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Powdery Mildew

Powdery mildew is a disease that afflicts a wide range of trees, vegetables, and flowers, and is caused by several different fungi, Podosphaera leucotricha on apples and Podosphaera clandestina on cherry trees. The fungus survives over winter in vegetative or fruit buds that were infected the previous season, and it does not spread and develop significantly until temperatures are over 50°F. The spores on infected shoots are transferred by wind and insects to surrounding foliage. No rainfall is necessary for this disease to travel. In fact, although high humidity will hasten its spread, rainfall will inhibit powdery mildew.

New infections cause yellow spots on the surface of leaves, but the obvious, telltale sign of powdery mildew is white, powdery coating on the undersides. Eventually leaves will dry and wither, depriving the tree of nutrition. Infected terminal shoots will have a silvery-gray color and a stunted or deformed appearance. A heavily infected tree will even show signs on the fruit, a "net" of russet on the skin.

In areas where pressure from powdery mildew is high, certain susceptible apple varieties should be avoided: Baldwin, Cortland, Mutsu (Crispin), Gala, Ginger Gold, Honeycrisp, Idared, Jonathan, Monroe, Paulared, and Rome are all highly susceptible. Inspect your trees when the buds are in the "tight cluster to pink" stage and remove visibly infected buds. Finally, a spray regime may be necessary to treat heavy infections. Sulfur is most commonly used.

For further information, see Cornell's IPM Factsheet on Powdery Mildew and the Cornell Guide to Growing Fruit at Home.

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