Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. UNITED STATES DEPARTMENT OF AGRICULTURE BULLETIN No. 712 Contribution from the Bureau of Plant Industry WM. A, TAYLOR, Chief Washington, D. C. PROFESSIONAL PAPER October 29, 1918 APPLE POWDERY MILDEW AND ITS CONTROL IN THE ARID REGIONS OF THE PACIFIC NORTHWEST. By D. F. FISHER, Assistant Pathologist, Fruit-Disease Investigations. CONTENTS. Page. Page Keonomic importance of powdery mildew.... 1 | Orchard spraying experiments—Contd. Review of the literature. ..:2.....5..022222222- 2 Spraying experiments in 1916.............. 15 Mhelcausaloreanism=ee sees ey eee 4 Spraying experiments in 1917.............. 17 Dissemination of the fungus...................- Ou inj unys tonnuitandtolarer sence eee ee 20 Description oftheidisease.: 5. 2-20 -4)2. 3s5- 22 GHieSpray; materials saan eee nee eee 21 Susceptibility of varieties...:...............-.- 7 | Summary of controli measures: .-:..-<....-/.-- 23 Importance of the disease in the Pacific North- IDoBENBI MAS OA Se sebodosenossoooaseooseconeseoc 23 WES SSS bho SRSA AE Ee mn Ae Ria Mace aatns G || -esebhawhave Gb-qoeiaboaVs otis Roe ae See okoncasode 24 SPLAy MUMy en see ee ae be 9 | General notes on the control of the disease...-.- 25 Orchard spraying experiments..............-. HOE} EXC ONCLUSIONS Ata a Sk eee Rah ae Shenae Renate on arate 26 Spraying experiments in 1915.............. UP) "| Ure MPRABIRG CHRO S Sees Conde doe oleeake san sescade 28 ECONOMIC IMPORTANCE OF POWDERY MILDEW. Apple powdery mildew is usually considered as of only minor im- portance and principally affecting nursery stock in the eastern part of the United States, only becoming serious in restricted localities. Occasionally it is severe in Utah, Colorado, New Mexico, and the Kast on old trees, but in the apple-growing districts of the Pacific coast the conditions are such that it often becomes serious. The arid climate of the hot interior valleys of the Pacific Northwest has proved an effective safeguard against fungous diseases in general, but apple powdery mildew is endemic. In these regions, where deciduous-fruit growing, and especially apple growing, has become such an important industry, orcharding is generally carried on under very intensive conditions and the in- 1 Acknowiedgment is due Dr. Charles Brooks, of the Office of Fruit-Disease Investiga- tions, Bureau of Plant Industry, for many helpful suggestions during the ceurse of the work, and to Messrs. H. J. Newcomer, of the Bureau of Entomology, and M. M. Brown and L. C. Carey, formerly assistants in spraying experiments, who actively assisted in the field work. ; 2 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. dustry is highly specialized. In the Wenatchee Valley, Washington, © where the experimental work herein reported was carried on, the arable land is almost entirely taken up with apple orchards; and, having a high valuation, it is cut up into small holdings, the average orchard area being somewhat less than 10 acres for each owner. Great attention 1s given to every detail of the orchard operations; pests and diseases are vigorously combated, and every effort is made to produce a maximum amount of the perfect high-colored fruit which has made this region notable. Imperfections and markings affecting only the appearance of the apples cause a loss of grade and consequent financial loss to the grower. If, as in the case of apple powdery mildew, there is added to this effect a lessened pro- duction, every effort to remove the cause is justified. The seriousness of apple powdery mildew is greatly modified by the character of the season in any given year. The spread of in- fection is greatly facilitated by a rainy sprig. During seasons when conditions for spore germination are most favorable the — disease assumes epidemic proportions and is the cause of serious loss. Tt not only attacks the foliage and tender wood growth, but also destroys fruit buds and directly attacks the fruit. In the Wenatchee Valley the russeting of the fruit is considered one of the most serious effects of the disease. REVIEW OF THE LITERATURE. The first mention of the disease in this country was made by Bessey (1)* in 1877. He reported a serious outbreak of powdery mildew (Podosphaera kunzet) on seedling apples and cherries in the college nursery in 1871. The fungus was first described as Sphaerotheca leucotricha by Ellis and Everhart (2) in 1888. Galloway (8) carried on the first extensive ox presiiiieneal work and published the first recommendations for control. In 1889 he re- ported that the disease occurred abundantly through all the region east of the Mississippi. He found that it was confined to attacks on young trees in the nursery, especially seedlings, the leaves becom- ing dry and brittle and of so little use that the trees were rendered worthless for budding. He stated that the disease is spread by wind dissemination of spores, aided by insects, rain, and other agents. He pointed out that the ascospores are of no practical importance and that the fungus winters over in mycelial form. In a later publi- cation (4) he reported the successful use of ammoniacal copper- carbonate spray in controlling the disease. 1The serial numbers in parentheses refer to ‘‘ Literature cited’’ at the end of this bulletin. APPLE POWDERY MILDEW. 