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Downy mildew
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. The prime example is ''Peronospora farinosa'' featured in NCBI-Taxonomy〔(NCBI-Taxonomy ) – ncbi.nlm.nih.gov〕 and HYP3.〔(HYP3 ) – ncbi.nlm.nih.gov〕 This pathogen does not produce survival structures in the northern states of the USA, and overwinters as live mildew colonies in Gulf Coast states. It progresses northward with cucurbit production each spring. Yield loss associated with downy mildew is most likely related to soft rots that occur after plant canopies collapse and sunburn occurs on fruit. Cucurbit downy mildew only affects leaves of cucurbit plants.
== Symptoms ==
Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface.〔Schilder, Annemiek. (Downy mildew - Plasmopara viticola. ) ''MSU Plant Pathology.''〕 As lesions mature, they expand rapidly and turn brown. The under surface of infected leaves appears watersoaked. Upon closer inspection, a purple-brown mold (see arrow) becomes apparent. Small spores shaped like footballs can be observed among the mold with a 10x hand lens. In disease-favorable conditions (cool nights with long dew periods), downy mildew will spread rapidly, destroying leaf tissue without affecting stems or petioles.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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