated the lenticels and killed spots in the outer layers of the bark. It is much easier to find treated cankers when the trees are wet with dew or rain because then the cankers (especially on smooth- barked branches) are very dark compared with the healthy bark. However, when wet it is not as easy to determine whether or not the disease has been arrested as when dry. The Zinc CJiloHde Solution, — Directions for preparing the zinc chloride solution will be found on page 48. This solution is a caustic and penetrating disinfectant. However, even if it penetrates to the wood in case of an old canker or a small branch it does no injury to the branch below the area on which it is applied. After the zinc chloride has become neutralized by the juices of the bark the healing process is rapid and new buds may start where the solution was applied. Even where the cambium layer has been discolored by the zinc chloride, there are often enough live cells uninjured to regenerate a new cambium layer. When the zinc chloride penetrates into the wood it may move upward for some distance but the cambium layer over this darkened wood usually regenerates. The zinc chloride seldom moves downward in the wood. In the bark it may work downward or upward but only for a short distance beyond where it entered these tissues. Except on small branches and with very long cankers there is seldom, if ever, more damage done by the zinc chloride than the blight had done at the time of application. There is very little danger to persons using the zinc chloride solu- tion. It is so caustic and distasteful that it is not likely to be taken internally. It was formerly used in human surgery as a disinfectant and for use in cauterizing or 'burning out' cancers and tumors. It is very painful and irritating to the eyes and a large quantity acci- dentally spilled into the eyes might cause serious injury. The material irritates the skin of some persons more than others. It will burn the skin if the clothing becomes saturated and is allowed to remain in contact with the skin for some time. However, some operators have their fingers wet with the solution day after day without serious annoyance. Antidote. — In case of accidental swallowing of zinc chloride solu- tion use whites of eggs beaten up in water, or olive oil, or a large amount of water or milk. Do not use emetics. Send for a doctor. In case any large quantity gets into the eyes wash them out with olive oil 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 41 or whites of egg^s. Washing soda in water will neutralize the zinc chloride on the hands and should also be used if any large amount is spilt upon the clothing. Cotton clothing is not greatly damaged by zinc chloride, but woolens, shoes and leather gloves are. Rubber shoes and rubber gloves are not harmed. Brushes will last longer if washed after using, each evening, in a solution of washing soda. DIRECTIONS FOR THE CONTROL OF PEAR BLIGHT IN CALIFORNIA To be successful in the control of pear blight it is necessary that the grower have a thorough understanding of the disease and its relation to his general scheme of orchard management. The recom- mendations which follow are based on results of experiments by the author and on observations of results secured by successful California growers. 1. Use of Blight -Resistant Roots and Bodies. — All pear trees planted in districts where blight is apt to be a serious factor should have blight-resistant bodies. It would be very desirable also to have blight-resistant roots, but at the present writing definite advice on this point cannot be given (p. 18). 2. Cidtural Methods of Blight Control. — As suggested before (pp. 16-17) the grower should try to discourage excessive succulent growth by careful regulation of irrigation water and a system of pruning as light as is consistent with the production of regular crops of fruit of marketable quality. Blight operators should go ahead of the pruning crew because if active cases are present, the pruners may scatter the disease. Water- shoots about the body of the trees and suckers should be removed only during the dormant season. Cutting these off during the blight season will make wounds into which the rain may carry blight from the infec- tions above. Also the removal of these sprouts encourages new buds to push out about the body of the tree immediately after harvest (of Bartlett and other early varieties), and these are very readily inocu- lated with blight by insects. Cankers on these new growths are diffi- cult to find, especially in the fall when there is no gumming. On account of blight it is desirable, in training young trees, to leave six or seven permanent scaffold branches at a height of four or five feet from the ground. Then if blight kills one or more of these scaffold branches the tree may be readily rebuilt and brought back to its original bearing capacity. 