The first symptoms of powdery mildew are light spots on leaves. When the mildew growth becomes more dense, infected areas look as though they have been lightly sprayed with white paint or "powdered". The white substance observed on leaves is composed of millions of spores of the powdery mildew fungus. Heavily infected leaves turn yellow and may eventually brown and die. Infected plants are weakened, causing them to be more susceptible to other stresses, such as drought or low temperature injury.
Powdery mildew occurs most commonly on Kentucky bluegrass in shady areas. Slow or nonexistent air circulation, shade, high humidity, and temperatures of 60-70 F are the components needed for disease development. The disease is often particularly severe under these conditions when the lawn is heavily fertilized with nitrogen.
The best strategy for controlling powdery mildew is to plant shade-tolerant cultivars in shady areas. Examples of shade-tolerant bluegrasses with disease resistance include Glade, Eclipse, and Sydsport. For cultural control, avoid excessive nitrogen fertilization and selectively prune trees or shrubs to increase air circulation and sun penetration. Fungicide applications are rarely necessary, but may be applied if the disease is severe.
This article originally appeared in the October 11, 1996 issue, p. 161.