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Source: A Jones.
Fortunately, this does not often occur before fall, except in nurseries.
Mature leaf spots are easily recognized by their grayish white centers with sharply defined margins.
Appearing first on upper leaf surfaces as small, brown lesions, they enlarge to 1∕8 to ¼ inch in diameter. The borders become dark brown, and small black pimples appear in the centers.
Sexual spores are produced on overwintered, fallen leaves and are carried by air currents to newly formed leaves. About a month after infection, new spores are generated in the centers of the grayish white leaf spots, from which they are washed by rain to other leaves. These secondary infections usually peak in late summer or early fall.
Routine fungicide sprays normally control this disease in Pennsylvania.
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