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Downey mildew


Photo by Tristan Mueller

Pathogen(s) causing disease:

Peronospora manshurica

Symptoms:

Appear on the upper surface of young leaves as pale green to light yellow spots which enlarge into pale to bright yellow spots. Spots look slightly gray and fuzzy, especially during periods of high relative humidity. Younger leaves are more susceptible to downy mildew than older leaves. The center of spots eventually turns brown, bordered by yellow margins. Pods can also be infected without obvious external symptoms. Infected seed has a dull white appearance and is partially or completely covered with a pale coating of fungal spores.

Conditions:

Occurs wherever soybeans are grown. The downy mildew fungus survives in crop residue and on the surface of seed.

Management:

Do not plant seed from infected fields. If contaminated seed is planted the next season, the fungus can infect the seedling systemically and cause stunting and mottling of the leaves. The downy mildew fungus survives in crop residue and on the surface of seed. Therefore, crop rotation or deep burial of infested crop residue is an effective way to reduce inoculum. Numerous sources of resistance to P. manshurica are present in soybean germplasm although soybean varieties in grown in Iowa are not generally characterized for downy mildew resistance.

Sources:

U of WI
ISU

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