Powdery mildew is a common disease that appears as a white powdery substance on the leaf surface. The powdery appearance comes from millions of tiny fungal spores, which are spread in air currents to cause new infections. It attacks all kinds of trees. Trees most commonly affected by powdery mildew are linden, crabapple, catalpa
and chokecherry, but almost any tree or shrub can get powdery mildew.
Powdery mildew is a product of moist conditions and is usually seen in the wetter spring and fall seasons. This humidity-loving fungi can be controlled if moisture can be controlled.
Don't plant trees in heavily-shaded areas and provide plenty of space for air movement and growing room. Prune the tree for effective air movement.
If necessary, spray with wettable sulfur at the recommended rate specified on the label. Sulfur may injure tender foliage, especially in hot weather so be careful. Do not use sulfur on walnuts, as injury may occur. Other fungicides can also be used under certain conditions. Check with your local extension agent for these chemicals and application rates.