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. The fungi often survive the winter tucked away in the scales of the buds. Therefore, the first infections of the mildew often are seen on the tips of new shoots. Mildews may also form a minute ball about the size of a grain of pepper, filled with microscopic spores.