Answer: 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 soluble solids. 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.
To control, you'll need to spray your cherry trees next spring. Start fungicide applications at petal fall, or after the first leaves have unfolded, and repeat applications as directed on the fungicide label. For now, be sure to rake and remove fallen leaves and plan on spraying your trees with a fungicide next spring to keep them from developing a fungal infection.
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