3 Burrill (5) mm 1892 changed the name of the fungus to Sphaero- theca malt (Duby), identifying it with HLrysiphae mali of that au- thor, but this fungus was later identified as Pryllactinia corylea (Pers.) Karst. Grout (7) accepted Burrill’s identification. He found that the perithecia matured late in the season and were usually found on the shoots, seldom on the leaves, which he suggested might account for the rarity of their collection and the confusion in nomenclature. In 1894 Pammel (6) called attention to the disease in Iowa, stating that 1t was of common occurrence. He recommended the use of Bordeaux mixture instead of ammoniacal copper carbonate as a spray, In a later publication (8) he discussed the confusion in nomen- clature and suggested that it is doubtful whether perithecia of Pod- osphaera oxyacanthae were ever actually seen on the apple in this country. The early confusion of the identity of the fungus was cleared up by Salmon (9) in 1900. He definitely Sed the fungus to Podosphaera leucotricha. The first report of the disease from the State of Washington was ~ by Lawrence (10) in 1905. He described the different species of powdery mildews occurring in the State but did not recognize Posdosphaera leucotricha. In 1910 Stewart (11, p. 318-321) reported that he had found no other species than Podosphaera leucotricha on apples in New York. He found the disease chiefly on water sprouts and nursery stock, but also found it on the terminal growth of trees 36 years old. He noted that the Chenango and Black Ben Davis varieties are particularly susceptible. In 1914 Ballard and Volck (12) published a very complete descrip- tion of the occurrence and control of the disease in the Pajaro Valley of California, where the climatic conditions are particularly favor- able to the development of the disease in serious epidemic form. They found that fully 90 per cent of the foliage of susceptible varie- ties, such as Yellow Newtown and Yellow Bellflower, was frequently diseased. They found that Bordeaux mixture gave very poor mildew control. Copper acetate and copper oxychlorid were effective against the mildew, but caused injury to the fruit and foliage. Dilute lime- sulphur solution and solutions of sulphids in general were lkewise injurious. About 125 different materials were tested for foliage spraying, and as a result it was found that sulphur in some finely divided form was the most efficient. Specific directions were given for the preparation of an “iron-sulphid” spray whereby the soluble sulphids so injurious to foliage in that climate were eliminated. At- tention was called to the importance of pruning out diseased twigs as a supplemental aid in control. The important relation between 4 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. tree vigor and susceptibility to mildew was shown and experiments were reported which demonstrated the possibility of foliage stimula- tion through spraying with various compounds. _THE CAUSAL ORGANISM. The cause of apple powdery mildew is the fungus Podosphaera leucotricha (KE. and E.) Salm. Another species of Podosphaera, P. oxyacanthae (DeC.) De- Bary, has been reported as the cause of this mildew in some eastern localities, but the writer has never found it on the apple in the Pacific Northwest. The two species can not be distinguished ex- cept in their perfect stage, when marked dif- ferences are observed. , : In the case of Podo- Fic. 1.—Perithecium and ascus of Podosphaera Sphaera leucotricha there nana ant B) Same forme! 6 are two. types. of append ages on the perithecia. The basal appendages are short, twining, and rootlike, while the apical appendages are long, slender, stiff, and rarely divided at the tips. Perithecia of P. oxyacanthae have appendages placed more or less equatorially and all dichotomously branched at the tips. They are usually all of one type, but rootlike basal appendages are also sometimes found. The distinction between the two species is shown in figures 1 and 2. It is probable that much of the confusion which has arisen in regard to the iden- tity of the species involved is reat due to the fact that in most lo- Fig. 2—Perithecium and ascus of Podosphaera ealities the perfect stage is pro- oxycanthae (DeC) DeBary. (The former is 5 magnified 80 times and the latter 420 times.) duced but sparingly. How- ever, in the Wenatchee Valley this stage is abundant, and repeated examinations of perithecia have never revealed other than Podo- sphaera leucotricha on the apple. P. oxyacanthae has, however, fre- quently been found by the writer on the native bitter cherry (Prunus emarginata (Dougl.) Walpers) and on the western chokecherry (Prunus demissa (Nutt.) Walpers) growing in close proximity to apple trees. APPLE POWDERY MILDEW. 5 DISSEMINATION OF THE FUNGUS. In the Wenatchee Valley the spread of the fungus during the growing season is largely dependent on the prevalence of dews. Light rains occasionally occur, but moisture for most of the spore germination is found in the dews, which are of almost nightly oc- currence. While the humidity of the air in the Wenatchee Valley is usually very low, it is much higher in the orchards which are being irrigated. The daily range of temperature varies between 25 and 30 degrees F. during most of the early part of the season, and often it is even greater; consequently, dews are frequent and occur in the orchards when other sections are free from them. Spore dissemination is doubtless accomplished by the agency of the wind. During the early part of the season strong winds are of common occurrence and often blow steadily for days. The branches of the trees are whipped about, and the conidia are scattered around and lodged on tender young leaves and shoots. With the coming of nightfall and dews, favorable conditions for spore germination arise and infection proceeds. Beetles were often found feeding on the mildew, and they may be responsible for some of the spread of the fungus; but they were never found in sufficient numbers to indicate that they were important agents in its dissemination.