UNIVERSITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA MONILIA BLOSSOM BLIGHT (BROWN ROT) OF APRICOTS BY B. A. RUDOLPH BULLETIN No. 383 February, 1925 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1925 MONILIA BLOSSOM BLIGHT (BROWN ROT) OF APRICOTS* By B. A. RUDOLPH CONTENTS PAGE Summary 4 Introductory 7 The Disease: Its characteristics and its cause 7 History of the disease 15 Importance of the disease in California 18 Possible reasons for the increase of the disease in California 18 Experiments for the control of Monilia blossom blight 19 Introductory statement 19 Scope of the experiments 19 The time the sprays were applied 20 The number of trees used in the experiments 20 How the sprays were applied 20 Method of determining results 20 Observations made in private orchards 21 Explanation of terms 21 Dormant sprays 23 Experimental 23 Results obtained in private orchards with dormant sprays 23 Delayed dormant and red bud sprays 23 Successful results obtained in private orchards 23 Unsuccessful results obtained in private orchards 24 Results obtained experimentally with red bud sprays 24 The most favorable period for securing good results with one spray 26 Bloom sprays 28 Experimental 28 Results obtained with full bloom sprays in private orchards 28 Spraying before the full bloom. Results obtained in private orchards 28 Multiple spraying 30 Multiple spraying in the pre-bloom period 31 Multiple spraying throughout the susceptible period 32 Discussion - 34 Reasons for the failure of single sprays to control the disease 34 Reasons for the failure of dormant sprays to control the disease 34 Reasons for the failure of full bloom sprays to control the disease 34 Conditions under which red bud sprays succeed 35 Reasons for the success of the multiple spray treatment 35 * The term Monilia blossom blight is suggested for adoption as a common name for the blossom and the twig blight phase of the disease to distinguish it from "Brown Rot," a decay of the ripening fruit, produced by the same organism. The term brown rot is so universally used elsewhere to designate the rot of the fruit that its use in California to designate the disease of the blossoms and twigs is very confusing. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION PAGE Orchard sanitation 35 The relation of careful pruning to spraying in the control of the disease 35 Ordinary pruning better than no pruning 38 The practicability of careful pruning 38 The disposal of infectious material 38 Experiments to produce the apothecial form 38 Depleted vigor of the trees conducive to Monilia blossom blight r 39 The relation of organic fertilizers to the propagation of the brown rot fungus. . 39 Ornamental plants as "carriers" of the brown rot fungus 39 Fungicides 40 Bordeaux mixture 40 Sulfur-containing compounds 41 Lime-sulfur damage 41 Effect of pure sulfur on ripening apricots 42 Oil sprays 43 Value as dormant sprays 43 Red bud oil sprays 44 Fungicidal dusts 44 Experimental dusting for the control of the disease 44 Results obtained with fungicidal dusts in private orchards 46 When to spray for Monilia blossom blight control 46 The preparation of Bordeaux mixture 47 The preparation of the stock solutions 47 How to mix the spray 48 Commercially prepared Bordeaux sprays 49 Bibliography 50 SUMMARY Monilia blossom blight is due to the attack of a fungus of the genus Sclerotinia. The fungus reproduces itself by means of spores which are produced in abundance, before the blossoms open, on the old blighted twigs and decayed fruits left in the trees. A second reproduc- tive form is known, but it rarely occurs in California, The blossoms may be attacked at any time after the buds have cracked and show the folded white petals beneath the red sepals, until the full bloom or even a little later when the petals have been shed. Having killed a blossom the fungus passes into the twig and kills it. Later the fungus attacks the ripening fruit producing a characteristic "brown rot. n The disease was first described in Europe in 1796, in America in 1881, and it was known to exist in California before 1900. The disease has become exceedingly destructive in California in recent years. This is possibly due to the planting of less resistant varieties. Bulletin 383] BROWN ROT OF APRICOTS 5 Over one hundred different treatments, involving the use of most of the better known fungicides, oil emulsions, coal-tar derivative sprays, fungicidal dusts and various proprietary preparations pre- sumably possessing fungicidal properties, have been tried out experi- mentally during the past four years for the control of the disease. These materials were used separately or in combination, in single or multiple applications at various times either before, during or after the bloom. In addition to these experiments, records of the methods employed and the control obtained in approximately two hundred private orchards were compiled. From the data accumulated it has been concluded that : (1) In orchards already seriously infected and situated in localities where normal climatic conditions render the disease difficult to con- trol, a dormant spray of any standard fungicide may decrease the amount of disease noticeably, but seldom sufficiently to be of practical value. (2) A spray applied in the spring when the buds are expanding, or still later in the red bud stage, affords the best control obtainable with any one spray. Frequently such a spray controls the disease. In orchards situated in localities normally damp during the susceptible period and where little effort has been made to remove the great accu- mulation of old infectious material from the trees, the control obtained by such a spray may be entirely unsatisfactory, although, relatively speaking, the amount of disease may be much diminished. (3) The more closely the application of the spray approximates the period when the buds are cracking, showing the folded white petals beneath the red sepals, the greater the control is apt to be, but there is no certainty that, even at this favorable period, one application of any fungicide will give a practical control of the disease under adverse conditions. Relatively speaking, the control will be greater at this time than at any other, but from the practical standpoint, it is likely to be a failure. (4) A spray applied at any time after the blossoms are open usually affords some relief, but from the practical standpoint, the control afforded generally is a failure, and, the longer the spray is delayed, the greater the failure is likely to be. Similarly, a full bloom spray usually affords some relief, but the results are invariably very unsatisfactory from the practical standpoint and particularly so if the susceptible period is damp. (5) All sprays may help a little regardless of the time of appli- cation. 