Downy mildew is a widespread disease that occurs during periods of high humidity and moderate temperatures. The disease is typically superficial and causes no yield loss, but can cause defoliation of plants and reduced yields under rare conditions. Downy mildew can be distinguished from other foliar soybean diseases by the tufts of tan-colored fungal growth on the underside of infected leaves.
The initial symptoms of downy mildew are small, light green spots (not water-soaked) on upper leaf surfaces. The spots enlarge and turn pale to bright yellow. They may coalesce into large irregular brown areas. Tan to gray tufts of fungal growth often develop on lower leaf surfaces, especially under wet and humid conditions. Infection of pods and seed can also occur. Seeds may become covered with a whitish coating of fungal hyphae and spores.
Conditions and Timing that Favor Disease:
Downy mildew can occur on plants of all ages, although the disease is most common after flowering begins. Young leaves are most susceptible and infected leaves are often seen on the tops of plants. Dew and high humidity favor disease development, as well as mild temperatures.
Downy mildew is caused by the fungal-like organism Peronospora manshurica. Common snap bean is also a host. The fungus overwinters on infected leaves and seeds, and can be transmitted by seed.
Select resistant soybean varieties and use appropriate seed treatments. Bury infested residue where feasible and where disease has been severe. Rotate soybeans with a non-bean crop for at least one year.