As its name suggests, powdery mildew is visible as a light gray or white dusty coating on leaves, stems, flowers, or fruits. Depending on the plant species, affected leaves may be distorted, and tiny dark dots may appear in the white coating. Powdery mildews can occur on nearly all kinds of plants, but each plant is affected by a different powdery mildew fungus. For example, the fungus that causes powdery mildew on lilac will not infect roses.
Powdery mildew is usually favored by moderate temperatures and humid (but not wet) conditions. It tends to be more severe in the shade and in areas with little airflow. Because of this, putting plants in full sun and cultural practices that promote airflow (such as pruning and appropriate spacing) can help to minimize powdery mildew problems. Keeping plants in good vigor helps them to resist infection. On some plants such as lilac, powdery mildew does not cause serious damage to the plant and can be tolerated. On other plants, damage may be severe and chemical controls may be desired. Many fungicides are labeled for use against powdery mildew on ornamentals, but they are most effective when used preventatively (before infection occurs) and good coverage of the plant is essential.
Powdery Mildew in Turf Grass
The first symptoms of powdery mildew are light spots on leaves. When the mildew growth becomes denser, 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.