42 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 3. Insect Control. — We are not in a position at this writing to give definite advice regarding control of insects responsible for spreading blight (p. 15). 4. Calendar of Blight-Control Opey^ations. — Four general opera- tions in the control of blight during the various seasons of the year are recommended as f ollow^s : a. The Winter Cleanup. During the fall and v^inter, search for holdover cankers. If, during the late fall, leaves on a branch or tree color up early, gouge into suspicious-looking spots in the bark of the branch and even into the trunk and roots belov7, if no blight is dis- covered in the branch. However, other diseases or conditions than blight may cause the leaves on a branch or tree to change color early in the fall. If many trees are found with root cankers, the root-crown of all trees to a depth of 10 or 12 inches under ground should be inspected by gouging. This should also be done after any year in which fall blight has been prevalent in the orchard. In some old orchards many roots have a reddish color which is sometimes mistaken for blight. In these conditions, however, there is not the sappy or oily appearance of blighted tissue. All cankers discovered should be treated with the zinc-chloride solution if the roots or branches affected are not too badly injured. Treated roots and crowns should be left exposed for several days before re-covering with soil because it takes several days for the zinc chloride to penetrate. In the winter time, during foggy weather, active cases should be left exposed after treat- ment for several weeks to make sure that the blight has been killed. These cases should be marked for future inspection. Just before blossoming and during the blossoming period, search for holdover cankers and inspect once more all cankers treated the season before to detect any new activity. Oozing cankers during the blossoming season are responsible for infections each spring. &. Spring Operations. During the latter part of the blooming period, begin to inspect the trees for blighted blossoms and twigs, especially about the trunks and larger branches. Throughout the spring and summer try to cover the orchard once a week to cut out infected twigs and to treat cankers, in order to keep ahead of the insect spreaders of the disease. The greater the number of infections in a tree, the greater the number of new infections that may be car- ried to healthy twigs. Furthermore, success in saving the larger branches with the zinc chloride solutions depends upon finding the cankers while they are still young. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 43 In removing' twigs and branches, cut to some lateral (if possible) a foot or more below the last signs of the disease. Examine every piece cut off to make sure that the cut was made far enough below the disease. If the disease was within 8 or 10 inches of the main branch, the branch should be treated with zinc chloride solution. Do not allow the cut piece to fall through the tree because it may scratch and infect other parts. Trees severely killed back by blight throw out a great many succu- lent branches and these are very susceptible to blight. On this account it is desirable to cure as many diseased branches as possible. Yet dur- ing a severe blight epidemic it will not pay to waste much time treat- ing small branches with zinc chloride because a difference of two or three days in getting over the orchards may make the difference between saving or losing many large branches. The removal of late and out-of -season blossoms while cutting off blighted twigs assists very materially in lessening the number of infec- tions during seasons of blight epidemics. Some growers begin remov- ing these late and out-of -season blossoms just as soon as they discover that the blight is becoming epidemic in their orchards. Disinfect all cuts and wounds and keep the tools and shears smeared with the mercury-glycerine disinfectant (formula No. 3, p. 49). A water solution without the glycerine is also effective, but the glycerine disinfectant remains on the shears longer without drying up and hence is more efficient in the hands of careless operators. If cuts are not disinfected, ants and other insects may re-infect them immediately, especially in damp weather in the spring and early summer. If there are many twigs to cut, a piece of cheap sponge bound tightly to the wooden part of the shear handles (fig. 9) and saturated with the disinfectant makes a very quick method of apply- ing