downy mildew

noun
1.
Also called false mildew. any fungus of the family Peronosporaceae, causing many plant diseases and producing a white, downy mass of conidiophores, usually on the under surface of the leaves of the host plant.
2.
Plant Pathology. a disease of plants, characterized by mildewed, distorted, and yellowed foliage, caused by any of several fungi of the family Peronosporaceae, as of the genera Peronospora and Phytophthora.

Origin:
1885–90

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World English Dictionary
downy mildew
 
n
1.  a serious plant disease, characterized by yellowish patches on the undersurface of the leaves, caused by the parasitic fungi of the family Peronosporaceae, such as Peronospora destructor: affects onions, cauliflower, lettuce, etc
2.  any of the fungi causing this disease

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American Heritage
Science Dictionary
downy mildew   (dou'nē)  Pronunciation Key 
A disease of plants caused by oomycete organisms of the order Peronosporales and characterized by gray, velvety patches of spores on the lower surfaces of leaves. Downy mildew of the grapevine nearly destroyed the French wine industry in the 1870s, spurring the development of the first chemical pesticides used on plants.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

downy mildew

disease of plants, especially in cool humid regions, caused by several fungi, including species of Basidiophora, Bremia, Peronospora, Phytophthora, Plasmopara, Pseudoperonospora, and Sclerospora. White, gray, bluish, or violet downy patches of mildew form mostly on the undersides of leaves in damp weather. Pale-green to yellow or brown areas usually develop on the upper leaf surface opposite the downy growth. Affected leaves often wilt, wither, and die early. Stems, flowers, and fruits are sometimes infected. Seedlings may wilt and collapse. Garden plants, bush fruits, vegetables, and certain trees, shrubs, field crops, and weeds are susceptible.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Downy mildew is a major disease in all sunflower growing regions.
The diseases anthracnose, downy mildew, and powdery mildew can result in yield
  and fruit quality losses when foliage is damaged.
Breeding new hop varieties with high alpha resin contents and resistance to
  downy mildew.
Spinach downy mildew: overview of races and the development of molecular
  markers linked to major resistance genes.
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