Home gardeners often choose hydrangeas for the large, colorful blooms they produce. While hydrangeas are generally healthy and vigorous, they are susceptible to a fungal disease known as powdery mildew that causes mildew to develop on the leaves, stems and flowers of the plant. Powdery mildew is not usually fatal to hydrangeas, but often becomes unsightly.
Powdery mildew affects many types of plants, flowers, trees and shrubs. Fungi cause this disease. All powdery-mildew-causing fungi produce similar symptoms, no matter which species of plant they infect. Warm days and cool nights are favorable weather conditions for powdery mildew. This disease requires humidity to germinate; growing hydrangea in shady areas increases risk of disease development. Powdery mildew fungi do not need water to germinate, and frequent rain showers often halt disease development.
According to the Alabama Cooperative Extension System, powdery mildew is most common in greenhouse-grown bigleaf hydrangea. Powdery mildew is also prevalent in H. paniculata, H. serrata and H. macrophylla. This disease is not often seen in oakleaf hydrangea or H. quercifolia. Home-grown hydrangea also develop powdery mildew, but the symptoms are not usually as severe as greenhouse-grown varieties.
Powdery mildew causes hydrangeas to develop small tufts of gray or white powdery mildew on the leaves of the plant. When the weather conditions are favorable, these areas enlarge until they cover the entire leaf surface. The leaves of infected hydrangea are often discolored, and the plant becomes stunted. Powdery mildew fungi also cover the flowers and new shoots of infected plants. Hydrangeas suffering from powdery mildew may not produce large blooms, and purple splotches may appear on infected hydrangea leaves.
Good air circulation is important in preventing powdery mildew development. Space your hydrangeas far enough apart to promote air circulation. Avoid planting your hydrangea in areas of heavy shade, as this promotes disease development. Avoid overhead watering and provide your hydrangeas with water early in the day, so plants dry out completely before the nighttime temperatures turn cool. Remove any infected plant tissue from your garden, as powdery mildew fungi can overwinter in fallen debris. Very severe cases of powdery mildew often require the application of fungicides for control. Fungicides are most effective if applied at the first sign of disease and continued every seven to 14 days while weather conditions are favorable for powdery mildew development.