the disinfectant to the cut ends. Breaking off diseased twigs and branches instead of cutting is not advisable because the broken ends are not easily disinfected and so insects may immediately infect the wounds with blight bacteria. Do not use zinc chloride for disinfecting cuts for it is too penetrat- ing and besides if used too freely in the tops of the trees it is apt to be spattered onto the fruit. c. Blight During the Harvest Season. Do not stop the blight work during the harvest season. Keep a competent man inspecting the trunks and main branches because at this time the most serious infec- tions are apt to be overlooked. In some seasons, however, the summer and fall infections occur mostly in the new sprouts and off-season 44 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 blossoms which appear immediately after the crop is off (of the Bart- lett variety). At this time of year the most serious losses of trees often occur on account of trunk and root-crown infections which do not ooze out gum freely in the fall. d. Fall Operations. If many cases of blight are discovered in trunks and root-crowns at any time during the summer or fall all the trees should be inspected immediately by gouging. Failure to do this might cause very serious losses. Each inspection cut as well as the tools must be disinfected (formula 3, p. 49). The U-gouge (fig. 14) is one of the best gouges for this purpose. An attempt should be made to find all cankers of blight before late fall and winter because it is much more difficult to cure active cankers during this season than it is during the spring, summer, and early fall months. 5. Disposal of Blight Cuttings. — During hot weather the infected twigs on the ground dry rapidly and probably are not a serious source of infection. During the spring and early summer it is well, as a precautionary measure, to gather and burn all twigs and branches which are removed on account of blight. These cuttings should never be thrown in large piles and left unburned because they will not dry out sufficiently to kill the bacteria for a long time. Especially before an irrigation the fresh clippings should be gathered because it is probably possible to infect the root-crown and suckers by bacteria in the irrigation water. 6. Treatment foi' Badly Blighted Trees. — The fact that severe epidemics of blight occur only one or two years at a time makes it possible for the grower to repair injured trees and rebuild the orchard. In case a tree is girdled by blight and the cambium is killed in the upper ]3art of the trunl^ or about the crotches, the trunk should be cut off below the canker but as far above the graft union as is safe. This should be done as soon as it is discovered, else the roots will be starved and not be able to support a vigorous new growth. If this occurs in early summer new sprouts from the trunk may grow out immediately after sawing off the trunk, but if it is late in the fall, the new sprouts will not grow out until the following spring. Several sprouts should be saved about three inches apart in order that the stump may heal completely across. If not enough sprouts occur, grafts should be put in the following winter (February or March) to fill out. The cut end of the stump should be covered with the Bordeaux-oil paint to prevent wood rots. In figure 15 perhaps too many sprouts were left as scaffold branches for the future tree. 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 45 Fjg. 15. — New branches growing from lower part of the trunk after removing the top, which was killed l)y blight. Fig. 16. — Bearing pear trees which had been injured \>y tdighl ni\ igorated by grafting root suckers into trunk and branches above the injury. 46 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 In case the girdling extends to the graft union the tree can be cut off below the graft union and scions of the Bartlett or desired variety set in about three inches apart the following spring. However, most growers prefer to take out trees which are killed to the graft union and replant with new trees the following winter. It is usually Fig. 17. — A three-year-old inarch with three inarched seedlings. easy to grow new trees in a young orchard, but in an older orchard it requires considerable care because the older trees rob the young trees of moisture unless irrigated often. In case a tree is injured only on one side in the trunk a sprout from below can be used as a bridge graft (fig. 16) to increase the sap flow on that side. If the injury extends down to the root a seedling tree can be planted along side the old one and the top grafted into the 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 47 old tree above the wound. This is called inarching (%. 