This fungal disease attacks a wide variety of plants, including all sorts of beans, clematis, dahlia, grape, hydrangea, rose, strawberry, tomato, and zinnia, and trees such as apple, maple, oak, peach, and sycamore. It is favored by moist air, shade, and poor air circulation, but needs dry leaves to become established. The first symptoms are small gray or white circles on leaves, stems, and flowers; then entire leaves and blooms become powdery white and distorted. Some plants remain vigorous despite the infection, but others decline or fail to set fruit. Some flowering plants can become so disfigured that they must be removed from the garden.
To prevent powdery mildew, plant resistant varieties and routinely spray plants with jets of water to wash off fungus spores. Increase sunlight to plants by avoiding overcrowding. In the fall, discard infected flowers, fruits, and plants. Sulfur may help; on roses and other flowering plants, try a baking soda and summer oil spray. Some gardeners report success with the antitranspirant sprays sold to protect tender plants from cold. Such sprays keep the surface temperature of treated leaves somewhat higher than that of the surrounding air; apparently they also prevent mildew spores from attaching to foliage.
Chemical controls include triforine and thiophanate-methyl.