1. any of various parasitic fungi of the ascomycete order Erysiphales, which produce a powderlike film of mycelium on the surface of host plants.
2. Plant Pathol. a disease caused by powdery mildew, characterized by yellowing and death of the foliage and a white mealy growth of fungus on the surface of above-ground parts.
[ 1885-90, Amer. ]