17). Perhaps the best way to ^raft in the top of the seedling is to cut out a slice of bark of the seedling a considerable distance below the tip and cut out a piece of bark in the old tree (to the cambium layer) wide enough to receive the seedling. Nail the seedling down with a small brad and wax over with grafting wax. The top is not cut off until after the graft has united well to the old tree three or four weeks after the operation. Keep sprouts off the seedlings. In case no sprouts are available below, a bridge graft can be used to increase the sap flow. The scions for the bridge graft should be of the previous season's growth and about the diameter of a lead pencil. The two ends of the scion are whittled flat on one side for about 1% inches. Sections of the bark are then cut out at both ends of the girdle to fit these flattened ends of the scion. When in place, the cambium at the edges of the scion will be in contact with the cambium layer of the tree. The ends are then fastened down with brads and the union covered with wax to exclude air. 7. The Blight Crew. — Great care should be exercised in choosing men for blight work. Not all men have the patience and habitual carefulness to become good blight men. Some will not believe that blight is infectious. Such men do more harm than good in blight work. The crew should be under the supervision of a competent blight foreman. Some owners of large orchards divide the orchard into sections with one competent operator to supervise a small group of blight cutters in each section. The zinc chloride treatment of cankers on the scaffold branches, trunks, and roots should be performed by the supervisor of the crew. All the men should carry the zinc chloride solution for use in the upper parts of the tree either on cankers or in cases where it is doubtful whether or not the disease has yet passed from the twig into the main branch (fig. 12C). The men should be careful in the tree tops not to spatter the solution on the fruit. 8. Proprietary Remedies. — The pear grower is cautioned regarding the adoption of new kinds of blight remedies. There is perhaps no other orchard-tree disease that has been so widely exploited by quack 'tree-doctors' as has pear blight. However, some of these remedies have been offered in good faith by the orginators. This disease has so many peculiar variations in behavior that it is very easy for orchardists as well as experimenters to be misled regarding the efficiency of a remedy. A remedy should be tried throughout a period 48 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 of at least two years and with virulent types of cankers before passing final judgment upon it. It should be kept in mind that a remedy which works in one district may not work in another. In California, manufacturers and dealers in remedies for plant diseases and insect pests must be under license by the State Depart- ment of Agriculture. A license is not issued unless the remedy has considerable promise, and if it proves worthless the license will be revoked or at least will not be renewed. 9. Community Cooperation. — The matter of community coopera- tion in blight control is very important but pear growers have experi- enced much difficulty in getting together on a working program aifecting a whole community or district. The County Horticultural Commissioners can do very little in the enforcement of horticultural laws unless the growers are whole- heartedly behind the movement. There are in most districts many absentee owners, and small non-commercial plantings. There are also ornamentals in home yards and native rosaceous plants which may harbor the disease over winter or assist in its spread at blossoming time. The expense of a campaign to take care of all sources of infec- tion would be greater than some counties would care to provide for without the hearty cooperation of most of the growers and farm.ers in the district affected. FORMULAS Formula No. 1 — 43 Per Cent Zinc Chloride. — Both strengths are prepared by dissolving dry zinc chloride powder in the following solvent in an enameled kettle : Solvent : 1 gallon denatured alcohol 1 pint water 3 ounces concentrated hydrochloric acid. Dissolve 6 pounds of zinc chloride in the above quantity (9 pints) of solvent. In dissolving, the solution becomes very hot. Stir thoroughly, while hot, and crush all hard lumps with a wooden paddle, until thoroughly dissolved. Avoid as much as possible inhaling fumes, for the chemicals used in denaturing the alcohol are injurious. Cool as rapidly as possible, covering with a lid to prevent evaporation. Add a few drop of laundry blueing to give it a bluish color. Keep in large bottles, well corked. If much of the