'powdery mildew' definitions:

Definition of 'powdery mildew'

noun

Any of various fungi of the genus Erysiphe producing powdery conidia on the host surface

From: GCIDE

Mildew \Mil"dew\, n. [AS. melede['a]w; akin to OHG. militou, G. mehlthau, mehltau; prob. orig. meaning, honeydew; cf. Goth. milip honey. See Mellifluous, and Dew.] (Bot.) A growth of minute powdery or webby fungi, whitish or of different colors, found on various diseased or decaying substances. [1913 Webster]
powdery mildew a fungal disease of plants caused by an ascomycete of the order Erysiphales, characterized by a powdery white film on the surface of the affected plants. It is damaging to, e.g., roses and lilacs. Also, a fungus that causes such a disease. [PJC]