- noun a fungal disease (Erysiphe graminis) affecting cereals and grasses. Another form also affects sugar beet and brassicas.
- a disease caused by the fungus Podosphaera necator that, unlike other mildews, does not need damp conditions to thrive. It occurs in hot weather on the upper surface of leaves of vines. Powdery mildew has two forms: primary mildew, which forms on young leaves in spring, and the more serious secondary mildew, which makes leaves dry and fall off in summer.