CHERRY LEAF SPOT 695 maturely defoliated produced fewer blossoms the following year, the flowers were poorly developed and slower in opening, fewer cherries ripened, and the cherries were smaller. Many fruit spurs died, and the crop was greatly reduced on the spurs that survived. By reducing shoot growth and spur Cherry development, the defoliation lowered the yield for several years. Following the worst outbreak of Leaf Spot cherry leaf spot on record in the Cum- berland-Shenandoah Valley in 1945, thousands of sour cherry trees died and F. H. Lewis many others had severe injuries. In Virginia on trees defoliated in Cherry leaf spot, caused by the para- May and June of 1945, the average sitic fungus Coccomyces hiemalis^ is one of weight of the buds in late summer was the major factors that determine the 90 milligrams.' The buds on trees that cost of producing cherries and the had retained their foliage averaged 147 yield and quality of the fruit. milligrams in weight. The smaller buds The disease occurs on the sour did not have enough vitality to survive cherry, Prunus cerasus, sweet cherry, P, the winter. All unsprayed trees died. avium, and the mahaleb cherry, P, None of the trees died in one orchard mahaleb, wherever they are grown where sprays had delayed defoliation under conditions that favor the sur- 4 weeks or more. vival of the fungus. That includes our Heavy early defoliation in West Vir- eastern and central producing areas ginia in 1945 stimulated the produc- and the more humid areas in the West. tion of secondary growth on 64 percent Because it has been most serious on of the terminals about 2 weeks after sour cherry in the Eastern and Central harvest. The secondary leaves were States, this discussion largely concerns soon lost to leaf spot, and some ter- the experimental work on sour cherry tiary growth developed. Following in those regions. this poor control of leaf spot, an esti- The losses are due primarily to the mated 72 percent of the branches were injury the disease does to the leaves, killed the following winter. Those trees which become yellow and drop. Fail- bore almost no fruit in 1946. ure to control leaf spot on sour cherry, Early defoliation in 1945 in Pennsyl- with consequent defoliation of the vania was followed by the death of trees before harvest, usually results in more than 25,000 trees, besides general a crop of low-quality and unattractive killing of shoots, spurs, and branches, fruit of light-red color. The fruit often and a light crop of poor fruit in 1946. is low in soluble solids, including sug- Delay of the first leaf spot spray ap- ars, has a flat, watery taste, and may be plication until 10 to 12 days after unsalable. While such fruit may mean petal* fall in one orchard of about 100 the loss of the crop for a season, that acres resulted in general leaf spot loss is sometimes less important than infection and death of all of the trees other losses brought about by the loss in the orchard worth, at that time, of the leaves. close to $100,000. In no case in the Studies by W. G. Dutton and H. M. area did an orchard defoliated in Wells, of the Michigan Agricultural June of 1945 escape without severe Experiment Station, after the early injury or death during the following defoliation of unsprayed trees in 1922, winter. Where defoliation was delayed showed that trees that had been pre- but virtually complete in July within 696 YEARBOOK OF AGRICULTURE 1953 3 weeks after harvest, severe injury Little information is available re- occurred, but most of the trees sur- garding losses from premature de- vived. If leaf spot was controlled until foliation by leaf spot on sweet cherries late September the trees were not in the orchard. Orchard trees of sweet injured. cherry commonly are less injured by In the block of young trees used for leaf spot than is the sour cherry. The experimental spraying in Pennsylvania effects of the disease appear to be like in 1945, about one-third of the leaves those on sour cherry. remained on unsprayed trees i week The part of the losses from cherry before harvest. The increase in trunk leaf spot attributable to the cost of the size of the trees during that summer control program varies greatly among was less than half as great as on trees the different producing areas. The where leaf spot was controlled. No cost evidently is least in some sections trees died during the following winter. of California and greatest in the sec- Killing of shoots and spurs was general tions of the East that have the longest on the unsprayed trees, and the growing season. Some growers spray bloom in 1946 was very light in com- one or two times; others do so eight parison with adjacent sprayed trees. or nine times each season, besides The 1946 crop of cherries on the trees cultivating the orchard in the spring. unsprayed in 1945 remained of poor The total cost of the control program color until just before harvest. Then often exceeds 75 dollars an acre each they darkened rapidly and unevenly year on sour cherries in the Cumber- and shriveled and dried during an land Valley of south central Pennsyl- abnormally short harvest season. The vania. Probably a fair estimate for yield.in 1946 averaged 36.Q pounds to the Great Lakes districts is 35 to 50 the tree; 56 percent of the cherries dollars. Those programs also control on the trees unsprayed in 1945 graded other diseases and insects. About one- No. I. Trees