The disease first emerges on upper sides of leaves as tiny, red to purple, circular spots. After the leaves become infected, they turn yellow and fall. Photo by K. Peter.
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 soluble solids. Buds and wood become susceptible to winter injury, which may show the next season as poor growth, dead spurs, and dead limbs.
The disease first emerges on upper sides of leaves as tiny, red to purple, circular spots. These enlarge to 1/8 to 1/4 inch in diameter and become red-brown to brown. By then, spots show brown on the undersides of leaves, and during wet periods tiny, whitish, feltlike patches appear in their centers. These contain the spores (conidia) of the causal fungus. On sweet cherry leaves the spots tend to be somewhat larger. Some spots may drop out, leaving a shotholed 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, sexual spores (ascospores) mature and are discharged. They are blown to young, expanded leaves where infection takes place through the stomates on the undersides. These first infections are often so few in number that they may be overlooked. However, conidia from the feltlike centers of spots on leaf undersides mature 10 to 15 days after the first infections. They are spread by rains. Each succeeding wave of infection becomes heavier, and severe defoliation begins.
The fungus overwinters in diseased leaves fallen to the ground. Rotary mowing the orchard after leaves drop in fall will hasten leaf decay and reduce the spore numbers in which the fungus can overwinter. Otherwise, fungicide applications are the primary means of control beginning at the bract leaf stage during bloom time and continued throughout the season as disease conditions persist. Two fungicide sprays postharvest are recommended to prevent premature defoliation. The goal is to have the trees hold onto their leaves through September. Premature defoliation in July and August can cause the tree to be more susceptible to winter damage, which ultimately could lead to tree death.
Specific chemical recommendations for home gardeners are in Fruit Production for the Home Gardener, and recommendations for commercial growers are in the Penn State Tree Fruit Production Guide.