- any fungus of the family Peronosporaceae, causing many plant diseases and producing a white, downy mass of conidiophores, usually on the under surface of the leaves of the host plant.
- a disease of plants, characterized by mildewed, distorted, and yellowed foliage, caused by any of several fungi of the family Peronosporaceae, as of the genera Peronospora and Phytophthora.
- 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
- any of the fungi causing this disease