* The perfect stage of Podosphaera leucotricha begins to appear as early as the middle of June, when black speckled patches of peri- _thecia form in the felted mycelium on twigs, leaf petioles, and mid- ribs (Pl. I, fig. 1), and occasionally on the fruit. The perithecia have been observed from year to year in an at- tempt to determine definitely whether they play any appreciable — part in the dissemination of the fungus. The dates of the earliest collection of perithecia in the Wenatchee Valley during the course of this investigation are as follows: In 1915, on June 22, three months after the appearance of the first leaves. In 1916, on June 18, two months after the appearance of the first leaves. In 1917, on June 29, two months after the appearance of the first Ieaves. Twigs bearing perithecia were brought into the laboratory at fre- quent intervals in the spring for examination to determine the date at which the spores were shed. None of the perithecia were found ruptured until fully a month after conidia had become abundant on the current season’s growth. Frequently no ruptured perithecia could be found until after new perithecia had developed on the cur- rent season’s growth. Early in the spring it was difficult to break 1Jdentification of the insects discovered feeding upon the mildew has been made by Mr. BE. J. Newcomer, of the Bureau of Entomology, as follows: The most abundant, a ladybird (Psyllobora borealis Casey) ; the next common, a small, pointed, black and brown beetle (Pentaria nubilia Le Conte) ; and the rarest, Anthicus nitudulus Le Conte. 6 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. the perithecia under a cover glass for microscopic examination, but after soaking in water for 24 hours they were much more easily broken, indicating that rainfall would facilitate natural ascospore dispersal.. However, rainfall adequate for this purpose is always lacking in the Wenatchee Valley at this season. Repeated attempts were made to inoculate tender young foliage with ascospores obtained from perithecia of the previous season’s growth, A suspension of spores was sprayed on Jonathan leaves with an atomizer. Some of the leaves were inclosed in glassine bags in order to increase the humidity, while others were left exposed. The work was done between the middle of May and the first of June, when the disease was actively spreading in the orchards. The results were always negative. It would appear from these studies that the ascospores play little, if any, part in spreading the disease and that they are unnecessary to the overwintering of the fungus. This con- clusion is in agreement with the ee of the investigations of Ballard and Noalele (12) in the Pajaro Valley of California and oe earlier work of Galloway (38). DESCRIPTION OF THE DISEASE. In the Wenatchee Valley the writer has found the disease in its conidial and perithecial stages on twigs, foliage, and fruit of the apple, and occasionally also on pear twigs and folhage. It sometimes attacks the blossoms, and in this case frequently the entire blossom cluster, with its attendant leaves, is involved. Affected flowers are dwarfed and hypertrophied and set no fruit. The petals are de- formed, are of a greenish or yellow color, and soon become covered with a powdery coating of spores. (PI. I, fig. 8.) The first appearance of the disease is usually manifested in small rayish or white feltlike patches of mycelium on the under sides of the leaves, which become crinkled and curled. (PI. II, fig.1.) Often the presence of the fungus can be detected before the appearance of the felted mycelium by the mottled color of the infected leaves, ac- companied: by a corrugation of their surface. The patches covered by the fungus rapidly enlarge, and generally the entire leaf becomes cov- ered with the felted mycelium and a powdery coating of spores. When very young leaves are attacked they have a tendency to increase in length but not in breadth and gradually become somewhat folded longitudinally. (Pl. I, fig. 1 and Pl. II, fig. 1.) Infected leaves be- come greasy and eventually so brittle and parched that they fall from the tree. The mildew spreads rapidly down the petiole of the leaf to the twig, which becomes covered with mycelium and spores. Twig growth is soon checked, and in severe infections the twig is killed outright. (Pl. II, fig. 2.) In the Wenatchee Valley, trees covered with terminals killed by powdery mildew are frequently seen after a season of severe infection. a ee ee a ee ree IMENT atc oe Sree oh y APPLE POWDERY MILDEW. a _A serious phase of the disease is found in the case of twigs stunted but not killed. New leaves are infected as rapidly as they appear. Buds formed on such twigs are infected by the overwintering myce- lum, and in the spring the fungus resumes activity when the leaves unfold, thus performing a function vital to the spread of the disease in carrying the fungus over from season to season. On such twigs the internodes are much shortened and the lateral buds are crowded to- gether. (PI. I, figs. 8 and 4.) These buds are characterized by a purplish red color and an elongated shape, and they are always de- layed in opening from a week to 10 days behind normal buds. (PI. II, figs. 4 and 5, and Pl. IIL.) In cases of severe infection the formation of new fruit buds is pre- vented and the following season is one of a light crop or no crop at all. Vegetative growth, however, is favored by the absence of a fruit crop; hence, during such a season the trees are given an opportunity to recuperate their vigor and resist the mildew for the time being. But the following year an increased crop is produced, and mildew, again being favored by reduced vegetative vigor, resumes virulent | activity and so tends to produce a periodicity of crops that is very undesirable. The effects of fruit infection are to dwarf the apple and produce a russeting of the skin beneath the mycelium. (PI. I, fig. 2.) Fruit infection usually occurs early in the season. Active infections on apples