Mildew is any of several parasitic fungi that attack a wide variety of crops, including fruits, vegetables, and grains. The term "mildew" is also applied to various fungus diseases that appear as discolored areas on leather, paper, fabric, and many other materials. Contrary to popular belief, these diseases are caused not by mildews but by related fungi.
Generally, mildews are divided into two groups: powdery mildews and downy mildews. Powdery mildews have a fine powdery appearance on the surface of the plant. They usually attack roses, gooseberries, and grasses, including the cereal grains. Downy mildews have a fuzzy appearance on the host plant. Downy mildew of grape (Plasmopara viticola) is one of the most destructive of all mildews. It has caused great damage to both American and European varieties of grapes. Other downy mildews attack melons and lima beans.
Both powdery and downy mildews cause dwarfing, deformation, and loss of affected plant parts. Most mildews can be controlled by applying a liquid or powder fungicide to the infested plants.
Downy mildews are classified in the class Phycomycetes, order Peronosporales. Powdery mildews are classified in the class Ascomycetes, order Erysiphales.