Cherry Leaf Spot

F. H. Lewis.

Cherry leaf spot, caused by the parasitic fungus Coccomyces hiemalis, is one of the major factors that determine the cost of producing cherries and the yield and quality of the fruit.

The disease occurs on the sour cherry, Prunus cerasus, sweet cherry, P. avium, and the mahaleb cherry, P. mahaleb, wherever they are grown under conditions that favor the survival of the fungus. That includes our eastern and central producing areas and the more humid areas in the West. Because it has been most serious on sour cherry in the Eastern and Central States, this discussion largely concerns the experimental work on sour cherry in those regions.

The losses are due primarily to the injury the disease does to the leaves, which become yellow and drop. Failure to control leaf spot on sour cherry, with consequent defoliation of the trees before harvest, usually results in a crop of low-quality and unattractive fruit of light-red color. The fruit often is low in soluble solids, including sugars, has a flat, watery taste, and may be unsalable. While such fruit may mean the loss of the crop for a season, that loss is sometimes less important than other losses brought about by the loss of the leaves.

Studies by W. C. Dutton and H. M. Wells, of the Michigan Agricultural Experiment Station, after the early defoliation of unsprayed trees in 1922, showed that trees that had been prematurely 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 development, the defoliation lowered the yield for several years.

Following the worst outbreak of cherry leaf spot on record in the Cumberland-Shenandoah Valley in 1945, thousands of sour cherry trees died and many others had severe injuries.

In Virginia on trees defoliated in May and June of 1945, the average weight of the buds in late summer was go milligrams. The buds on trees that had retained their foliage averaged 147 milligrams in weight. The smaller buds did not have enough vitality to survive the winter. All unsprayed trees died. None of the trees died in one orchard where sprays had delayed defoliation 4 weeks or more.

Heavy early defoliation in West Virginia in 1945 stimulated the production of secondary growth on 64 percent of the terminals about 2 weeks after harvest. The secondary leaves were soon lost to leaf spot, and some tertiary growth developed. Following this poor control of leaf spot, an estimated 72 percent of the branches were killed the following winter. Those trees bore almost no fruit in 1946.

Early defoliation in 1945 in Pennsylvania was followed by the death of more than 25,000 trees, besides general killing of shoots, spurs, and branches, and a light crop of poor fruit in 1946. Delay of the first leaf spot spray application until 10 to 12 days after petal fall in one orchard of about 100 acres resulted in general leaf spot infection and death of all of the trees in the orchard worth, at that time, close to $100,000. In no case in the area did an orchard defoliated in June of 1945 escape without severe injury or death during the following winter. Where defoliation was delayed but virtually complete in July within 3 weeks after harvest, severe injury occurred, but most of the trees survived. If leaf spot was controlled until late September the trees were not injured.

In the block of young trees used for experimental spraying in Pennsylvania in 1945, about one-third of the leaves remained on unsprayed trees 1 week before harvest. The increase in trunk size of the trees during that summer was less than half as great as on trees where leaf spot was controlled. No trees died during the following winter. Killing of shoots and spurs was general on the unsprayed trees, and the bloom in 1946 was very light in comparison with adjacent sprayed trees. The 1946 crop of cherries on the trees unsprayed in 1945 remained of poor color until just before harvest. Then they darkened rapidly and unevenly and shriveled and dried during an abnormally short harvest season. The yield in 1946 averaged 36.2 pounds to the tree; 56 percent of the cherries on the trees unsprayed in 1945 graded No. 1. Trees on which leaf spot was controlled best in 1945 had an average of 107 pounds each, 79 percent of which graded No. 1.

Those examples illustrate the fact that the losses from premature defoliation in one year by leaf spot on sour cherries may reduce quantity and quality of fruit for 2 years or more or may weaken a tree so that it cannot survive the following winter. Such severe attacks are not general: The disease usually is kept under fair control.

The losses from premature defoliation by cherry leaf spot on nursery stock of the sour cherry, sweet cherry, and Prunus mahaleb are usually caused by failure of many of the buds to grow on the weakened rootstocks and failure of the trees to grow to salable size in a year. Failure to control leaf spot on the rapidly growing seedlings of sweet cherry has been a reason why some eastern nurserymen have been reluctant to propagate sour cherry on rootstocks of sweet cherry.

Little information is available regarding losses from premature defoliation by leaf spot on sweet cherries in the orchard. Orchard trees of sweet cherry commonly are less injured by leaf spot than is the sour cherry. The effects of the disease appear to be like those on sour cherry.

The part of the losses from cherry leaf spot attributable to the cost of the control program varies greatly among the different producing areas. The cost evidently is least in some sections of California and greatest in the sections of the East that have the longest growing season. Some growers spray one or two times; others do so eight or nine times each season, besides cultivating the orchard in the spring. The total cost of the control program often exceeds 75 dollars an acre each year on sour cherries in the Cumberland Valley of south central Pennsylvania. Probably a fair estimate for the Great Lakes districts is 35 to 50 dollars. Those programs also control other diseases and insects. About one-third to two-thirds of the cost could be eliminated if leaf spot were absent.

LEAF SPOT normally appears on sour cherry on the upper surface of the leaf as a small interveinal spot of dying tissue of variable color. The spot rapidly enlarges, becomes brown to purple, and dies from the center outward. The spots are irregular or round and may occur over the entire surface. The individual spots never become large, but they may merge and so kill large areas of the leaf. The appearance of many spots on the leaf usually precedes rapid yellowing and dropping. The spots may separate from the healthy tissue, drop out, and make a shot-hole condition.

The appearance of the spot on the upper surface usually is accompanied or preceded by a pink mass of fungus spores on the lower surface. The mass may be more or less columnar, following its extrusion through a small hole in the leaf surface or it may be a somewhat hemispherical mass, following weathering and drying. It may be absent or difficult to locate after a long period of dry weather or if the fungus in the lesion is killed by a fungicide.

Leaf spot infection on the fruit stems (pedicels) and fruit are unusual and often hard to identify. Such lesions are usually small and brown, without the spore masses of the fungus on them.

Upper left: Brown rot on sour cherry. Upper right: Leaf spot, a destructive cherry disease. Lower left: Brown rot on peach. This is the most common disease of peaches in the more humid sections of the United States. It destroys many bushels yearly. Lower right: Pear black spot, a disease common on unsprayed pear leaves and fruit.