Mildew, any of several species of fungi or protists that form a whitish or grayish growth on living plants and on such organic materials as leather. (The growth itself is also called mildew.) Powdery mildews are fungi that form a powdery coating on the surfaces of leaves, flowers, and fruits. These mildews usually stunt the development of the plants they grow on. Downy mildews are protists that form downy spots on the underside of leaves, on other parts of plants, and on other organic surfaces. These mildews cause serious losses of such crops as grapes, lima beans, and melons. Mildews on crops are usually killed with sulfur- or copper-based fungicides.

Certain mildews attack and discolor leather, paper, wood, and cloth in damp and poorly ventilated places. These materials can be made mildewproof by applying such chemicals as creosols and phenols.

Mildews typically reproduce by means of spores. The spores are dispersed by the wind and, upon landing on a suitable surface, develop into new growths of mildew.

Powdery mildews belong to the family Erysiphaceae of the division Ascomycota. Downy mildew belong to the family Peronosporacae of the phylum Oomycota.