6 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION (6) Regardless of whether or not the trees have been sprayed, there will be little or no disease even though the fungous spores are present in the greatest abundance, unless rain, heavy dew or fog occurs during the susceptible period. Moisture is the great determining factor in the infection of the flowers. Many growers not recognizing this fact credit the control of the disease in dry seasons to weak mixtures of standard fungicides or to other spray materials which under more normal conditions prove utterly unsuited to the purpose. (7) In badly diseased orchards situated in localities which are favorable to the ready development of the disease because the weather is normally humid during the blossoming period, and in cases where little or no attempt has been made to remove the great accumulation of old infectious material from the trees, multiple spraying throughout the entire susceptible period is the only means likely to secure a prac- tical control. AVith this method several sprays are applied, starting with the red bud spray and following with others in quick succession as the flowers continue to open until the trees have reached full bloom, or even a little later when the petals have been shed. (8) The degree of control in badly diseased orchards is propor- tionate to the number of sprays applied during the susceptible period of the blossoms. (9) Orchard sanitation (the careful removal of all infectious material from the trees and the ground) is as vital as spraying in the control of the disease, but should be regarded as an auxiliary to spray- ing operations, rather than as a separate means of controlling the disease. (10) The extent of the control by sprays is largely affected by the degree to which orchard sanitation has been observed. (11) Manures, decaying and freshly cut grasses and alfalfa in the orchard do not harbor the fungus over winter. Many ornamental plants were examined, but the disease was found only in the Japanese quince (Chaenomales japonica). (12) The fungicide best adapted to use on apricot trees and for the control of the disease is home-made Bordeaux 8-8-50. Bordeaux 5-5-50 or 6-6-50 frequently gives excellent results in orchards less severely affected or situated in localities where climatic conditions are more favorable to the control. (13) Bordeaux does not interfere with the fertilization of the apricot blossoms whatever the time of application, fertilization appar- ently taking place before the red buds open. Bulletin 383] BROWN ROT OF APRICOTS 7 (14) Having brought the disease within the limits of practical control by the multiple spray method, fewer sprays, and in most instances, only one spray of Bordeaux 8-8-50 or weaker strength will be needed in succeeding years to hold the disease in check. (15) Lime-Sulfur, Soda-Sulfur and other sulfur-containing com- pounds are distinctly unsuited for use on apricots because they cause a physiological injury to the trees commonly referred to as "sulfur poisoning. ' The foliage on such trees is stunted and yellowish, the fruit is dwarfed, of poor color and quality, and ripens late. Even pure sulfur applied with water to the ripening fruit may cause it to be stunted and late in ripening. Apricot trees treated with sulfur or sulfur-containing compounds sometimes fail to bloom the following season. (16) The more commonly used oil emulsions, miscible oils and coal-tar derivative sprays which are applied in winter for the control of insect pests have some fungicidal effect, but insufficient for the control of the disease in badly diseased orchards in humid localities. The relative degree of control they afford is sometimes quite marked, but from the practical standpoint they are generally unsatisfactory. Applied in the spring they usually burn the buds or lower the crop yield by interfering with the pollination of the flowers. (17) Fungicidal dusts are ineffective in the control of the disease, particularly in orchards in localities where the disease is difficult to control. Thirteen different fungicidal dusts were used in single and multiple applications throughout the susceptible period, but none were found equal to a single application of liquid Bordeaux under similar conditions. Generally the results obtained from the dusts were very poor from both the relative and practical standpoints. INTRODUCTORY THE DISEASE : ITS CHABACTEKISTICS AND ITS CAUSE Monilia blossom blight (brown rot) is due to the attack of a fungus of the genus Sclerotinia. This fungus has two reproductive forms, the imperfect stage (Monilia) and the perfect (Sclerotinia). The latter stage is exceed- ingly rare in California, the former very common. Monilia blossom blight never kills a tree ; it is primarily a disease of the second-year or fruit wood which it attacks through the blossoms. 8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The severity of the attack depends on the weather. In climates favorable to the disease its severity increases yearly if not controlled. In the spring- the apricot blossoms are attacked at any time after the buds have cracked and show the folded white petals beneath the red sepals until the petals have fallen. The killed flowers remain fixed to the twigs until eventually dislodged by storms. > Fig. 1. — Apricot fruit spurs killed by Monilia blossom blight. The dead flowers do not fall off. Note the production of gum at the junction of living and dead tissue. From Bull. 326, Calif. Agr. Exp. Sta. From the flowers the fungus passes into the spurs, killing them back as much as five inches in one week and a foot or more before it ceases to be active. Fruiting twigs may be killed back five feet by numerous infections along their entire length or by girdling from a single infection at the base. Bulletin 383] BROWN ROT OF APRICOTS 9 Copious amounts of amber- or honey-colored gum are character- istically produced at the junction of the living and the dead tissue. It is harmless and being readily soluble in rain water may not always be present. (Fig. 1.) In humid weather the dying flower parts usually become covered with fungous spores (the Monilia stage) within about twenty-four hours after infection. Fig. 2. — Production of Monilia spores from cankers on almond twigs killed the previous spring by Monilia blossom blight. The spore pustules are recog- nized as roundish light-colored bodies. Natural size. Prom Bull. 326, Calif. Agr. Exp. Sta. Having killed a spur the fungus becomes dormant or inactive until the following winter when it is reactivated by wet weather. Spores are produced in almost inconceivable numbers on the dead wood. (Fig. 2.) If the wood dries out, spore production ceases for the time being. 