alcohol evapo- rates the strength of the solution becomes greater. Precipitates (sedi- ments) consist of impurities. It is not necessary to stir the solution 1928] PEAR BLIGHT CONTROL IN CALIFORNIA 49 before using because the sediments are not useful. Keep the dry zinc chloride powder well sealed for it absorbs moisture very rapidly. It would be impossible to weigh out the proper amount of moisture-laden zinc chloride because it may absorb more than its own weight of water. Small lots of dry zinc chloride may be transferred to Mason jars with rubber gaskets to keep out moist air. It is not necessary to use chemically pure materials for making up zinc-chloride solution. The 43 per cent solution can also be prepared by diluting 1 gallon of the 53 per cent solution (formula No. 2) with three pints of the solvent. Formula No. 2 — 53 Per Cent Zinc Chloride. — Dissolve 9 pounds of dry zinc chloride in 9 pints of the solvent. The 53 per cent solution is prepared in the same way as the 43 per cent solution, except that blueing is not added. The same precautions should be exercised in preparing and storing it. Formula No. 3. — Mercury -Glycerine Disinfectant. — 1 quart distilled water (or rain water) 3 quarts commercial glycerine. 16 tablets cyanide of mercury (or i/4 oz. of the crystals) 16 tablets bichloride of mercury (or % oz. of the crystals). Add the w^ater to the glycerine in a clean enameled kettle. Heat to 165° or 175° Fahrenheit. Remove from the stove. Dissolve the cyanide of mercury in this by stirring rapidly with a clean dry stick, glass rod, or enameled spoon. Next add the bichloride of mercury and stir until dissolved. The tablets are the 71/2 grain, or half -gram size, known also as 'large' tablets. The powder or crystal form of the pure compounds is usually much cheaper than the tablets. Glycerine should be purchased in 1-gallon or 5-gallon lots and should be the commercial and not medicinal grade for the latter is more expensive. This solution will keep for a long time in bottles, well stoppered. Mercury compounds are very poisonous. Keep the tablets under lock and key, for they are very attractive to children. In case of accidental poisoning the following is the simplest of the official treat- ments : Administer white of eggs and then induce vomiting by use of mustard water (2 to 4 teaspoonfuls of mustard thoroughly stirred into a cupful of warm water) . Send for a physician or rush the patient to the nearest hospital. This disinfectant solution is economically and efficiently applied with a i^aint brush (about II/2 inches wide). Swabs waste too much material. When doing frequent inspection work with a jackknife the pocket sponge-bottle (fig. 14) is very convenient. The sponge is kept 50 CALIFORNIA AGRICULTURAL EXTENSION SERVICE [CiRC. 20 saturated with the disinfectant and carried in the pocket bottle with the lid off, thus being" always in readiness for use. Operators become careless about disinfecting if the disinfectant is not in handy form. Formula No. 4. — Bordeaux-Oil Paint. — The Bordeaux-oil paint is made by stirring raw linseed oil into any one of the Bordeaux powders found on the market — not the 'two-package' Bordeaux. It has been used by a number of fruit growers in Oregon for five or six years and is recommended by the Oregon Experiment Station^ as more satisfac- tory than lead paints or the Bordeaux paste made with water. It makes a covering which lasts for at least four years and prevents infections of wood-rotting fungi. The paint can be made up in a metal container or in jars. Add the raw linseed oil slowly, while stirring, until a thick smooth paint is formed. Pouring back and forth from one container to another will help even the mixture. It is desirable that the paint be a little thick when first prepared because it becomes thinner on standing. Apply with a paint brush. Wash the brush with coal oil, benzene, or gasoline before setting it away to dry. ACKNOWLEDGMENTS The writer wishes to express his appreciation to the following who have aided in this work at one time or another: Dr. Warren P. Tufts, upon whose suggestions the experiments in treatment of pear-blight cankers were begun, for valuable suggestions and for assistance in preparing the manuscript and illustrations; to the members of the Pomology Division at Davis and to Dr. C. S. Bisson and Dr. W. W. Robbins for valuable suggestions; to the following County Farm Advisors who have been very helpful in studying the results secured by pear growers in the use of the zinc-chloride treatments: Roy D. McCallum, B. J. Jones, and W. D. Norton; to the following pear growers for valuable suggestions and trials in their orchards : Howard Reed, Hay ward Reed, Geo. F. Otis, Leland L. Stoll, Mercer Runyan, David Elliott, and Frank A. Cleland. Thanks are especially due to the Earl Fruit Company and to the California Pear Growers Associa- tion for renting, for the College of Agriculture, a pear orchard near Marysville in which much of the earlier experimental work was done. 9 Zeller, S. M. Cankers of apple and pear in Oregon and their control. Oregon Agr. Exp. Sta. Cir. 73:1-29. 