The disease is most severe on leaves and can 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 might show the next season as poor growth, dead spurs, and dead limbs.
The disease first emerges on the upper sides of leaves as tiny, red to purple, circular spots. These enlarge in diameter and become red brown to brown. By then, spots show brown on the undersides of leaves; during wet periods, tiny whitish, felt-like patches appear in their centers. These contain the spores (conidia) of the fungus. On sweet cherry leaves, the spots tend to be somewhat larger. Some drop out, leaving a "shot-holed" appearance. After the leaves become infected, they turn yellow and fall.
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, expanded leaves where infection takes place through the stomates (air pores) on the undersides. These first infections often are so few in number that they might 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 increase the spread of the disease.
Rotary mowing the orchard after leaves drop in the fall will hasten leaf decay and reduce overwintering inoculum. Otherwise, fungicide applications are the primary means of control. Begin fungicide applications when the first leaves have unfolded.