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Spores are reproductive bodies. The Monilia spores are produced in chains. Hundreds, possibly thousands of these spore chains are grouped in dense clusters, visible as tiny powdery pustules or cushions on the old decayed fruits and twigs. At first the pustules are bluish- or ash-gray, becoming buff to brown as they age and dry out. When a spore is ripe it falls away from the chain, and new ones are constantly being produced from the base. (Fig. 3.) y r\ Mi- IP Fig. 3. — Photomicrograph of Monilia spores produced in culture on Ciapeck agar. A spore germinates if sufficient moisture is present and sends forth a fungous thread which quickly penetrates and kills the petals and reproductive flower parts, finally passing into the spur. Large, ugly cankers may be produced in the older wood, the attack usually being made from the base of a killed spur. The cankers ordinarily are inconspicuous until the bark dries and peels away in summer revealing deep gummy lesions. These ordinarily heal over by the end of the year and cause no further trouble. New tender foliage may be attacked in very wet weather, but the resistance of the new growth to the disease is usually marked. Repealed efforts to induce the fungus to attack healthy bark of either first or second year wood by binding spore pustules or spore covered fruits to the twigs with moist bandages failed. Bulletin 383] BROWN ROT OF APRICOTS 11 Occasionally, in very wet weather the young fruits in the jackets are attacked, but after the old flower parts have fallen away they rarely suffer much. Attacks at this stage are usually made through insect wounds and abrasions. A rot of small green apricots produced by Sclerotinia libertiana Fcl., a closely related fungus, is frequently confused with brown rot. Fig. 4 — Brown Eot of ripe apricots. The disease passes from fruit to fruit in clusters, through the skin. Note enormous production of Monilia spores on the surface. Natural size. From Bull. 218, Calif. Agr. Exp. Sta. 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Ripe fruit is much less resistant to brown rot. According to Cordley, 33 Smith, 19 Cooley, 69 Valleau 77 and others the fungus can penetrate the healthy skin of peaches, plums and cherries, and the same is probably true of the apricot. One diseased fruit will occasion the destruction of an entire cluster of apricots, the fungus passing through the skin of each with ease. Fig. 5. — Apricot "mummy" produced by Brown Rot. Spore pustules of the fungus may be clearly seen on the fruit near the stem. From Bull. 326, Calif. Exp. Sta. Once attacked, the green or ripe fruit rots quickly. A minute, circular brown spot appears which rapidly enlarges, finally enveloping the entire fruit. The common term "brown rot" is derived from the appearance of the decay of the fruit. Fungus spores are produced on the rotting fruits in enormous quantities. (Pig. 4.) Most affected fruits fall, but some remain in the trees where they shrivel to a tough, leathery condition and as such are known as "mummies. " (Fig. 5.) Bulletin 383] BROWN ROT OF APRICOTS 13 From the old mummies on the ground, the so-called apothecial or perfect stage is produced. Hard, irregular-shaped, black bodies, some- what resembling black rubber when fresh and frequently as large as a grain of corn, develop in the decayed fruit tissue. They are known Fig. 6. -Prune "mummy" with apothecia of the brown rot fungus. (Courtesy C. Brooks and D. Fisher, U. S. D. A.) as sclerotia and constitute a resting stage. In this form the fungus can safely withstand the cold of winter and the heat and drought of summer. In the spring, at blossom time, apothecia are produced from the sclerotia. They are little fleshy fungous bodies, trumpet- or morning glory-shaped, which consist of a stalk several millimeters to 14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION several inches long and a cup or trumpet-shaped structure at the top, two or three millimeters to several centimeters in diameter. (Fig. 6.) Roberts 95 and Ezekiel 96 have shown that apothecia may be produced in the spring following the decay of the fruit, although ordinarily they do not appear until the second year. The apothecia are lined on their upper or inner surface with innumerable, minute, cylindrical sacks called asci. (Fig. 10.) Each ascus contains eight spores which are released when ripe and carried by air currents to the blossoms. The flowers are quickly attacked if Fig. 7. — Flowers and twigs of the Japanese Quince (Chaenomeles japonica) attacked by Monilia blossom blight. Note heavy production of Monilia spores on the dead blossom. Twice natural size. moisture conditions are favorable, and within a day or two the dead and dying parts become covered with masses of spores of the chain type already described. Thus the cycle is complete. The apothecial stage is the great source of trouble in the eastern states, also in certain parts of Oregon. In California it is exceedingly rare and of no consequence in the propagation of the fungus. Also, since the Monilia stage insures abundant reproduction in the spring it is superfluous. Pollock 84 has shown that while the mummies eventually rot away in the soil, the sclerotia may persist there for ten years and annually send up their crop of spore-bearing apothecia. Bulletin 383] BROWN ROT OF APRICOTS 15 HISTOEY OF THE DISEASE The first published account of brown rot of fruit appeared in 1796 in Europe when Persoon 1 gave the name Torida fructigena to a fungus which he found on decayed fruit of Prunus domestica (European plum), Amygdalus persica (peach) and Pyrus communis (French pear). Many investigators, including Albertini and Schweinitz, 3 Fuckel, 12 Saccardo 14 and Rabenhorst 10 have used this name. In 1801 Persoon 2 changed the name of the fungus to Monilia fructigena which name is still in use. In 1817 Kunze and Schmidt 4 referred the fungus to Oidium fructigenum, and among those who retained this name are Ehrenberg, 5 Fries, 7 Duby, 8 Cooke 13 and Smith. 