1. Also called false mildew. 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.
2. Plant Pathol.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.