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Cool, moist conditions are required for this disease to occur. The fungus only infects young emerging leaf tissue in the spring, entering the stomates and growing between the leaf cells. The new spores of the fungus formed on the surface of the spots are blown to new buds and remain dormant there until the next spring. Thus, the fungus has only one infection period in the spring and does not continue to cause new spots to form later during the growing season.
Fungicide application is not necessary because the leaves are seldom severely spotted and do not fall prematurely. Although infections may be extensive some years, little damage actually results.
To prevent spotting, fungicide must be applied prior to bud break, late in dormancy. Once bud break has occurred and symptoms are visible, it is too late to spray.
Prepared by Gary W. Moorman, Professor of Plant Pathology
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