Powdery mildew infects many species of plants, including lawn grasses. Although the powdery white substance is similar on various species, it's actually different fungal organisms that cause the disease, so you need not worry about the mildew on your grass spreading to trees, shrubs, and flowers. The fungus that causes powdery mildew on grass, Erysiphe graminis, is most severe on Kentucky bluegrass, although Bermudagrass and fescues can get it too. Shaded lawns are most affected. The disease is more unsightly than damaging. It slows the growth of the grass plant, weakening it and making it more susceptible to other problems. Excessively fertilized lawns are more susceptible to powdery mildew. Fertilize and irrigate only moderately in shaded areas to help prevent the disease. Reduce shade and improve air circulation by pruning surrounding trees and shrubs. If the problem persists, consider overseeding with grasses more tolerant to mildew. An alternative is to replace the grass with a shade-tolerant ground cover.