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downy mildew[¦dau̇n·ē ′mil·dü]
a dangerous plant disease caused by parasitic fungi of the class Phycomycetes of the family Perono-sporaceae. It attacks the green parts of the plant, primarily the leaves. Spots form on the diseased parts of the plant; the lower portions of the spots are covered with a whitish, grayish, or violet film, which is the sporangiosphore of the fungus (zoosporangia with sporangiosphores). Zoosporangia may appear repeatedly during the growing season; disseminated in the air, they are the principal source of infection. High humidity and soil with high moisture content foster the development of downy mildew. In most of the causative agents, the mycelium lives one year, dying along with the affected parts of the plant. However, sometimes it is perennial, surviving in bulbs, roots, and other wintering organs. All species of the family Peronosporaceae are obligate parasites, extremely specialized in their nutrition (requiring a specific host). The most injurious downy mildews are those that infect grapes, cabbages, turnips, onions, alfalfa, cucumbers, sunflowers, beets, tobacco, and several other cultivated plants.
Control measures include the implementation of correct cultivation procedures, the selection of resistant varieties, and dusting or spraying the plants with fungicides.
P. N. GOLOVIN