Powdery mildews are the only fungal leaf disease in which the fungus grows on the surface of the plant. There it forms a superficial white powdery coating on leaves, buds, shoots and flower petals. The entire surface of the leaf may be covered with a cottony spider webbing, most of which consists of spores ready to be blown to an uninfected leaf. The fungus can easily be wiped off the surface.
Powdery mildew is a disease of mid- to late summer.
Perhaps the most common hosts are roses and lilacs. Unlike other leaf disease fungi, powdery mildews are most serious in shady, damp locations where plants are crowded, air circulation is poor and relative humidity is high. Powdery mildews flourish when days are warm and nights are cool and when dew forms on the leaves. Many plants can be infected by powdery mildews, but powdery mildew on daffodils is usually specific to daffodils and will not infect lilac or rose, and vice versa. Powdery mildews are caused by over l,600 species of fungi.