1926. STATION PUBLICATIONS AVAILABLE FOR FREE DISTRIBUTION BULLETINS No. No. 253. Irrigation and Soil Conditions in the 3 89. Sierra Nevada Foothills, California. 390. 262. Citrus Diseases of Florida and Cuba Compared with those of California. 391. 263. Size Grades for Ripe Olives. 268. Growing and Grafting Olive Seedlings. 392. 277. Sudan Grass. 393. 278. Grain Sorghums. 394. 279. Irrigation of Rice in California. 283. The Olive Insects of California. 304. A Study of the Effects of Freezes on 395. Citrus in California. 310. Plum Pollination. 396. 313. Pruning Young Deciduous Fruit Trees. 397. 324. Storage of Perishable Fruits at Freez- ing Temperatures. 398. 328. Prune Growing in California. 400. 331. Phylloxera-resistant Stocks. 402. 335. Cocoanut Meal as a Feed for Dairy 404. Cows and Other Livestock. 405. 340. Control of the Pocket Gopher in 406. California. 407. 343. Cheese Pests and Their Control. 344. Cold Storage as an Aid to the Mar- . keting of Plums, a Progress Report. 408. 347. The Control of Red Spiders in Decid- 409. uous Orchards. 348. Pruning Young Olive Trees. 349. A Study of Sidedraft and Tractor Hitches. 410. 350. Agriculture in Cut-Over Redwood Lands. 353. Bovine Infectious Abortion, and As- 411. sociated Diseases of Cattle and New- born Calves. ^ 412. 354. Results of Rice Experiments in 1922. 357. A Self-Mixing Dusting Machine for Applying Dry Insecticides and Fun- 414. gicides. 358. Black Measles, Water Berries, and 415. Related Vine Troubles. 416. 361. Preliminary Yield Tables for Second- Growth Redwood. 417. 362. Dust and the Tractor Engine. 363. The Pruning of Citrus Trees in Cali- 418. fornia. 364. Fungicidal Dusts for the Control of 419. Bunt. 366. Turkish Tobacco Culture, Curing, 420. and Marketing. 367. Methods of Harvesting and Irrigation 421. in Relation to Moldy Walnuts. 422. 368. Bacterial Decomposition of Olives During Pickling. 423. 369. Comparison of Woods for Butter Boxes. 424. 370. Factors Influencing the Development of Internal Browning of the Yellow 425. Newton Apple. 426. 3 71. The Relative Cost of Yarding Small and Large Timber. 427. 373. Pear Pollination. 374. A Survey of Orchard Practices in 428. the Citrus Industry of Southern California. 375. Results of Rice Experiments at Cor- 429. tena, 1923, and Progress in Experi- 430. ments in Water Grass Control at the 431. Biggs Rice Field Station, 1922-23. 377. The Cold Storage of Pears. 432. 380. Growth of Eucalyptus in California Plantations. 433. 382. Pumping for Draininge in the San Joaquin Valley, California. 434. 385. Pollination of the Sweet Cherry. 386. Pruning Bearing Deciduous Fruit 435. Trees. 3 87. Fig Smut. 388. The Principles and Practice of Sun- Drying Fruit. Berseem or Egyptian Clover. Harvesting and Packing Grapes in California. Machines for Coating Seed Wheat with Copper Carbonate Dust. Fruit Juice Concentrates. Crop Sequences at Davis. I. Cereal Hay Production in Cali- fornia. II. Feeding Trials with Cereal Hays. Bark Diseases of Citrus Trees in Cali- fornia. The Mat Bean, Phaseolus Aconitifo- lius. Manufacture of Roquefort Type Cheese from Goat's Milk. Orchard Heating in California. The Utilization of Surplus Plums. The Codling Moth in Walnuts. The Dehydration of Prunes. Citrus Culture in Central California. Stationary Spray Plants in California. Yield, Stand, and Volume Tables for White Fir in the California Pine Region. Alternaria Rot of Lemons. The Digestibility of Certain Fruit By- products as Determined for Rumi- nants. Part I. Dried Orange Pulp and Raisin Pulp. Factors Influencing the (Quality of Fresh Asparagus after it is Har- vested. Paradichlorobenzene as a Soil Fumi- gant. A Study of the Relative Value of Cer- tain Root Crops and Salmon Oil as Sources of Vitamin A for Poultry. Planting and Thinning Distances for Deciduous Fruit Trees. The Tractor on California Farms. Culture of the Oriental Persimmon in California. Poultry Feeding: Principles and Prac- tice. A Study of Various Rations for Fin- ishing Range Calves as Baby Beeves. Economic Aspects of the Cantaloupe Industry. Rice and Rice By-Products as Feeds for Fattening Swine. Beef Cattle Feeding Trials, 1921-24. Cost of Producing Almonds in Cali- fornia : a Progress Report. Apricots (Series on California and Prices). The Relation of Rate of Maturity to Egg Production. Apple Growing in California. Apple Pollination Studies in fornia. The Value of Orange Pulp for Milk Production. The Relation of Maturity of fornia Plums to Shipping Dessert Quality. Economic Status of the Grape Industry. Range Grasses of California. Raisin By-Products and Bean Screen- ings as Feeds for Fattening Lambs. Some Economic Problems Involved in the Pooling of Fruit. Power Requirements of Electrically Driven Manufacturing Equipment. Investigations on the Use of Fruits in Ice Cream and Ices. The Problem of Securing Closer Relationship Between Agricultural Development and Irrigation Con- struction. Crops Cali- Cali- and bulletins- No. 436, I. The Kadota Fig. II. Kadota Fig Products. 