on which leaf spot was third to two-thirds of the cost could controlled best in 1945 had an average be eliminated if leaf spot were absent. of 107 pounds each, 79 percent of which graded No. i. LEAF SPOT normally appears on sour Those examples illustrate the fact cherry on the upper surface of the leaf that the losses from premature de- as a small interveinal spot of dying foliation in one year by leaf spot on tissue of variable color. The spot sour cherries may reduce quantity rapidly enlarges, becomes brown to and quality of fruit for 2 years or purple, and dies from the center more or may weaken a tree so that.it outward. The spots are irregular or cannot survive the following winter. round and may occur over the entire Such severe attacks are not general: surface. The individual spots never The disease usually is kept under fair become large, but they may merge and control. so kill large areas of the leaf. The The losses from premature defolia- appearance of many spots on the leaf tion by cherry leaf spot on nursery usually precedes rapid yellowing and stock of the sour cherry, sweet cherry, dropping. The spots may separate from and Prunus mahaleh are usually Caused the healthy tissue, drop out, and make by failure of many of the buds to grow a shot-hole condition. on the weakened rootstocks and failure The appearance of the spot on the of the trees to grow to salable size in a upper surface usually is accompanied year. Failure to control leaf spot on or preceded by a pink mass of fungus the rapidly growing seedlings of sweet spores on the lower surface. The mass cherry has been a reason w^hy some may be more or less columnar, eastern nurserymen have been re- following its extrusion through a small luctant to propagate sour cherry on hole in the leaf surface or it may be a rootstocks of sweet cherry. somewhat hemispherical mass, follow- CHERRY LEAF SPOT 697 ing weathering and drying. It may be Higgins. In no case did the Coccomyces absent or difíicuit to locate after a long isolate from any two Prunus species period of dry weather or if the fungus show exactly the same host relation- in the lesion is killed by a fungicide. ships. Further, the same isolate com- Leaf spot infection on the fruit stems monly infected different hosts with (pedicels) and fruit are unusual and different degrees of severity, varying often hard to identify. Such lesions are from slight flecking to abundant usually small and brown, without the production of typical leaf spots. He spore masses of the fungus on them. tentatively grouped the fungi as The symptoms of leaf spot on other follows according to the plant from species of cherry are somewhat like which they were obtained. Prunus those on sour cherry. On sweet cerasus^ P. avium, P. mahaleb^ and P. cherry, the spots are often larger and pennsylvanica; P. domestica; P. virginiana; more nearly circular in shape than and P. serótina. Prunus mahaleb was those on sour cherry. The spore masses susceptible to isolates of all four of the fungus, particularly on sweet groups. P. cerasus was infected only by cherry seedlings, are often present in isolates from Group i. large numbers on the upper surface of J. B. Mowry, of the Indiana Agri- the leaf. Prunus mahaleb has some cultural Experiment Station, reported tendency to show a chlorotic ring in 1951 the inoculation of 66 species, around the young lesion, and the dead varieties, and hybrids of Prunus with spots rarely drop out. Other species single-spore cultures of Coccomyces. He like the chokecherry, Prunus virginiana, added Prunus fruticosa to Keitt's Group are more apt to show shot hole than I. He obtained infections on sour cher- the sour cherry. ry and Prunus mahaleb with the isolate from P. serótina^ and on both sweet and THE FUNGUS that causes leaf spot on sour cherry with the isolate from P. sour cherry and sweet cherry in the pennsylvanica. Seedlings of P. cerasus y United States conforms generally with P. insititia, P. mahaleb, and P. ienella the description of Coccomyces hiemalis. were susceptible to most isolates Probably the fungus is the only tested. Seedlings of P. besseyi, P. japón- common one on Prunus mahaleb. ica, P. pumila, P. pérsica, P. salicina, B. B. Higgins, of the Cornell P. serótina, P. spinosa, and P. virginiana University Agricultural Experiment were susceptible to relatively few iso- Station in New York, in 1913 and 1914 lates. Seedlings of P. glandulosa and divided the various isolates or collec- P. maritima were resistant to all seven tions of Coccomyces that he studied isolates. into three species based on both mor- While future work may be needed to phologic and host-range differences: clear up some aspects of this situation Coccomyces hiemalis on sweet cherry it seems clear that the Coccomyces fungi {Prunus avium), sour cherry {P. cerasiis), which cause leaf spot on the cultivated and pin cherry {P. pennsylvanica) ; cherries, Prunus cerasus, P. avium, and Coccomyces prunophorae on the plums P. mahaleb, form a group which con- Prunus americana^ P. domestica^ and P. forms in general with Higgins' de- insititia; and Coccomyces lutescens on the scription ojf Coccomyces hiemalis. It is wild black cherry {Prunus serótina)^ evident from the work I have de- the chokecherry (P. virginiana)^ and scribed and that of R. O. Magie in Prunus mahaleb. Wisconsin in 1935 that this fungus is G. W. Keitt in Wisconsin in 1918 able to cause leaf