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powdery mildew[′pau̇d·ə·rē ′mil‚dü]
any one parasitic fungus of the order Erysiphales (Perisporiales) of the class Ascomycetes. The multicellular mycelium usually develops ectotrophically, on the surface of various plant organs. It rarely develops endotrophically, in the cells of leaf parenchyma, or mesophyll (in the genera Leveillula, Phyllactinia). The fungi feed by means of special outgrowths of the mycelium, or haustoria. During the vegetative period, they reproduce asexually by means of conidia. The fruiting bodies, or cleistothecia, develop most often on aging mycelium; they are dark in color, lack stomata, and have hyphal processes (whose structure has species or generic specificity). The cleistothecia have one or more sacs (asci), which form as a result of the sexual process.
There are 20 genera of Erysiphales, including about 100 species. They prefer dry places and are especially abundant during a dry summer. The mycelium forms a white coating on the green parts of a number of wild and cultivated grains, causing the disease powdery mildew. Species of fungi that cause powdery mildew of oak, grape, hops, apple, and other plants are widely distributed.
M. A. BONDARTSEVA
(also oidium disease), any one of a group of plant diseases caused by powdery-mildew fungi (Erysiphales). Powdery mildew usually appears at the beginning or middle of summer in the form of a powdery, cobwebby, or tomentose mealy-white film that spots leaves, shoots, stems, petioles, fruit stalks, and sometimes fruits. The film gradually spreads, covering most of the green organs, which become brownish with innumerable black spots. Diseased leaves and shoots become fragile and wither prematurely. The wood of trees and shrubs infected with powdery mildew does not mature; as a result, the plants freeze in the winter. Those shoots that remain alive exhibit poor growth in the next vegetative period. Sick fruits usually split; the flesh is exposed and quickly rots.
All the causative agents of powdery mildew are exoparasites of plants. Many powdery-mildew fungi are parasites of agricultural crops. The most harmful species are Uncinula necator, which parasitizes grapes; Sphaerotheca mors, which infests gooseberries; Erysiphe graminis, which parasitizes bread grains; Sphaerotheca pannosa forma persicae, which affects peaches; Erysiphe communis, which infests sugar beets; and Sphaerotheca pannosa forma rosae, which parasitizes roses.
Control measures include destroying affected plant residue, pruning and burning diseased shoots, deep autumn plowing, applying phosphorus and potassium fertilizers, using crop varieties that are more resistant to powdery mildew, and spraying or dusting plants with sulfur preparations or with dinocap (kara-thane). Sulfur preparations are not used on gooseberry, since they cause shedding of leaves. For control of powdery mildew on roses, sodium carbonate or a solution of green soap may be used.
REFERENCEGolovin, P. N. Muchnistorosianye griby, parazitiruiushchie na kurturnykh i poleznykh rasteniiakh. Moscow-Leningrad, 1960.
D. N. TETEREVNIKOVA-BABAIAN