17 This name, however, has now gone out of general use. In 1822 Persoon 6 renamed the fungus again, calling it Acrosporium fructigenum, a classification which has never been accepted by scientific writers. In 1833 Wallroth 9 referred the fungus to both Oospora Candida and 0. fructigena, but in 1875 von Thiimen 15 changed the name to Oidium wallrothii and still later to 0. fructigenum. In 1851 Bonorden 11 described for the first time another brown rot fungus occurring ' ' on rotting fruit ' ' which he named Monilia cinerea. The differences between this fungus and the one described by Persoon were considered too meager, however, by some investigators to permit of its being regarded as a distinct species. In 1898-99 Woronin 32 proved that the two species are distinct. In 1893 Schroter 27 referred the two species of Monilia to the genus Sclerotinia, being confident from his studies of them that a second reproductive form, the apothecial stage eventually would be dis- covered. In 1904 Aderhold 43 confirmed these predictions when he found the perfect stage of Monilia fructigena in Europe. In the United States brown rot has been very troublesome, and for nearly half a century there has been a steady output of publications dealing with the pathological, physiological, morphological and etio- logical aspects of the disease and its therapeutics. The better known papers are those of Peck 16 (1881), Arthur 18 (1886), E. F. Smith, 19 Galloway 20 (1889), Humphrey 21 (1890), Smith 22 (1891), Halsted, 23 Chester, 24 Jones 25 (1892), Humphrey 26 (1893), Taft, 28 Bailey 29 (1894), Kinney, 30 Goff 31 (1987), Cordley 33 (1899), Quaintance, 34 Waugh, 35 Bioletti 36 (1900), Norton, 37 Clinton, 38 Bioletti 39 (1902), Starnes, 40 Alwood and Price, 41 Card and Sprague 42 (1903), Clinton, 44 Sheldon 45 (1905), Heald 46 (1906), Scott, 47 Faurot, 48 Stone and Monahan, 49 Kern 50 (1907), Dandeno, 51 Reade, 52 Blake 53 (1908), Pollock, 54 Scott 16 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION and Ayers 55 (1909), Lewis, 56 Pollock, 57 Scott and Ayers, 58 Stewart, 59 Scott and Quaintance 60 (1910), Scott and Quaintance 61 (1911), Demaree, 62 Whetzel 63 (1912), Cook, 04 Jehle, 65 Matheny 66 (1913), Conel, 68 Cooley, 69 Orton, 70 Jehle 71 (1914), Bailey, 72 Jackson, 73 Posey, 74 Stakman and Rose, 75 Heald, 76 Valleau, 77 Hawkins 78 (1915), Bartram, 79 Brooks and Cooley, 80 Brooks and Fisher, 81 Hessler 82 (1916), Pollock 84 (1918), Willaman, 92 Home, 87 Willaman, 93 Howard, 89 McCubbin, 86 Cook, 90 Anderson, 94 Cook 91 (1920), Roberts, 95 Ezekiel, 96 Howard and Home 97 (1921), Barss, 98 Ezekiel, 99 Brooks and Fisher, 100 Norton, Ezekiel and Jehle 101 (1923), Norton, and Ezekiel, 102 Brooks and Fisher 103 (1924). In 1902 Norton 37 discovered the apothecial stage of the brown rot fungus in this country and named it Sclerotinia fructigena because the imperfect stage occurring here was then commonly regarded as Monilia fructigena. Subsequent studies of this form by other investi- gators led to the general opinion that what Norton really found was the perfect form of S. cinerea and not S. fructigena as he supposed. Also it was concluded that S. fructigena never has occurred in this country and that the fungus occurring here should never have been called by any other name than S. cinerea. Accordingly the great majority of writers have referred to the American brown rot fungus as Sclerotinia cinerea until as late as 1920. In 1920 Wormald 88 demonstrated that while S. cinerea of Europe and America are practically identical morphologically, biologically they may be distinguished with comparative ease and suggested the new name S. cinerea, forma americana for our brown rot fungus. In 1923 Norton and Ezekiel 102 presented a paper before the American Phytopathological Society at Cincinnati confirming Wor- mald 's work. A brief abstract of their paper appeared in Phyto- pathology in 1924 suggesting that the American brown rot fungus be renamed Sclerotinia americana Wormald, since in their opinion it can no longer be considered identical with 8. cinerea of Europe. Whether this very recent reclassification will meet with the general approval of other investigators will depend largely to what extent Wormald 's work has been confirmed by Norton and Ezekiel whose publication giving the details of their work has not yet appeared. Posey 74 and Barss 98 believe there are two distinct forms of the brown rot fungus in Oregon, and in recent years Wormald 85 has shown that even the two accepted species of the brown rot fungus in Europe have each at least two distinct biologic strains. Bulletin 383] BROWN ROT OF APRICOTS 17 Other forms of the brown rot fungus have been described, possibly the best known of which is Sclerotinia laxa (Ehrenb.) Ader. and Ruhl. Many investigators, however, insist it is identical with S. cinerea. Possibly the first published account of the disease in California, that of Professor F. T. Bioletti 36 of the University of California, appeared in the Pacific Rural Press following a severe outbreak of the disease in ripening Moorpark apricots at Niles. He briefly discussed the cause of the disease saying, "It is particularly injurious to peaches in the eastern states and to prunes and apricots on the Pacific slope. ' ' He was fully aware that the disease also attacks the blossoms and young growth of the peach and concluded that "the same probably occurs in the case of apricots and other fruits. ' "* Since 1902 frequent references to brown rot may be found in the Annual Report of the California Agricultural Experiment Station. In 1916 the California Agricultural Experiment Station undertook experimental spraying for the control of the disease. No conclusions were drawn because climatic conditions prevented an attack. In 1918 Circular 204 appeared with recommendation for the control of the disease, but investigational work was not resumed until 1920. In the meantime the annual loss in the San Francisco Bay region due to the killing of the blossoms had reached alarming proportions. Orig- inally observed as a disease of ripening fruit, the apricot industry in the coastal regions was now menaced by the blossom blight. Because of the thoroughness and the rapidity with which the blossoms are destroyed after damp cold spells, much of the earlier Monilia blossom blight damage was mistaken for frost injury. For similar reasons many growers attributed the disease to sour sap, sour soil and other causes. The results of the first year's experiments at the Deciduous Fruit Station are contained in Bulletin 326, and the present paper covers the experimental control work up to the present time. * In a recent personal interview Professor Bioletti stated that while he had never seen specimens of diseased apricot blossoms when he wrote his paper, it was common hearsay at the time that they were attacked. 