spot on several other published the results of more than Prunus species, including the plums, 1,000 cross-inoculation tests and added under more or less ideal conditions. others in 1937. He used isolates of No evidence has been found that C. Coccomyces from all three of the groups lutescens is of any significant importance of cherries and plums set up by on cherries. There is no known evi- 698 YEARBOOK OF AGRICULTURE 1953 dence that the common wild cherries, conidia, or summer spores, are borne P. pennsylvanica and P. serótina^ are of in large numbers and are spread from any importance as a source of the leaf leaf to leaf by water. The rapid spread spot fungus in sour cherry orchards. of leaf spot in the summer and fall is Coccomyces hiemalis belongs to the usually due to the rapid increase and order Phacidiales of a group of fungi spread of the fungiis by means of re- referred to as Ascomycetes because peated generations of conidia through- they bear the spores of the perfect or out the summer and fall. sexual stage in a club-shaped organ Besides the regular, or normal, asco- called an ascus. Dr. Higgins first de- spores and conidia, the fungus pro- scribed it in 1913. He found the per- duces conidia in the overwintering fect stage of the fungus on the leaves of fruit body after the ascospores are sweet cherry, P. avium, and showed discharged in the spring. The conidia that it was the fungus that previously cause leaf spot if placed on a suscep- had been called Cylindrosporium, tible leaf under favorable conditions, Coccomyces hiemalis passes the winter but may be of little significance in the in the old leaves on the ground as a normal reproduction of the fungxis. partly formed, round or somewhat The fungus also produces small spores, elongated, dark-colored fruit body, called microconidia, on the leaves in which normally extends from the fall. Their function in the reproduction lower to the upper epidermis of the of the fungus is unknown. leaf but remains covered above and below by the epidermis. AN APPROACH to the problem of con- The fruit body, or stroma, begins to trol of a disease of this type is based on swell toward the lower leaf surface the knowledge that we are dealing during the first warm days in the with two plants, the cherry tree and spring. Club-shaped asci then form the fungus Coccomyces in this case, both within the stroma. There follows the of which have their normal manner of formation of eight two-celled asco- development and sensitivity to various spores within each ascus. As the asci influences. The modern orchard sets enlarge rapidly within the stroma, the up a nearly ideal situation for the covering of the fruit body is lifted until reproduction of both plants. Our pur- it ruptures. The ascos pores within the pose is to interfere in some way with asci mature shortly afterwards—nor- the reproduction of the fungus without mally when the sour cherry is in the seriously injuring the cherry tree. pink, or early-bloom, stage of growth. A healthy sour cherry blooms while The ascospores are discharged through the leaves are still small. The leaves the end of the ascus in wet weather and are folded along the midrib while are carried upward by wdnd. If they small and begin to unfold during the lodge on a susceptible leaf under favor- latter part of the blooming period, able conditions, the ascospores germi- usually while the flower petals are nate and leaf spot results in i to 2 weeks. falling. The growth of the spur leaves Penetration of the leaf by the germ is rapidly completed after petal fall, tube from the ascospore occurs through but growth of new leaves on the ter- the stomata of the leaf. minals continues until midsummer. After the fungus invades the leaf, a The leaf spot fungus rarely infects disk-shaped mass of fungus mycelium the very young leaves, apparently be- is formed beneath the epidermis of the cause the stomata through which the leaf. Secondary spores, the conidia, are fungus enters the leaf are not mature borne on the surface of this mass. until about the time the leaf unfolds. When they have accumulated in suffi- Once unfolded, however, the leaves cient numbers, the epidermxis of the are susceptible to infection throughout leaf is ruptured and the conidia appear the summer and fall. in a pink to whitish-pink mass. The As the fungus overwinters in the old CHERRY LEAF SPOT 699 leaves on the ground, any factor that not feasible, but it has not been gener- reduces the prevalence of the fungus ally accepted in commercial practice. one year will reduce the likelihood of The objections to it are usually the serious trouble with the disease the extra cash outlay required, the shortage following spring. Thus few infections of labor, and inability to reduce safely one year make it less likely that leaf the number of summer sprays in dis- spot will be a problem the following tricts where neighboring growers allow year. Too much faith in this fact, how- the fungus to overwinter undisturbed. ever, has led to trouble. The fungus often spreads rapidly in the fall and THE DIFFERENCE in sensitivity of the may overwinter at a high population tree and the leaf spot fungus to the level although it was not a problem action of chemicals often permits a earlier in the growing season. Too, if high degree of control with sprays the ascospores are abundant enough to applied to the tree. Such sprays are establish the fungus