Problem: Powdery Mildew (Apple)
This plant disease is caused by a fungus (Podosphaera leucotricha) that thrives in both humid and dry weather. The fungus spends the winter in leaf and flower buds. In the spring, spores are blown to the emerging young leaves, which are very susceptible to infection. The fungus saps plant nutrients, causing distortion and often death of the tender foliage. The fruit yield may be greatly reduced, because infected blossoms do not set fruit. Young, developing apples that are attacked become dwarfed and turn russet color. The fruit can be eaten if peeled, however. Powdery mildew is favored by warm spring days and cool nights, reduced light, and lack of rainfall.
Spray infected trees with a fungicide labeled for edibles, following label directions. On small trees with limited disease, prune out and destroy infected shoots when they are first noticed.