Cherry Leaf Spot

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.

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