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Mildew is the name given to a group of fungous diseases which attack leaves, shoots, flowers and fruits. The true or powdery mildews (Erysiphaceae) appear as a thin white powdery coating on the surface of the plants. The disease is usually accompanied by distortion and dwarfing, and often death of the affected parts. In some cases, however, as in the maple mildew. the affected areas of the leaves retain their chlorophyll and remain green in the autumn long after the remainder of the leaf is dead and yellow. The mycelium is always superficial, forming spots or more or less extended areas on the affected organs. The injury is done by numerous haustoria, which penetrate the cells of the host and absorb nutriment for the mycelium, and also serve as organs of attachment. During the summer mildews are propagated by 1-celled spores, many of which are cut off in succession from erect simple branches all over the diseased surface. Other spores, by means of which the fungus passes through the winter, are produced in sacs inclosed within hollow spherical receptacles, called perithecia. These appear as minute black or dark brown specks over the diseased area. They are produced in the autumn, and remain on the fallen leaves; but the spores within them do not ripen until the following spring, when they are liberated by the decay of the perithecia.

In the United States, considerable injury is caused by the following species: The rose mildew, Sphaerotheca pannosa, on roses under glass; Erysiphe graminis on wheat and other grasses; the vine mildew, Uncinula spiralis, producing the powdery mildew of grapes; Podosphaera Oxycanthae on apples and pears; and Sphaerotheca Castagnei, the hop mildew. The most successful mode of combating the mildews is by dusting with sulfur or spraying with bordeaux mixture. Either of these fungicides kills the mycelium and spores of the fungus.

The downy mildews or false mildews belong to the Peronosporaceae, a group of fungi widely separated from the true mildews. The mycelium is parasitic within the tissues of the host, only the fruiting branches appearing at the surface. The fruiting branches have a characteristic form and method of ramification for each genus of the group. The spores, when they lodge on new host-plants, either produce an infecting thread directly, or, in most cases, the content of the spore is discharged in the form of swarm-spores, which swim about for a time and finally come to rest and produce the infecting mycelium. Resting-spores are produced sexually in this group within the tissues of the host.

This family contains about ten genera, of which the following are most commonly known: Phytophthora infestans, the potato blight; Plasmopara viticola, the downy mildew of grapes; Bremia lactucae, often causing great damage to lettuce in forcing-houses; Pythium Debaryanum, causing damping-off of seedling cucumbers and various other seedling plants; and Cystopus candidus, the common white rust of crucifers. Modes of combating these diseases are set forth for each specific case in the experiment station literature of the various states.
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