18 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION IMPOETANCE OF THE DISEASE IN CALIFOENIA No accurate estimate of the annual loss occasioned in California by Monilia blossom blight can be given. Unquestionably the loss in fruit alone, to say nothing of the great cost of fighting the disease, has amounted to hundreds of thousands of dollars annually in the past. Several typical examples of the destructiveness of the disease may be given : In one orchard in the Santa Clara Valley the disease was so bad in 1920 that only $750 was realized from about nine acres of large Blenheim apricot trees, of which $250 was paid to pick the occa- sional scattered fruits. Apricots that year brought $100 a ton. Experienced fruit buyers estimated that at the prevailing price the orchard should have produced $12,000 worth of fruit. In 1921 at Berryessa in one nine-acre orchard of large Blenheim apricot trees only one ton of fruit was picked. Before Monilia blossom blight had invaded this orchard it regularly produced upwards of ninety tons a year. Many fine old apricot orchards have been dug up in the past five years, others have been virtually abandoned, and many hundred acres of trees have been worked over to more resistant varieties of prunes. In addition to the very great losses occasioned by the killing of the blossoms, large losses have been sustained by the rotting of the ripe fruit. Discussion of this phase of the disease is reserved for a. future paper. Possible Reasons for the Increase of the Disease in California Several possible reasons for the increase of the blossom blight phase of the disease in comparatively recent years may be advanced : It may be due to a change of climate. This is probably the least tenable of all hypotheses. But, if for any reason the coastal regions are damper or more foggy today than they were twenty years ago, the increase in the amount of the disease may be explained on this basis. A second possible reason for the increase of the disease may be found in the fact that great stretches of land which once grew hay and grain are now solidly planted with apricots, thus facilitating the spread of the disease. Possibly the most plausible reason for the increase lies in the fact that varieties more susceptible to the disease are now being grown. Bulletin 383] BROWN ROT OF APRICOTS 19 Comparatively a few years ago the Moorpark was widely cultivated. Today it is virtually a thing of the past. It is an excellent eating and drying apricot, but it bears irregularly, tends to ripen unevenly, also its flat-sidedness detracts from its appearance when canned, and it "cooks up" making a cloudy syrup. It is therefore unsuited to can- ning purposes and has little demand. The Blenheim apricot, par- ticularly adapted to coastal conditions, conforms more closely to can- ning requirements. It bears heavily and regularly, ripens evenly, also it is round and firm and does not produce a cloudy syrup when canned. The Moorpark is the most resistant and the Blenheim the most suscept- ible of all varieties. Similarly the Hemskirk, resembling the Moor- park in many ways is no longer planted to any extent. Other varieties, better adapted to the climate of the interior valleys, such as the Tilton are more resistant than the Blenheim, but under certain conditions they may suffer severely from the disease as is true of even the Hemskirk and Moorpark. Still another possible reason for the increase of the disease may be that the fungus has become better adapted to the attack of the blossoms, or a new strain of the organism may have been introduced into the state. Posey 74 and Barss 98 claim to have found a new species in Oregon. EXPERIMENTS FOR THE CONTROL OF MONILIA BLOSSOM BLIGHT INTEODUCTOEY STATEMENT Since 1921 extensive experiments for the control of the disease and observations of the results obtained by private orchards with sprays have been made. Scope of the Experiments. — The experiments involved the use of most of the better known liquid fungicides as well as oil emulsions, miscible oils, coal-tar derivative sprays, fungicidal dusts and a number of proprietary spray materials presumably of a fungicidal nature. About one hundred distinct treatments were tried, involving the use of these materials, separately or in combination, singly or in multiple applications. The value of orchard sanitation as a means of fighting the disease was investigated. 20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The experiments were made in Santa Clara, Alameda, San Benito, Monterey and Santa Cruz counties. Duplicate experiments were also conducted by the farm advisors in most of these counties to verify the results obtained in the main experiments. Orchards used in the experiments were of the Blenheim variety, old or fully mature, of uniform size and in a very seriously diseased condition. The Time the Sprays were Applied. — The sprays were applied when the trees were fully dormant in winter, in the spring when the buds were swelling, when the trees were coming into bloom, and when the petals had fallen. The Number of Trees Used in the Experiments. — Ten to thirty trees were included in each sprayed plot, except in several instances where fewer than ten trees were used. The actual number of trees used in each experiment is given in the tables. Check plots, usually consisting of ten trees each were left at suitable points in the orchards by comparison with which the relative degree control afforded by any treatment was determined. How the Sprays were Applied. — Bean spray pumps and spray guns were used to apply the different sprays at a pressure of 200-300 pounds. The tanks were carefully cleansed after each treatment. Care was taken to cover all parts of the trees with as little waste as possible. Method of Determining Results. — After the disease had run its course four men provided with hand tally-counting machines counted the infections that had developed in the sprayed trees and check trees. At least two counts were made from each tree by different men, and if the figures varied too greatly recounting was resorted to indefinitely until a satisfactory count was obtained. In general the counts ran very close together even in trees having several hundred or more infections. A difference of twenty in the count obtained in any tree having upward of two hundred infections was considered too great and a recount made. The counts taken on trees having little disease were usually identical or very nearly so. Only freshly killed spurs, one or more inches in length, were counted unless unmistakably specimens of Monilia blossom blight. The average number of infections per tree was obtained by dividing the total number of infections counted in the plot by the number of trees in the j>lot. This method determined the degree of relative control afforded by the treatment. The degree of practical control ;iikI the ultimate success of any treatment was determined by observing Bulletin 383] BROWN ROT OF APRICOTS 21 how closely the average number of infections per tree approximated a certain figure considered arbitrarily as the point of failure. Also the general effect of the treatment on the health of the trees. For instance, the degree of control, relatively speaking might be very great, but from the practical standpoint very poor. Too many infections may develop in a tree in spite of a spray to permit of even a fair-sized crop, yet when compared with unsprayed trees the control has been great. Generally speaking, any mature apricot tree