on the tree in the now the principal means of control in spring, it may spread rapidly in wet commercial orchards and nurseries. season. The time when the first fungicidal In dooryard or garden trees outside spray is needed depends on the pres- of orchard districts, the fungus can be ence of leaves large enough to be sus- destroyed by raking and burning the ceptible, the presence of mature asco- old leaves on the ground in fall and spores of the fungus, the presence of winter. That work is not practical in moisture for a sufficient length of time commercial orchards because of the to permit infection, and a tempera- labor involved, but part of the same ture at which the fungus will grow. purpose can be served by diski ag or All those conditions normally are plowing the old leaves under before met about the time the flower petals the ascospores mature in the spring or fall, and most spray schedules call for by spraying the old leaves with one or the first application then. That has more chemicals that destroy the fungus been satisfactory when the population in them. of the fungus is at a reasonable level Clean cultivation of the orchard and the ascospore-induced infections would be desirable if the sole aim were are few. At times, however, the petals the destruction of the leaf spot fungus, are slow in falling and leaf growth is but that is often not practical because rapid during late bloom. Such a situa- of the cost, the shortage of labor, and tion, coupled with a high population the desirability of maintaining some of the fungus and wet weather near sort of trashy soil cover in the orchard. petal fall, may permit many infections. A considerable reduction in the popu- The application of a spray just as the lation of the fungus can be obtained first flowers opened on sour cherry in merely by disking both w^ys along the Pennsylvania in 1947 reduced the per- tree rows in the orchard. centage of infected leaves 3 weeks later Good results have been obtained in to 1.9, compared to 24.5 with sprays reducing the production of ascospores started at petal fall and 92.7 on trees in the old leaves on the ground by not sprayed at all. means of fungicidal ground sprays. Contrary situations do occur. Few The most common spray material used or no infections may occur until a for the purpose has been the sodium month or more after petal fall, perhaps salt of dinitro-o-cresol, sold under the because of a scarcity of the fungus. trade names of Elgetol and Krenite Following the fungicidal spray at and used at one-half gallon of the paste petal fall, additional sprays are applied to 100 gallons of spray and applied at as needed. On sour cherry that nor- the rate of about 500 gallons the acre. mally means a second spray about 10 The mixture seems to have special days after the first, two sprays in June, merit in orchards where cultivation is and the last just after the fruit is picked 700 YEARBOOK OF AGRICULTURE 1953 in July. More sprays timed at closer photosynthetic activity, the size of the intervals and applied before the long fruit, the solids and acid content of the rain periods during the season are juice, the color of the fruit, the yield, often needed on rapidly growing nur- canning quality, and so on. The aim sery trees, on orchard trees growing in in any control program is to balance areas with a long growing season, and the various factors as precisely as pos- in places where an attempt is to be sible in order to control the disease made to control a high population of with the least injury to the tree and the fungus without the use of a copper fruit. fungicide. A combination of all these Sour cherries of acceptable size factors has led to the general use of commonly run about 100 to 125 fruits eight or nine sprays each season in a pound. Variations in size of 10 or south central Pennsylvania. 15 cherries a pound have been general It is necessary generally to spray sour in Pennsylvania with different fungi- cherry trees every year regard less of the cides. The solids content of the juice is size of the fruit crop. To newly planted normally about 14 percent, with i .5 to trees the first spray is applied when the 4.0 percent variation common between first leaves unfold with additional fungicides. The acid content of the sprays at intervals as needed to keep fruit has varied from 0.8 to i .5 percent the new leaves protected. with different fungicides. The weight Workers in the different cherry dis- of pits has varied from 6.5 to 8.3 per- tricts disagree as to the chemicals to cent of the total fruit weigjht. The color be used for control of leaf spot: The of the fruit has varied from a very severity of leaf spot varies widely light to a very dark red. among areas, the length of the season Those characteristics are related in over which protection must be pro- that any fungicide that reduces size of vided is different, and the severity of the fruit usually increases the percent- the injury to the tree by the fungicide age of solids and acid in the fruit and varies between districts. The effects of total weight of pits in a ton of fruit. the fungicide on the tree, however, The color of the fruit does not seem to usually are different in degree only. be closely related to the other effects. Some general statements therefore All the variations are important be- apply in most cases. cause one or more of them affects in The early work on fungicides for