Downy mildew
Downy mildew is a fungal disease of leaves and sometimes of pods and seed. The fungi that cause downy mildews as a group are considered to be obligate parasites, meaning that they are restricted typically to one specific host. The disease is more common on herbaceous plants and is favored by temperatures cooler than 65 degrees and by 100% relative humidity. Symptoms include the yellowing of leaves and the advanced development of white fungal growth on the undersides of older leaves. Because downy mildew symptoms are mild and generally restricted to older growth, they present few problems in the landscape. Downy mildew can, however, become systemic in small seedlings and lead to rapid death, however this is rare and more commonly associated with infected seed that has been planted. The three most important downy mildew diseases are those of impatiens, snapdragon and grape.
Specific diseases

Other images

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Downy mildew on black-eyed Susan (Rudbeckia fulgida var. sullivantii 'Goldsturm')
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White pustules of downy mildew on underside of black-eyed Susan (Rudbeckia fulgida)
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Leaf spots caused by downy mildew on upper leaf surface of black-eyed Susan (Rudbeckia fulgida)
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Leaf spots caused by downy mildew on upper leaf surface of black-eyed Susan (Rudbeckia fulgida)
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Leaf spots caused by downy mildew on upper leaf surface of black-eyed Susan (Rudbeckia fulgida)
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White pustules of downy mildew on underside of black-eyed Susan (Rudbeckia fulgida)
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White pustules of downy mildew on underside of black-eyed Susan (Rudbeckia fulgida)
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Downy mildew on leaves of verbesina (Verbesina)
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Characteristic pattern of white pustules on leaves of verbesina (Verbesina) caused by downy mildew
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Downy mildew on upper leaf surface of Solomon's seal (Polygonatum)
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Downy mildew on underside of leaf of Solomon's seal (Polygonatum)
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Downy mildew on winter squash leaves. G. Holmes, Cal Poly, Bugwood.org
Pests and Problems

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