Green Industry Resources
Hort Update- Seasonal Information for the Green Industry
Controlling Cherry Leaf Spot
Cherry leaf spot is caused by the fungus Blumeriella jaapii, formerly Coccomyces hiemalis, a common pathogen of sour cherries and less commonly sweet cherries.
The names 'yellow leaf' and 'shothole disease' have sometimes been used because the leaves turn yellow soon after symptoms develop and the center often drops out of the brown, circular lesions after they have developed. Characteristic symptoms include numerous, tiny purple spots on the upper leaf surface that enlarge and become necrotic (dead). Affected leaves often turn yellow and fall off early in the season, but frequently the area around the infected spots remains green; giving the leaf a mottled appearance. On sweet cherry leaves, the spots are often larger and nearly circular in shape. Spore masses of the fungus appear in the spots on both sweet and sour cherries. On the fruit stems, infections sometimes girdle the stem, causing a fruit drop.
This organism overwinters on fallen cherry leaves and in the spring produces large numbers of spores from last year's infected leaves. The spores are moved by air currents and rain. In the presence of spring moisture, the spores initiate new infections on young leaves. This disease is more severe on sour cherries than sweet cherries, and the resulting losses are associated with a weakening of the tree due to early summer defoliation.
Control of cherry leaf spot is greatly enhanced by raking up and destroying infected leaves in the fall or early spring before leaves emerge. Preventive fungicides are recommended at petal fall, shuck fall and two weeks later. Later in the year homeowners may consider making an after harvest fungicide application. A combination of Benomyl (50% WP) and Captan (50%WP) at rates of 1/4 Tablespoon and 2 Tablespoons respectively per gallon of water have good effectiveness and will help reduce the rate at which pathogens may develop resistance to benomyl products.