Powdery mildew is another fungal disease which afflicts many types of trees and shrubs. The fungi grow in dark round structures primarily on leaves, though sometimes seen on branches as well. These structures contain spores which are released into the air when the structures are broken. Humid conditions favor the growth of powdery mildew and it is spread by the wind. Roses, viburnums and azaleas are also susceptible to powdery mildew.
Symptoms: White and gray powdery patches and spots on leaves, stems, and buds. In extreme situations, the entire plant may seem dusted with talcum powder. Leaves may be twisted and distorted.
More information: OSU Extension, University of Minnesota Extension