after the skin has become hardened have not been observed, and it is therefore probable that danger of such injury is past before the apple is half grown. Infected apples often show elongated stems, while the basins are generally russeted. SUSCEPTIBILITY OF VARIETIES. The varieties of the apple found to be most susceptible to the dis- ease in the Wenatchee Valley are Pryor Red, Jonathan, Newtown, Black Ben Davis, Grimes, Esopus (Spitzenburg), Fameuse (Snow), and Stayman. No variety appears to be immune, but among the least susceptible are Winesap and White Pearmain. Of these varie- ties the Pryor Red and Fameuse are not commercially important in the Wenatchee Valley. | IMPORTANCE OF THE DISEASE IN THE PACIFIC NORTHWEST. Apple powdery mildew occurs in all of the apple-growing regions of the Pacific Northwest, but its economic importance in any of them varies from year to year. Investigations by the writer have shown that the disease is of little economic importance in the Willa- mette Valley of Oregon and the sections of Washington west of the Cascade Mountains. In these sections infection seems to be confined 8 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. % to watersprouts and terminals and has not been ebserved to be as severe as that often met with in the irrigated regions. Where apple scab is prevalent, as in these more humid sectionsof the Northwest, the ordinary sprayings with lime-sulphur solution for scab control are effective against mildew, and its control in these sections, there- fore, presents no unusual difficulty. In such districts as the Spokane Valley, Wash., the Hood River valley, Oreg., and other sections where natural rainfall can be relied upon for part of the mois- ture necessary for tree growth and where spraying for apple scab is generally practiced, the growers and horticultural inspectors are agreed that apple powdery mildew is of little or no economic im- portance. However, in the hot interior valleys, where irrigation is entirely depended upon for moisture supply and where fungicidal spraying has been regarded as unnecessary, the disease has demanded serious attention, and the development of a safe and effective spray- ing schedule has been attended with peculiar difficulties. Reports of the State horticultural inspectors indicate that serious loss is often experienced. Mr. C. W. Gilbreath, State horticultural inspector in the Walla Walla (Wash.) district, is authority for the following statement regarding the disease: The damage from apple powdery mildew in this district grew heavier each succeeding year and reached its maximum in 1915. As a resuit many orchards showed a reduction in crop of 25 to 50 per cent. Jonathan and Newtown are the most susceptible varieties. That the disease is equally serious in the Yakima Valley, Wash., where the largest acreage of apple orchards in the Northwest is found, is indicated by the following statement of Mr. H. E. Water- bury, State horiticultural inspector for the district: Mildew is prevalent all over the valley on certain varieties, such as Jona- than, Spitzenburg, Newtown, and Rome. The loss occasioned is chiefly loss in vitality, which can scarcely be measured. I do not know of any orchards that do not have some mildew. There has been considerable loss from the so-called mildew scratches (russeting) on the fruit, which has reduced the grade. In some places the Black Ben Davis was so badly marked that fully 25 per cent had to be culled. Observations by the writer in the Wenatchee Valley indicate that orchards often have shown a reduction of fully 75 per cent of the crop as a result of mildew infection and that frequently nearly all of the new wood growth is attacked, while foliage infection may reach 75 to 90 per cent. Its damage was most severe in many parts of the valley in 1914 and 1915, but each year sees appreciable losses in some orchards due to mildew infection. In 1915 many crops of Jonathan and Black Ben Davis were complete failures, due to the severity of the 1914 infection, while many other varieties were seriously damaged through fruit russeting, foliage infection, and twig blighting. Alarm over impending damage became so general Bul. 712, U. S. Dept. of Agriculture. PLATE |. fe APPLE POWDERY MILDEW-—I. Fic. 1.—A late stage of foliage infection on Pryor Red watersprouts. Note that many of the leaves have been shed and that those remaining are covered with conidia and are somewhat folded longitudinally. The twigs are covered with mycelium, embedded in which may be seen the dark patches of perithecia, usually located at the base of a leaf and extending up on the petiole. Fic. 2.—Mildew russeting of a Black Ben Davisapple, August, 1916. Fia. 3.— Mildewed blossom cluster froma Pryor Red tree. Note the abundance of conidia over leaves and blossoms and the deformity of the blossoms. Bul. 712, U. S. Dept. of Agriculture. APPLE POWDERY MILDEW-—II Fic. 1.—Portion of the terminal growth of a Black Ben Davis tree, showing an early stage of foliage infection. Fie. 2.—Branch of a Jonathan tree, Showing a terminal spur killed by an infection of the previous year and three spurs blighted during the current season. This branch had been sprayed with lime-sulphur solution and the mildewed leaves were severely burned, while the healthy foliage was unharmed. Fic.3.—Branch of a Pryor Red tree, showing two years’ growth, with lateral buds on the portion infected during the first year pushed out into spur growth. Note the shortened internodes. Fic. 4.—Terminal growth on a Pryor Red tree, Showing shortened internodes on theinfected portion. Thenoninfected buds are swelling, while those infected are still dormant. Fiq@. 5.—Portion of a branch of a Pryor Red tree, show- ing noninfected cluster buds opening and considerable expansion of healthy foliage, while in- fected leaf buds near the tip of the branch arestill nearly dormant. Fic. 6.