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Mildews are whitish fungi that grow on moist surfaces. Some mildews are parasites growing on the surface of plant foliage or fruits. Other mildews grow on the moistened surfaces of materials made from plant or animal tissues, such as wood, paper, clothing, or leather.

The downy mildews are in the fungal family Peronosporaceae. These fungi can only exist as parasites, and under conditions favorable to their growth they can be important plant pathogens. Heavily infested leaves have whitish mycelium emerging through the small pores on their surface known as stomata, and through other exterior lesions. It is this downy mycelium that gives the fungus its common name. Various species are economically important, for example, Plasmopara viticola is a downy mildew of cultivated grapes (Vitis vinifera).

The powdery mildews are in the family of fungi known as the Erysiphaceae. These can be important parasites of grasses and other plants, especially under humid conditions. Severely infested plants can have a whitish or grayish bloom of mycelium and spores over much of their above-ground surfaces. One of the most important species is the powdery mildew of grasses (Erysiphe graminis), which affects a wide range of grasses grown as food for humans or as fodder for livestock.

When they are perceived to be pests, mildews are sometimes treated with a pesticide called a mildewcide. Commonly used chemicals for this purpose include benzoic acid, formaldehyde, cresols, phenols, sulfur powder, and organic compounds containing mercury, lead, zinc, or copper. Infestations of mildews on books, walls, and other organic-rich surfaces can sometimes be treated by wiping with a dilute solution of domestic bleach (sodium hypochlorite).

See also Fungicide.

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