of average size having one hundred killed fruit spurs is seriously diseased from the practical standpoint, and this figure was used as a basis to determine the efficacy of the spray treatments. Such a tree presents a sorry sight, and an orchard which will average one hundred infections per tree not only fails to produce a satisfactory yield, but its value is greatly depreciated. Similarly, no treatment was considered successful which injured the trees, regardless of whether the control was good or bad. Fungi- cides containing sulfur were regarded as failures, therefore, as this element proved harmful to apricot trees. Observations Made in Private Orchards. — Counts were made of the infections in approximately two hundred private orchards in Napa, Santa Clara, San Benito and Monterey counties in the past four years. In no instance were counts made unless the owner could give accurate data of the exact strength of spray materials used and when they were applied. A number of these orchards were sprayed under the direct supervision of the University, which in some instances also furnished the materials used. The use of brackets throughout the tables of this paper indicates that the blocks examined adjoined each other and were of the same size, age and variety. The Explanation of Terms. — The following explanation of terms is given to facilitate reading the experimental data. Bordeaux Mixture. — The well-known fungicide made of copper sulfate (Bluestone) and unslaked lime. Figures are generally placed after the name of the spray to indicate the strength of the mixture; the first figure indicates the number of pounds of bluestone used, the second the number of pounds of lime and the last the number of gal- lons of water. Bordeaux 4-4-50 is known as "standard strength," Bordeaux 5-5-50, "winter strength" and Bordeaux 8-8-50 "double standard strength." Fungicidal Dusts. — Powders or dusts having fungicidal prop- erties, blown directly upon the trees with a bellows or power blower. 22 UNIVPJRSITY OF CALIFORNIA EXPERIMENT STATION These materials are usually composed largely of some inactive, harm- less substance known as a "filler" or "carrier" which carries the fungicide with which it is mixed to the trees. Fillers commonly used are air-slaked lime, kaolin, talc and sublimed sulfur. Lime-Sulfur Solution 1-10 (Winter Strength). — Ordinary com- mercial lime-sulfur solution, 32°-34° Baume, used at the rate of one gallon of the solution to nine gallons of water. Self-Boiled Lime-Sulfur 8-8-50 (Standard Strength). — In this mixture the 8 pounds of superfine sulfur is partially cooked by the heat generated when the 8 ponds of unslaked lime is allowed to slake in part of the water. At the end of 5 minutes of boiling, the mixture is quickly cooled by the addition of the rest of the water. Orchard Sanitation. — Here used, the term refers to removing and destroying all twigs, fruit spurs and mummied fruits which have been killed by the fungus and which might serve as a source of reinfection. Dormant Spray. — One applied when the trees are wholly inactive or dormant in winter. Delayed Dormant Spray. — One applied very early in the spring when the buds are swelling but before the red bud appears in the case of the apricot. Red Bud Spray. — One applied after the full red bud has appeared but before the flowers are open. Pre-Bloom Spray. — Any spray applied before the flowers open. The dormant, delayed dormant and red bud sprays are pre-bloom sprays. Bloom Sprays. — Sprays applied as the trees are coming into bloom. A ' ' half -bloom spray ' ' is one applied when half the flowers are open. A ' ' full bloom spray ' ' is one applied when the trees are in full bloom. Sprayed in the Jacket. — Sprayed after the bloom has passed, but before the old flower parts have fallen away. Multiple Spraying. — The use of more than one application of spray to control fungus and insect pests. Check Trees ; Check Plot. — Untreated trees or plots of trees by comparison with which the degree of control afforded by any treat- ment applied under identical conditions may be determined. Bulletin 383] BROWN ROT OF APRICOTS 23 DOEMANT SPEAYS Experimental. — Dormant sprays were tested in 1921 in the John Fahey orchard at Sunnyvale which consists of Blenheim apricots, 25 years of age, of very uniform size and pruned in the usual way. The sprays used were Bordeaux 5-5-50 and Lime-Sulfur solution 1-10. TABLE 1 Eesults Obtained with Dormant Sprats in the John Fahey Orchard in 1921 Material used Number of trees in plot Maximum number of infections per tree Average number of infections per tree Per cent of disease com- pared with check plot Bordeaux 5—5—50 10 10 10 205 414 245 102 296 184 34.4 Check plot Lime-sulfur solution 1-10 62.1 The table shows that while only 34.4 per cent as many infections developed in the plot sprayed with Bordeaux as in the unsprayed plot adjoining, the actual number of infections exceeded the number regarded arbitrarily as indicative of failure from the practical stand- point. The control with lime-sulfur was even poorer. Both treatments were regarded as failures from the practical standpoint. A similar experiment was made in 1922 in the 22-year-old Blenheim apricot orchard at Berryessa belonging to Mrs. G. Vennum with similar results. Results Obtained in Private Orchards with Dormant Sprays. — In private orchards fungicidal sprays are rarely used during the dormant period, but a few orchards were visited which had been sprayed with fungicides at winter strength during the dormant period. In these the average number of infections per tree was invariably high, and the degree of control a failure from the practical standpoint. DELAYED DOEMANT AND EED BUD SPEAYS Successful Results Obtained in Private Orchards. — In the work of Howard and Home 97 the only control obtained was with delayed dormant or red bud sprays, either of which proved equally efficacious. Observations made in private orchards during the past four years largely confirm their work. See Table 2. The following tables show a degree of control that was highly satis- factory from either the relative or the practical standpoint. 24 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION Unsuccessful Results Obtained in Private Orchards* — While a spray applied just before the blossoms open is the best, such a spray is not always successful. In badly infected orchards situated in regions normally damp during the blossoming period, either the delayed dormant or the red bud spray may fail to give the practical control sought for. See Table 3. TABLE 2 Results Obtained with Delayed Dormant or Red Bud Sprays in Private Orchards Orchardist Locality and year Treatment Number of sprayed trees. Age of trees, variety. Average number of infections. Number of unsprayed trees. Age of trees, variety. Average number of infections. I. Higgins , Mt. View Bordeaux 5-6-50. 