Infection by this fungus can cause small spots on the foliage of red oaks. The spots are only on the upper surface of the leaf and are blackish-brown. The centers of spots are lighter in color and are generally 1 mm in diameter. Symptoms are first visible in early June and increase in number until the middle of August. By that time the foliage is severely damaged and the tree is a pale yellow.
Defoliation occurs when infection is severe.
Sanitation is important in the prevention and control of this problem. Remove all diseased leaves. Spraying is seldom required. Trees which have been severely damaged by the disease should be given additional fertilizer to stimulate new growth. Powdery mildew occurs on all groups of oaks.