turn the yield and grade of raw fruit, cherry leaf spot was primarily con- the amount of waste and yield of cans cerned with bordeaux mixture. Many of fruit the ton of raw fruit at the can- tests of lime-sulfur solution, elemental ning factory, and the attractiveness of sulfur preparations, copper com- the product to the consumer. pounds, and organic fungicides fol- The effects of the fungicide on fruit lowed. Most commercial growers now quality and yield are of major impor- use sulfur on sweet cherries and one tance in commercial cherry growing of the proprietary copper compounds and canning where differences of 10 or on sour cherries, although a consider- 15 percent may mean the difference able amount of bordeaux mixture is between profit and loss. It should be still used and the organic fungicides kept in mind, however, that failure to are gaining in usage each year. control leaf spot is usually much more The choice of a fungicide for use on serious than the injurious effects of the sour cherries is partly determined by fungicide applied for its control. the effect of the fungicide on the tree Bordeaux mixture,, at a concentra- and fruit. Fungicidal sprays cause tion of 2 pounds of copper sulfate and various types of leaf and fruit injury 6 pounds of hydrated spray lime to 100 such as leaf scorching, leaf spotting, gallons of water, has been one of the leaf yellowing and dropping, and fruit most efiective fungicides. Concentra- scald. They also affect leaf size and tions varying from about i.5-3-100 to CHERRY LEAF SPOT 701 6-8-100 are now used. It has caused the Great Lakes districts where injury severe leaf injury when used during by them is at a minimum, and in other wet or abnormally dry weather or on areas where small orchards or garden foliage on which aphid honeydew was trees do not justify extra labor and present. It has dwarfed the fruit more expense with a more complicated spray than any other treatment. The fruit schedule in an effort to obtain has been dark red in color with a high maximum crops of perfect fruit. They content of solids and acid. have not been satisfactory in south The low cost and high degree of central Pennsylvania because of exces- effectiveness of bordeaux mixture has sive leaf and fruit injury. With the made it one of the best m.aterials for relatively large number of sprays control of cherry leaf spot on sour required there for leaf spot control, cherries in the nursery, on nonbearing crop reductions of 10 to 20 percent by orchard trees where no more than four a copper fungicide have been frequent or five sprays are required, and in the in hot, dry harvest seasons. sprays before bloom, at petal fall, and None of the copper materials may be after harvest on bearing trees. It has used on sweet cherries without danger given satisfactory results when used all of injury. through the season on bearing trees in northeastern Wisconsin and compara- FERBAM preparations sold under ble conditions. In other areas the use of such trade names as Fermate, Ferra- bordeaux mixture during the period dow, and Karbam Black, have been of rapid fruit growth has caused used both alone and with elemental excessive dwarfing of fruit. sulfur, usually one of the sulfur pastes. The proprietary copper compounds, The usual concentration has been 1.5 of which Copoloid, Copper Hydro, pounds to 100 gallons of water of a Copper A, Cupro-K, and Bordow are product containing about 75 percent examples, have been used at rates of 8 active ingredient. Two pounds has to 12 ounces of actual copper plus 3 been the minimum effective concen- pounds of hydrated spray lime in 100 tration in Pennsylvania wdth the gallons of water. Properly used, any sprays started before bloom and one of them has given fair to good continued at 7- to 14-day intervals control. As a group, they have been until harvest. One and one-half pounds less effective than bordeaux mixture, has been adequate when used with but they have caused less leaf injury, one of the elemental sulfurs. and the number of leaves remaining on Ferbam has not usually caused any the tree has often been as high with one visible injury to the tree or reduction in of them as with bordeaux mixture. fruit size. The large fruits have been They have sometimes been associated comparatively low in solids content, with an injury on the fruit that has largely or entirely because of their occurred as a black line around the size, and have been unsatisfactory to stem of the fruit and is very objection- some canncrs because of this. able in canned cherries. The copper compounds have been F, H. LEWIS is a professor of plant the most frequently used materials for pathology at Pennsylvania State College leaf spot control on sour cherries since and pathologist at the Pennsylvania State about 1940. They have represented a College Fruit Research Laboratory, Arendts^ compromise between the older fungi- ville, Pa. He is a graduate of Clemson cides, bordeaux mixture and lime- Agricultural and Mechanical College and sulfur solution, in that they have given obtained his doctor's degree in plant pathol-- less fruit dwarfing than bordeaux ogy from Cornell University in ig43. He mixture and better leaf spot control has worked on cherry leaf spot and the than lime-sulfur. They still have effects of fungicides on cherry fruit quality considerable merit for use in some of in New York and Pennsylvania since ig40. 201500=—na- -4()