Powdery mildew forms a white to gray, fungal coating on the above-ground parts of the plant. Lower leaves are usually the most severely infected because of the high humidity in the lower canopy. As disease lesions age small black fruiting bodies (cleistothecia) develop with in white infected areas. Powdery mildew is favored by wet and humid days with moderate temperatures of 600 F or above. Powdery mildew is disseminated by airborne spores.