—An apple, showing injury resulting from the application of sulphur-dust sprays in the Yakima Valley in 1917. Fic. 7.—Pryor Red apple, showing sulphur injury, the type of injury which may be expected to follow applications of sulphur sprays after the advent of hot weather in the arid valleys of the Pacific Northwest. i Bul. 712, U. S. Dept. of Agriculture. PLATE Ill. APPLE POWDERY MILDEW-—III. Branch from a Jonathan tree, showing a terminal twig killed by the infection of the previous year. Note that the petals are beginning to fall from the healthy blossoms on the lower por- tion, while the blossoms from the infected buds near the terminal twig are not yet open, APPLE POWDERY MILDEW. 9 in 1915 that for the first time in the Wenatchee Valley systematic spraying for a fungous disease was undertaken. In general, the methods followed were based upon the recommen- dations of Ballard and Volck (12) for the Pajaro Valley of Cali- fornia; but since the climatic conditions under which they found soluble sulphids injurious to foliage were not prevalent in the Wenatchee Valley, the somewhat laborious process of removal of these compounds from the iron-sulphid spray was not attempted and a modified formula was used. (See formula 7, p: 11.) A con- siderable quantity of commercial spray preparations was also used. ..ccording to 4 report of Mr. O. T. Clawson, then State horticul- tural inspector for the Wenatchee district, the following quantities of spray materials were sold for mildew spraying in 1915: Commercial” sulpmuts pastes 228 255 oa ee 17, 850 pounds. Lime-sulphur solution ee See ee 11, 600 gallons. Iron sulphate (for making iron-sulphide spray)____ 31, 000 pounds. SPRAY INJURY. Tt has not been difficult to control the mildew effectively with any one of several of the sulphur spray materials commonly employed for spraying, but their use during the intensely hot summer weather in the arid. districts of the Northwest has invariably been attended with severe fruit burning and some foliage injury. The importance of climatic factors on spray practices in mildew control has been pointed out by Ballard and Volck (12). However, the difficulty experienced in the Pajaro Valley of California, i. e., defoliation and the dropping of fruit caused by sulphur injury, is seldom met with in the irrigated regions of the Northwest, appar- ently because of the absence of the foggy weather which is so prev- alent in the Pajaro Valley. Sulphur burning on the fruit is found to be the chief reaction induced by climatic conditions in the irri- gated sections of the Northwest. Fruit dropping following sulphur injury is only occasionally met with, and foliage injury usually is of minor importance when it occurs at all. However, sulphur spotting of the apples has often resulted in more loss than the mildew would have caused. Growers have therefore largely abandoned attempts to spray for the disease until a safe spraying program could be developed. Sulphur spotting is always confined to apples on the south and southwest sides of the trees and involves only those apples exposed to the direct rays of the sun during the hottest part of the day. Apples shaded by the foliage are never affected. In the Wenatchee Valley no injury has resulted from sulphur sprays applied before the middle of May. Sprays applied after this date have generally resulted in the fruit burning as soon as hot weather starts, and sprays applied early in June have invariably 6265018“ Bull, 712-9 ; 10° BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. resulted in injury. In some cases intense burning sunlight has not occurred until fully two weeks after the spraying was completed, but still severe burning of the fruit resulted. The adoption of adequate spraying methods is largely dependent on the safety with which they may be used. The data secured on injury in connection with the use of various sprays during the course of the experiments reported below will therefore be of value in pointing out the weather conditions under which burning occurs, the type of injury resulting, and the time at which the sprays may be safely applied. These data are presented in connection with the ex- periments herein reported. ORCHARD SPRAYING EXPERIMENTS. The orchards used were adjacent to each other and for the purposes of this work can be considered as one. The trees used were a solid block of the Pryor Red and Jonathan varieties, the experimental plats each consisting of adjacent rows of seven trees each, while the Black Ben Davis plats comprising a hike number of trees were located in double rows extending across the orchard. The trees were 14 years old at the beginning of the experiments in 1915. The Pryor Red trees have a very open habit of growth, and al- though they had been cut back severely to bring them into more workable shape they still had a spread of about 30 feet and were about 380 feet high. When in full foliage they required about 25 gallons of spray material per tree. The Black Ben Davis trees were not spread out as much, but were more compact and dense. They required about 20 gallons of spray material each. The Jonathan trees were smaller than those of the Black Ben Davis and more open, requiring about 10 to 15 gallons of spray material per tree. All had been infected with mildew for several years, the dis- ease being especially severe in 1914, when the crop of the Pryor Red trees was reduced to an average of 2 bushels each, although the trees were otherwise capable of settene at least 30 ner The Black Ben Davis and Jonathan trees had produced a somewhat heavier crop than those of the Pryor Red, but in every case the foliage and twig growth had been seriously infected and many terminal fend had been killed. The experiments were undertaken to determine— (1) The most effective fungicides for use against apple powdery mildew. (2) The most desirable dates for spraying. (3) The possible toxic effects of the various spray materials tested. A high-power spraying outfit of 200-gallons capacity was em- ployed, a pressure of 200 to 250 pounds was maintained, and eddy- 1These experiments were carried out at Wenatchee, Wash., in the orchards of the V. & W. Land & Improvement Co. and of Mr. A. P. Kornbau, to whom acknowledgment is due for many courtesies extended during the course of the investigations and for active cooperation at all times, H ¥ t ; 4 APPLE POWDERY MILDEW. 