46 trees examined. 4 trees examined. 1922 Delayed Dormant. 15-year Blenheims. 9 infections, average. 15-year Blenheims. 94 infections, average. Higgins Mt. View, 1922 Bordeaux 5-6-50. Delayed Dormant. 51 trees examined. 15-year Blenheims. 7 infections, average. Malovina Unsprayed. 52 trees examined. 15-year Blenheims. 178 infections, average. Churin Mt. View, 1922 Bordeaux 5-6-50. Red Bud spray. 34 trees examined. 30-year Blenheims. 11 infections, average. Popovitch Unsprayed. 38 trees examined. 30-year Blenheims. 184 infections, average. G. H. Bacon Hollister, Bordeaux 5-5-50. 12 trees examined. 12 trees examined. 1923 Red Bud cracking. 20-year Blenheims. 7 infections, average. 20-year Blenheims. 104 infections, average. Corotto Hollister, 1923 Bordeaux 8-8-50. Red Bud spray. 12 trees examined. 10-year Blenheims. 15 infections, average. Picetti Unsprayed. 12 trees examined. 10-year Blenheims. 127 infections, average. Although the treatments caused a large dimunition of infection, it was insufficient to have any practical value. Results Obtained Experimentally with Red Bud Sprays. — Red bud sprays were tested in the John Fahey orchard in 1921. Mr. Fahey had just purchased the orchard which had received no sanitary prun- * The badly infected condition of these orchards at the time of spraying was due to the fact that the vital importance of cleaning up the trees was then little appreciated. Most of the orchards had been pruned in the customary manner, which is often insufficient for the ultimate control of the disease. Bulletin 383] BROWN ROT OF APRICOTS 25 TABLE 3 Unsuccessful Results Obtained with Sprays Applied just Before the Opening of the Blossoms in Private Orchards Orchardist Locality and year Treatment Number of sprayed trees. Age of trees. Average number of infections. Number of unsprayed trees. Age of trees. Average number of infections. Holmes Berry essa, 1921 Lime-sulfur solution 1-10. 43 trees examined. 25 years old. 35 trees examined. 25 years old. Red Bud spray. 246 infections, average. 311 infections, average. Bache Campbell, 1921 Bordeaux 4-5-50. Red Bud spray. 42 trees examined. 30 years old. 4 trees examined. 30 years old. 200 infections, average. 536 infections, average. Vennum Berry essa, 1921 Bordeaux 4-5-50. Red Bud spray. 36 trees examined. 22 years old. 35 trees examined. 22 years old. 168 infections, average. 180 infections, average. Flickinger Berry essa, Bordeaux 4-4-50. 49 trees examined. 49 trees examined. 1921 Red Bud spray. 20 years old. 121infections, average. 20 years old. 135 infections, average. Jensen* Hollister, 1922 Bordeaux 5-5-50. Red Bud cracking. 6 trees examined. 25 years old. 223 infections, average. "Buggo" Crude Oil 6 trees examined. Emulsion 30-200. 25 years old. Red Bud cracking. 373 infections, average. Check plot. 6 trees examined. 25 years old. 473 infections, average. Dry Lime-sulfur 6 trees examined. 10-40. 25 years old. Red Bud cracking. 401 infections, average. Bordeaux 5-5-50. 6 trees examined. Red Bud cracking 25 years old. (duplicate). 281 infections, average. King Hollister, Bordeaux 6-6-50. 9 trees examined. 1923 Red Bud spray. 11 years old. 348 infections. Lewis Unsprayed. 9 trees examined. 10 years old. 1,723 infections, average. Renzf Hollister, Bordeaux 7-8-50. 10 trees examined. 1923 Red Bud spray. 17 years old. 123 infections. Hamilton Unsprayed. 10 trees examined. 30 years old. 1,001 infections, average. * Figures compiled from demonstration plots of Mr. W. J. Tocher, Farm Advisor, San Benito County. t Renz's trees though younger than Hamiltons were equally large. 26 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION ing, and it contained therefore much infective material. Damp weather prevailed at times during the blossoming period. In those parts of the orchard where the disease had been the worst, Bordeaux gave the best results. See Table 4. These results were of no practical value. In all cases the infections were greatly in excess of the number regarded as indicative of failure. This failure was in part due to the condition of the trees. TABLE 4 Results Obtained with Single Eed Bud Sprays in the Orchard in 1921 : John Fahey Spray materials used Number of trees examined Average number of infections Bordeaux 4-5-50 plus Dormant Soluble Oil (a miscible oil) 5-200 27 11 21 283 Bordeaux 4—5—50 264 Bordeaux 4—5—50 (duplicate) 229 The Most Favorable Period for Securing Good Results with One Spray. — The best results with single sprays are obtained at the delayed dormant or the red bud stage, and the nearer the buds are to breaking and to showing the folded white petals beneath the red sepals, the better the control. In 1921 Mr. Chappel near Sunnyvale sprayed his orchard in the red bud stage with Bordeaux 30-30-200. In a block of 42 trees on a strip of gravelly soil where most of the buds showed the white of the petals, the average number of infections was 18 to the tree. In a near-by block of 122 trees on a heavier soil, where the buds were much less advanced, the average number of infections was 103. The develop- ment of the buds is earlier in light, gravelly soils than on heavy, clay soils. Later Mr. Chappel sprayed a neighboring orchard belonging to Mr. McDaniel. The soil here was also heavy, but the buds somewhat more advanced. In a block of 84 trees the average number of infections was 43. The general orchard conditions in the three cases were the same. The degree of control therefore was in proportion to the nearness of the buds to the point of breaking. Similar instances have been observed repeatedly. Bulletin 383] BROWN ROT OF APRICOTS 27 Fig. 8. — The best control of Monilia blossom blight of apricots obtainable with any one spray is secured at the time when the buds are cracking, showing the folded white petals beneath the red sepals, as shown here in the center twig. 28 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION BLOOM SPEAYS Experimental. — Extensive experiments were made at Hayward in 1923 and at Hollister in 1924 with sprays applied at various times during the bloom. In both years extremely dry weather prevailed in both districts and no conclusions could be drawn, as the disease did not appear. Results Obtained with Full Bloom Sprays in Private Orchards. — Observations made in private orchards during the past four years show that sprays applied during the full bloom have little practical value, although at times the decrease of infections may be great, just as in the case of pre-bloom sprays. TABLE 5 Typical Eesults Obtained in Private Orchards with Full Bloom Sprays Orchardist Locality and year Treatment Number of sprayed trees. Age and variety. Average number of infections. Number of unsprayed trees. Age and variety. Average number of infections. Butts* Campbell, 1921 Bordeaux 30-30-200. 