Cherry leaf spot, caused by the fungus Blumeriella jaapii, attacks the leaves, leaf stems, fruit, and fruit stems of tart, sweet, and English Morello cherries. The disease is most severe on leaves and may cause them to drop prematurely. When defoliation occurs before harvest, the fruit fails to mature normally, remaining light colored and low in sugar. Buds and wood become susceptible to winter injury, which may show the next season as poor growth, dead spurs, and dead limbs.

Symptoms

The disease first emerges on upper sides of leaves as tiny, red to purple, circular spots. These enlarge in diameter and become red brown to brown. By this time, spots are brown on the undersides of leaves, and during wet periods, tiny, whitish, felt-like patches appear in the center of these spots. These contain the spores (conidia) of the causal fungus. On sweet cherry leaves the spots tend to be somewhat larger. Some may drop out, leaving a shot-holed appearance. After the leaves become infected, they turn yellow and fall.

Disease Cycle

The fungus overwinters in diseased leaves on the ground. Around bloom or shortly afterward, spores (ascospores) mature and are discharged. They are blown to young, expanding leaves where infection takes place through the stomates (air pores) on the undersides. These first infections are often so few in number that they may be overlooked. Once unfolded, leaves are susceptible throughout the season, but as the leaves age, they become less susceptible. Each succeeding wave of infection becomes heavier, and severe defoliation begins. High humidity and rainfall increases the spread of the disease.

Disease Management

Rotary mowing the orchard after leaves drop in fall will hasten leaf decay and reduce the numbers in which the fungus can overwinter. Otherwise, fungicide applications are the primary means of control. Begin fungicide applications when the first leaves have unfolded.