11 chamber nozzles of the “ driving-mist ” type were always used. One line of hose was operated from the top of the sprayer, where special attention was directed toward covering the terminal branches in the tops of the trees and the outside parts of the trees. Another line of hose was operated from the ground and directed against the interior parts of the trees and upward against the under side of the leaves. In this manner an endeavor was made to cover completely all of the vegetative and fruit surface on each tree and obtain as complete pro- tection as the different spray materials could afford. The details of the experiments are presented in Tables I to IV. The materials tested are listed belcw, with a statement as to the manner of their preparation or dilution. In connection with each is given the formula number referred to in the spraying schedules. List of the spray materials tested, showing the composition of the formulas - : used. Formula 1—Commercial lime-sulphur, 34° Baumé, diluted 14 to 50. Formula 2.—Commercial lime-sulphur, 34° Baumé, diluted 1 to 50. Formula 3.—Commercial lime-sulphur, 34° Baumé, diluted 1 to 75. Formula 4.—Commercial lime-sulphur, 34° Baumé, diluted 1 to 100. Formula 5.—Self-boiled lime-sulphur, 8-8—50. Add 8 pounds of sifted sulphur to 8 pounds of stone lime, slaking in a barrel. Stir well, and add sufficient water from time to time to keep it from burning. After ebullition ceases, dilute to 50 gallons and strain into the spray tank, when it is ready for use. In practice it is best to make up enough for 150 to 200 gallons at once. Formula 6.—Iron sulphid (Ballard’s formula). Dissolve 10 pounds of iron sulphate in a barrel containing 50 gallons of water. Add lime-sulphur solution until no more precipitate forms. Allow the precipitate to settle and drain off the supernatant liquid. Again fili the barrel with water, stir up the precipitate, and allow to settle, and then drain off the clear liquid as before. Continue this washing process until the yellow color disappears from the clear liquid. Make up to 50 gallons for stock solution. For spraying, stir the stock solution well and dilute 10 gallons in 80 gallons of water to make 100 gallons of spray. Formula 7.—Iron sulphid (Wenatchee formula). Slowly- add 2 pounds of granulated iron sulphate to 100 gallons of water in the spray tank, keeping the liquid well agitated. The iron sulphate goes into solution very quickly. Add 3 quarts of lime-sulphur solution (at a strength of 34° Baumé) and use imme- diately. Formula 8—Colloidal sulphur.* Dissolve one-half pound of cheap glue in hot water and add to 50 gallons of water in the spray tank. With the agitator run- 1This formula was adapted from a rather meager description of a method for pre- paring colloidal sulphur contained in the Gardener’s Chronicle, Aug. 7, 1915, by J. M. Hector and 8. J. M. Auld, of University College, Reading, England. They employed this material against American gooseberry mildew at Heston, England, but a complete report of their work has not been published. In a letter to the writer, dated Mar. 10, 1916, Prof. Hector furnished the formula used by them. It differed from the one here described in that they employed gelatine instead of glue and hydrochloric acid instead of sulphuric acid, and they made up a stock solution, which was diluted for spray- ing. Prof. Hector stated that the chief objection to its general use was its expensiveness. However, by the substitution of glue and sulphuric acid, as employed in formula 8, its cost is greatly reduced. The formula as used at Wenatchee has been satisfactorily em- ployed against apple scab in the Willamette Valley, Oreg., and against prune brown-rot in Clarke County, Wash., in experiments carried on by the Office of Fruit-Disease Investi- gations. 12 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. ning, add 14 gallons of commercial lime-sulphur solution; then add commercial sulphuric acid until the yellow color is almost but not entirely replaced by white. It usually requires about 0.9 pint of sulphuric acid for 14 gallons of lime- sulphur solution. When arsenate of lead is to be used in combination with this material it should be added after the acid. Colloidal sulphur spray made up in this manner analyzes approximately as follows, depending upon the composi- tion of the lime-sulphur used: Polysulphid sulphur (calcium as sulphur), 0.09 per cent; thiosulphate sulphur (calcium as sulphur), 0.08 per cent; sulphate sulphur (calcium as sulphur), 0.08 per cent; free sulphur, 0.41 per cent; insoluble matter other than sulphur, 0.009 per cent. Formula 9.—Finely ground sulphur material of the following composition: Water, 41.44 per cent; sulphur, 55.15 per cent; organic matter (in the form of glue), 1.94 per cent; ash, 1.47 per cent. Dilute to 7 pounds to 50 gallons of water. Formula 10.—Finely ground sulphur material of the following composition: Water, 14.82 per cent; sulphur, 76.72 per cent; organic matter, 1.80 per cent; ash, 6.66 per cent. Dilute to 6 pounds to 200 gallons of water. Formula 11.