Overview of powdery mildew
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 turf will not infect roses.
Signs and symptoms of powdery mildew
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.
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.
Disease Cycle of powdery mildew
Powdery mildew occurs most commonly in shady areas. Slow or non-existent air circulation, shade, high humidity, and temperatures of 60-70° F are the components needed for disease development. In lawns, the disease is often particularly severe under these conditions when the lawn is heavily fertilized with nitrogen.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of powdery mildew
The best strategy for controlling powdery mildew in turf 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.