Trees, Shrubs, and Vines: Powdery Mildew

Problem
Leaves, flowers, and young stems are covered with a thin layer or irregular patches of a grayish white powdery material. Infected leaves may turn yellowish or reddish and drop. Some leaves or branches may be distorted. In late fall, tiny black dots (spore-producing bodies) are scattered over the white patches like ground pepper.

Analysis
This common plant disease is caused by any of several fungi that thrive in both humid and dry weather. Some fungi only attack older leaves and plant parts; other fungi attack only young tissue. Plants growing in shady areas are often severely infected. The powdery patches consist of fungal strands and spores. The spores are spread by the wind to healthy plants. The fungus saps the plant nutrients, causing discoloring and sometimes the death of the leaf. Certain powdery mildews also cause leaf or branch distortion. Since these powdery mildews often attack many different kinds of plants, the fungus from a diseased plant may infect other plants in the garden. For a list of powdery mildews and the plants they attack, see Powdery Mildews and Some of the Plants They Infect.

Solution
Several different fungicides, including those containing triforine, chlorothalonil, dinocap, and cycloheximide, are used to control powdery mildew. For control suggestions, look under your specific plant in the index.