—Sodium-sulphur material of the following composition: Sodium polysulphid, 56.84 per cent; sodium thiosulphate, 36.56 per cent; sodium sul- phate, 0.66 per cent; free sulphur, 3.38 per cent; iron and aluminum oxids, 0.14 per cent; water, 2.42 per cent. Dilute to three-quarters pound to 50 gallons of water. Formula 11a.— Same as formula 11, but diluted to 13 pounds to 50 gallons of water. Formula 12.—Sodium-sulphur material of the following composition: Sodium polysulphid, 55.06 per cent; sodium thiosulphate, 40.80 per cent; sodium sul- phate, 2.17 per cent; free sulphur, 1.56 per cent; water, 0.41 per cent. Dilute to 1 pound to 50 gallons of water. | Formula 12a.—Same as formula 12, but diluted to 13 pounds to 50 gallons of water. Formula 13—Barium-sulphur material of the following composition: Barium polysulphid, 67.44 per cent ; barium thiosulphate, 8.67 per cent; barium sulphate, 5.15 per cent; free sulphur, 17.28 per cent; water, 1.46 per cent. Dilute to 4 pounds to 50 gallons of water. Formula 14.—Bordeaux mixture, 4—4—50. Formula 15.—Ammoniacal copper carbonate prepared as follows: Dissolve 5 ounces of copper carbonate in 8 pints of ammonia (25 per cent solution, which should be diluted before using). Dilute to 50 gallons in spray tank. Formula L.—WLead-arsenate paste, 2 pounds to 50 gallons of water, or pow- dered lead arsenate, 1 pound to 50 gallons. Where used in combination with fungicides, this designation appears in the spraying schedule, together with the number of the fungicidal formula. Formula S.—Soap. The addition of 2 pounds of potash-fishoil soap to 50 gallons of spray for a spreader is indicated where this designation appears, together with the number of the fungicidal formula in the spray schedule. SPRAYING EXPERIMENTS IN 1915. The results of the spraying experiments conducted in 1915, together with the resulting spray injury and the control established, are shown in Table I. Aside from the foliage injury shown in Table I, very severe burn- ing of exposed fruits was noted on June 9 on the south and south- west sides of the trees. An estimate of this burning in the various plats was not attempted because of the almost entire absence of a APPLE POWDERY MILDEW. tS crop. Stayman Winesap (Stayman) trees, bearing a heavy crop and situated alongside the Black Ben Davis used in this experiment and sprayed by the orchard owner with iron sulphid (formula 7), showed on this date about 30 per cent of exposed fruit on the south and southwest sides very severely burned. Besides this, defoliation to the extent of 50 per cent in many instances had taken place, and a heavy drop of fruit had been caused. Subsequent observations indi- cated that this variety is particularly subject to injury of this nature. TABLE I.—Results of spraying experiments for the control of apple powdery mildew at Wenatchee, Wash., in 1915. Foliage injury. ¢ Spray g Jury formu- la.a Foliage in- fected. Dates of ap- plication. On Apr. 28.) On June 4. Variety and plat. On July 29.| Total. Per cent. Per cent. Percent. | Per-ct..| Per ct. 9 Black Ben Davis: No. GTi Vet 2 1.4 Apr. 24, May 1.5 crease. 10 -3 2 tne 1 5es s Apr. 14, “24, ont tt M a y 20, July 15. Apr. 14, 24, M ay 20, June 15, July 15. .| Apr. 34, May 20, June 15. INOmSi (Check ease oar aes Not sprayed. Pryor Red: M ay 20, crease ee ee ee ed Ges | Apr. 24 June 15. all IN oie ei eae 24, July May 20, June a 24, May 20, June 15. mi Apr. 24, May Less than ivape) & burning. crease. crease. ee in- } 12.6 12a 20, June 15. 12a L. July Ses 7 Apr. 24, May 20, June 15, July 15. Notisprayedsalec!2se8 eigisle abst ee eee Bn fee 14.9 10.6 splot (Checks) es ssc |teiace 20.1 a For composition of sprays used, see pp. 11-12. - © Buds in the ‘‘pink”’ (just before full bloom), Apr. 14; petals fallen, calyx not yet closed, ape 24; other ED GENO IS at the time of regular codling-moth’ sprayings. ¢ Foliage injury expressed in percentage of leaf-surface reduction. d On southwest side of trees. h Increase. e Very slight leaf spotting. % Defoliation. Ff Increase on southwest side of trees. 4 Defoliation (consisting only of infected leaves). g Leaf spotting and margin burning. k Leaf spotting. Since climatic factors are known to play such an important part in the development of spray injury, the following data will be of interest in connection with the injury noted. The records of the 14 BULLETIN 712, U. 8S. DEPARTMENT OF AGRICULTURE. Washington section of the Weather Bureau show the following con- ditions prevailing at Wenatchee during the period when spray in- jury occurred: Measurable precipitation. April, none. May, 1.77 inches, distributed over May 1, 2, 9, 10, 11, 14, 17, 23, 24, 27. June, 0.21 inch, distributed over June 10, 11, 12, 23. July, 0.28 inch, distributed over July 4, 8, 14, 26, 27. Mean maximum and minimum temperatures. = Maxi- Mini- = Maxi- Mini- Month, 1915. isn Sait. Month, 1915. in. arnt. IA Pree eee eee ee eee 70.3 ADP SUNG! oan eee eon eee 80.1 53.5 LE ie celne aan GBS te eNO SSSOEElS Se 68. 2 AG308| | eaiutliy;2 Sere eae ee 86. 