1. any of various fungi of the genus Erysiphe producing powdery conidia on the host surface
The Collaborative International Dictionary of English v.0.48:
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
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.
WordNet (r) 3.0 (2006):
n 1: any of various fungi of the genus Erysiphe producing
powdery conidia on the host surface