n. A minute parasitic fungus which frequently appears on the leaves, stems, and various other parts of plants or other decaying organic substances as a white frost-like down, or in spots or with various discolorations.n. A state of decay produced in living and dead vegetable matter, and in some manufactured products of vegetable matter, such as cloth and paper, by the ravages of very minute parasitical fungi.To taint with mildew.To become affected with mildew.n. A disease of cucumbers, melons, pumpkins, and similar plants, caused by Plasmopara Cubensis.