4 57.9 The maximum temperature was between 80° and 90° F. on April 16, 17, 18, and 19, on May 4, 6, 7, and 8, on June 3, 6, 7, 13, 14, 15, 20, and 21, on July 5, 7, 8, 17, 18, 26, 29, and 30; between 90° and 100° on June 4, 5, 22, 23, 29, and 30, and on July 1, 4, 19, 20, 24, 25, and 31; and above 100° on July 2, 3, 21, and 23. The above temperatures are shade temperatures. The tempera- tures on fruits exposed to the direct rays of the sun were naturally much higher. Tests made to indicate what this difference might be showed that the temperature was 20 to 30 degrees higher in the sun. From this it appears that the sun temperature ranged well above 100° F. on five successive days (June 3-7) just before the appearance of the fruit burning. No other excessive temperatures were recorded between the date of the last spraying and the appear- ance of the injury. With the continuance of the hot weather the injury was observed to increase to some extent, but most of the fruit exposed to direct sunlight was injured at the start. There was no indication that the scanty precipitation had any effect on spray in- jury either to fruit or foliage. The foliage injury appeared to be the result of toxic action of the wet sprays, probably intensified by the bright sunlight, since it began to appear so shortly after the application. It was a rather difficult matter to arrive at an accurate or repre- sentative measure of the control established, especially in the case of infected leaves and twigs, which constitute the principal mani- festation of the disease. The variation in the crops of fruit was so ereat among the different plats that no data could be secured on fruit russeting; hence the results had to be measured by the condition of the vegetative parts. An empirical estimate of the degree of infec- tion is largely influenced by the personal equation, and an inspection of all the foliage and twigs was manifestly impossible in an experiment of this magnitude. Actual counts were therefore made to show the APPLE POWDERY MILDEW. 15 amount of mildew on definite branches of each tree, thus presenting a fair average of conditions over the plat. This method removed the personal equation and allowed the results to be placed on a per- centage basis for comparison. In detail the method employed was as follows: An equal number of representative branches were in- spected in the tops and around the lower parts of the trees in each plat; each leaf cluster was taken as a unit, and if any of its leaves showed active mildew infection the unit was so counted. Several thousand such units were examined in each plat about a month after the last spraying. The results, as shown in Table I, indicate the ef- fectiveness of the sulphur sprays, and particularly of lime-sulphur solution, in controlling apple powdery mildew. On trees sprayed with lime-sulphur solution the mildew infection was reduced to a neghgible quantity, while the check trees were badly diseased. The less favorable results obtained from the use of the other materials can be attributed partly to the fact that they did not possess as ef- ficient wetting power as the lime-sulphur solution, a point which will be more fully discussed in a later paragraph. However, in the case of the sodium-sulphur sprays the poor resu!ts were doubtless in- fluenced largely by the relatively low concentration of polysulphids and free sulphur, which can be regarded as the active ingredients. SPRAYING EXPERIMENTS IN 1916. The results of the spraying experiments conducted in 1916, to- gether with the resulting spray injury and the control established, are shown in Table IJ. In 1916 most of the Pryor Red trees were sprayed with the same materials as in 1915, but the Jonathan and Black Ben Davis trees, which were not included in the 1915 experi- ments, were given two applications of iron sulphid, formula 7 (see p. 11) by the owner of the orchard. In the spraying experiments in 1916 only a negligible amount of foliage injury developed early in the season. Only about 5 per cent of tip burning was found prior to June 22 in plat 13. On that date, however, some injury appeared in nearly all plats. Sulphur spotting of the fruit began to appear on June 18, and by June 20 a consider- able portion of the exposed fruit which had been sprayed with sul- phur preparations was severely burned. As in 1915, the injury was confined to the portions of the tree receiving the direct rays of the sun in the hottest part of the day. The extent of the injury was esti- mated by actual counts of several hundred apples on the affected por- tion of each tree. It appeared to be most severe where the deposits of spray material were heaviest. In considering the fruit injury it was found that high temperatures had again prevailed between the time of the completion of the last spraying (June 10) and the date of the first appearance of the sulphur spotting on the fruit (June 18). 16 BULLETIN 712, U. S. DEPARTMENT OF AGRICULTURE. Thermograph records during the period from June 8 to 18 revealed the following temperatures: Maximum shade temperatures. Maxi- i 1 Maxi- Date, 1916. ane Date, i916. aT ye SHEL JUNCIS Fad. Sete Oe a A ea OO“ uMewa seers ie Se ee eee Pee eee 97 JUNG OSS se ee eee ee pate. Baia ney, ba th (PAM Nerd fib tray yen Lis) eat AES We pare ow op yee SARA EC OS ab, 100 JUTE OSS Sok es ES oe Roes § aoe Ea ae wo Si) Taner 6ik She. a ee Eee 102 Boa YS UT eae es tea gs EW Aull fc ib Baas bed roee y nee reo Ud mW ca ea 100 JUNE IIe Pht SAE a A 90 dunes Sh etd = VME ey ee eee 82 TUT Cal Ste meer hae ee ae ai gti ee ee 95 There was no measurable precipitation during this period. The re- sults again emphasized the danger of applying sulphur sprays when hot, burning ‘sunlight is expected to prevail. TABLE II.—Results of spraying experiments for the control of apple powdery mildew at Wenatchee, Wash., in 1916. Spray injury.c Spray | pates of — ——] Foli- Variety and plat. | formu- Tee one On June 22. On Aug. 1. Total. age in- y P la.a TO URINO OG ee ae eee ew Se a asp Foliage.| Fruit. |Foliage.| Fruit. |Foliage.| Fruit. Jonathan*® Per ct.| Per ct.| Per ct.| Per ct.| Per ct.| Per ct. | Per ct. 5S IND 05 ODS Sein INOSAe hotel eee May 10, June 1 18 (@) (d) i 18 1.1 5 SL 10 6 BWDP PE sass 4 INOS eases eee 6L May 10, June 1 10 (d) (2), 1 10 1.4 5 | CuBr 2258 em INOMSE ho c2 Nose May 10, June 2 10 (e) (e) 3 10 4 2L 10, July 21. 3 Apr 22 INOS 4a eee nes a 3L Aiey 10, June 3 12 (d) (a) 3 12 3 4 Apr. 22....... sf INOW Doers eek BAe ae aay 10, June 1 13 (d) (a) 1 13 5 sulAupraconte t=! ING. (G). 2) Rae ee SL May 10, June 5 21 (d) (a) 5 21 2 INowda(Check)e 22 ose sa INOb-Spray edie lec os