Downy mildew is a common disease of soybean that occurs wherever soybean is grown. Fortunately, it rarely affects soybean productivity in Iowa . However, seeds can become infected with downy mildew (Figure 1). These symptoms can be mistaken for infection by white mold (Sclerotinia sclerotiorum)
Downy mildew is caused by the fungus Peronospora manshurica . Symptoms first appear on the upper surface of young leaves as pale green to light yellow spots which enlarge into pale to bright yellow spots (Figure 2). The spots look slightly gray and fuzzy when viewed from below, especially during periods of high humidity. Younger leaves are more susceptible to downy mildew than older leaves.
The center of the spots eventually turn brown, bordered by yellow margins. During warm, wet weather, the pathogen can spread quickly. 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.
Management of Downy Mildew
Plant clean seed. 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.
Crop rotation and tillage. 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.
Soybean variety. 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.