437. Economic Aspects of the Dairy In- dustry. 43 8. Grafting Affinities with Special Refer- ence to Plums. 439. The Digestibility of Certain Fruit By- products as Determined for Rumi- nants. Part II. Dried Pineapple Pulp, Dried Lemon Pulp, and Dried Olive Pulp. 440. The Feeding Value of Raisins and Dairy By-Products for Growing and Fattening Swine. 441. The Electric Brooder. 442. Laboratory Tests of Orchard Heaters. 443. Standardization and Improvement of California Butter. 444. Series on California Crops and Prices: Beans. -(Continued) No. 445. Economic Aspects of the Apple In- dustry. The Asparagus Industry in California. The Method of Determining the Clean Weights of Individual Fleeces of Wool. 448. Farmers' Purchase Agreement for Deep Well Pumps. 449. Economic Aspects of the Watermelon Industry. Irrigation Investigations with Field Crops at Davis, and at Delhi, Cali- fornia. Studies Preliminary to the Establish- ment of a Series of Fertilizer Trials in a Bearing Citrus Grove. Economic Aspects of the Pear In- dustry. 446 447 450. 451. 452. No. 87. Alfalfa. 117. The selection and Cost of a Small Pumping Plant. 127. House Fumigation. 129. The control of Citrus Insects. 136. Melilotus Indica as a Green-Manure Crop for California. 144. Oidium or Powdery Mildew of the Vine. 157. Control of Pear Scab. 164. Small Fruit Culture in California. 166. The County Farm Bureau. 178. The Packing of Apples in California. 202. County Organization for Rural Fire Control. 203. Peat as a Manure Substitute. 209. The Function of the Farm Bureau. 212. Salvaging Rain-Damaged Prunes. 215. Feeding Dairy Cows in California. 230. Testing Milk,' Cream, and Skim Milk for Butterfat. 231. The Home Vineyard. 232. Harvesting and Handling California Cherries for Eastern Shipment. 234. Winter Injury to Young Walnut Trees During 1921-1922. 238. The Apricot in California. 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. 243. Marmalade Juice and Jelly Juice from Citrus Fruits. 244. Central Wire Bracing for Fruit Trees. 245. Vine Pruning Systems. 248. Some Common Errors in Vine Prun- ing and Their Remedies. 249. Replacing Missing Vines. 250. Measurement of , Irrigation Water on the Farm. 252. Support for Vines. 253. Vineyard Plans. 255. Leguminous Plants as Organic Fer- tilizers in California Agriculture. 257. The Small-Seeded Horse Bean (Vicia faba var. minor). 258. Thinning Deciduous Fruits. 259. Pear By-Products. 261. Sewing Grain Sacks. CIRCULARS No. 265. 266. 267. 269. 270. 273. 276. 277. 278. 279. 281. 282. 284. 286. 287. 288. 289. 290. 292. 293. 294. 296. 298. 300. 301. 302. 304. 305. 307. 308. 309. 310. 311. 312. Plant Disease and Pest Control. Analyzing the Citrus Orchard b> Means of Simple Tree Records. The Tendency of Tractors to Rise in Front; Causes and Remedies. ' An Orchard Brush Burner. A Farm Septic Tank. Saving the Gophered Citrus Tree. Home Canning. Head, Cane and Cordon Pruning of Vines. Olive Pickling in Mediterranean Countries. The Preparation and Refining of Olive Oil in Southern Europe. The Results of a Survey to Deter- mine the Cost of Producing Beef in California. Prevention of Insect Attack on Stored Grain. The Almond in California. Milk Houses for California Dairies. Potato Production in California. Phylloxera Resistant Vineyards. Oak Fungus in Orchard Trees. The Tangier Pea. Alkali Soils. The Basis of Grape Standardization. Propagation of Deciduous Fruits. Control of the California Ground Squirrel. Possibilities and Limitations of Coop- erative Marketing. Coccidiosis of Chickens. Buckeye Poisoning of the Honey Bee. The Sugar Beet in California. Drainage on the Farm. Liming the Soil. American Foulbrood and Its Control. Cantaloupe Production in California. Fruit Tree and Orchard Judging. The Operation of the Bacteriological Laboratory for Dairy Plants. The Improvement of Quality in Figs. Principles Governing the Choice, Op- eration and Care of Small Irrigation Pumping Plants. The publications listed above may be had by addressing College of Agriculture, University of California, Berkeley, California. 14m-6,'28