33 trees examined. 35-year Hemskirks. 2 trees examined. Small Hemskirks. 129 infections, average. 487 infections, average. Lester * Campbell, 1921 Bordeaux 30-30-200. 72 trees examined. 25-year Blenheims. 4 trees examined. 8-10-year Blenheims. 178 infections, average. 363 infections, average A DiFiori San Jose, 1921 Bordeaux 25-25-200. 42 trees examined. Old Blenheims. 114 infections, average. Huff Mt. View, 1922 Bordeaux 4-4-50. 20 trees examined. 15-year Blenheims. 18 trees examined. 15-year Blenheims. 257 infections, average. 351 infections, average. Hitchings Aromas, 1924 Bordeaux 30-32-200. 10 trees examined. 30-year Blenheims. 131 infections, average. * The check trees in the Butts and Lester orchards were distinctly smaller than the sprayed trees. There can be no doubt, therefore, that the spray helped a little considering the excessively high counts in the smaller trees. In every orchard listed above the trees showed an excessive number of infections, and the treatments must be regarded as of no practical value. Spraying Before the Full Bloom; Results Obtained in Private Orchards. — Observations show that sprays applied when the trees are half or two-thirds in bloom are ordinarily useless. In fact, the longer Bulletin 383] BROWN ROT OF APRICOTS 29 Fig. 9. — Apricots sprayed in full bloom, too late for control of brown rot. 30 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION spraying is delayed after the flowers start to open, the greater the failure is apt to be, since fogs, dew or rain wetting the open blossoms facilitate the attack of the fungus. TABLE 6 Results Obtained in Private Orchards with Sprays Applied when One-Half or Two-Thirds of the Blossoms were Open Orchardist Locality and year Treatment Number of sprayed trees. Age and variety. Average number of infections. Number of unsprayed trees. Age and variety. Average number of infections. Cinciamilla San Jose, 1921 Bordeaux 30-30-200. 2 / z bloom. 29 trees examined. 32-year Blenheims. 282 infections, average. Lowry Centerville, 1922 Bordeaux 5-5-50. ]/2 bloom. 25 trees examined. 25-year Blenheims. 10 trees examined. 25-year Blenheims. 318 infections, average. 324 infections, average. Love Aromas, 1922 Bordeaux 5-5-50. 14, bloom. 45 trees examined. 8-year Blenheims. 14 trees examined. 8-year Blenheims. 128 infections, average. 267 infections, average. Butts Campbell, Bordeaux 5-5-50. 9 trees examined. 2 trees examined.* 1922 J^ bloom. 35-year Hemskirks. 158 infections, average. 35-year Hemskirks. 1,377 infections, average. Kelly Hollister, Bordeaux 8-8-50. 12 trees examined. 12 trees examined. 1923 }/2 bloom. 12-year Blenheims. 96 infections, average. 12-year Blenheims. 281 infections, average. Nicholson Aromas, 1924 Bordeaux 7-7-50. H bloom. 10 trees examined. 38-year Early Golden. 5 trees examined. 38-year Early Golden. 419 infections, average. 555 infections, average. * The two check trees in this orchard were not pruned. The excessively great number of infections was made possible, no doubt, by the presence of so much infectious material left behind in the trees. Regardless of the stage of the bloom, the number of infections was very great, and no practical control was obtained. MULTIPLE SPEAYING All sprays diminish infection regardless of the time of their appli- cation, the best results being obtained just before the flowers open. The use of repeated or multiple sprays throughout the susceptible period is discussed here. Multiple spraying for disease and insect control is not new to California. The control of thrips, red spider, codlin moth, pear scab and peach blight frequently require more than one spray. Apple growers frequently spray as many as eight times for the control of codlin moth alone. Bulletin 383] BROWN ROT OF APRICOTS 31 Multiple Spraying in the Pre-Bloom Period. — Table 2 shows that under certain conditions one spray applied just before the blossoms open may control the disease. It is of interest to determine whether several sprays applied in the pre-bloom period can be relied upon regularly to control the disease. It seems improbable because the . :^( if* "It * A a mgmmmm^ ~'~ r7 ~' '*-' •A •!■ . / 0'. <.%t> r-. n (1 ft) ^ ^ \ / 4 * % d Fig. 10. — Photomicrograph of asci and ascopores of brown rot fungus (copied from Matheny). several sprayings are confined to a period when the disease is unable or little likely to make its attack, and no spray reaches the delicate petals or reproductive parts of the flowers later to be exposed. Even assuming sprays applied in the pre-bloom to have killed all the spores in the trees, there is nothing to prevent fresh spores being blown in from outside sources to attack the flowers as they open. 32 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION In 1921 extensive experiments with multiple sprays applied in the pre-bloom were made in the John Fahey orchard at Sunnyvale. TABLE 7 Eesults Obtained with Multiple Sprays Applied in the Pre-Bloom Period in the John Fahey Orchard at Sunnyvale Treatment Number of trees in plot Average number of infections per tree Bordeaux 4—5—50. Delayed Dormant I 7 1 8 1 8 r io l 5 f 5 1 5 r 5 Bordeaux 4—5—50 . Red Bud 207 Lime-sulfur solution 1—10. Dormant Bordeaux 4—5—50. Delayed Dormant 101 Bordeaux 4-5-50. Red Bud Self-boiled Lime-sulfur 8—8—50. Delayed Dormant Self-boiled Lime-sulfur 8-8-50. Red Bud 190 Lime-sulphur solution 1-10. Dormant Self-boiled Lime-sulfur 8—8—50. Delayed Dormant 187 Self-boiled Lime-sulfur 8-8-50. Red Bud Bordeaux 5—5—50. Dormant "Ortho" crude oil emulsion 15-200 plus lime-sulfur solution 1-10. Red Bud 117 Lime-sulfur solution 1-10. Dormant "Ortho" crude oil emulsion 15-200 plus lime-sulfur solution 1-10. Red Bud 358 Bordeaux 5—5—50. Dormant Bordeaux 4-5-50 plus "Dormant Soluble Oil" 5-200 (a miscible oil) Red Bud 165 Lime-sulfur solution 1—10. Dormant Bordeaux 4-5-50 plus "Dormant Soluble Oil" 5-200 (a miscible oil) Red Bud 217 The results of these experiments prove that in unsanitary orchards in localities normally damp during the blossoming period two or more sprays applied in the pre-bloom period may fail to give the practical control sought. Multiple Spraying Throughout the Susceptible Period of the Blossoms. — Multiple spraying throughout the susceptible period was tested in 1922 in the G. Vennum orchard at Berryessa, in the H. Owen orchard at Hayward in 1923, and in the H. Smith orchard at Hollister in 1924. No definite conclusions were drawn because the disease did not appear owing to an almost unprecedented drought during the susceptible period in the last two regions in 1923 and 1924, and because the Vennum orchard failed to bloom properly in 1922. Duplicate experiments conducted by farm advisors in other localities normally