a serious plant disease, characterized by yellowish patches on the undersurface of the leaves, caused by the parasitic fungi of the family Peronosporaceae, such as Peronospora destructor: affects onions, cauliflower, lettuce, etc
2.
any of the fungi causing this disease
Compare powdery mildew
downy mildew in American English
US
a disease of angiosperms characterized by the appearance of whitish or downy patches of fungus on the surfaces of plant parts, caused by various fungi (familyPeronosporaceae)