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Initial symptoms of downy mildew are pale green to light yellow spots or blotches on the upper surface of young leaves. These areas enlarge into pale to bright yellow lesions of indefinite size and shape. Eventually the lesions turn grayish brown to dark brown with a yellow margin. During periods of heavy dew or wet weather, a gray to purple fuzz that is visible growth of the downy mildew fungus develops on the lower leaf surface beneath the diseased areas.
The disease tends to occur under cool, moist conditions. The fungus survives in crop residue and on seeds, appearing as a white crusty growth. To control downy mildew, plant disease-free seed and rotate crops with at least one year between soybean crops. |
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