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Downy Mildew of Soybean

Causal Agent:

Peronospora manshurica; an oomycete

Symptoms and Signs:

  • Pale purple to gray, fuzzy spore mats on underside of leaves
  • Pale green to light yellow spots on upper surface of leaves
  • Infected seeds may be cracked with a dull white appearance (see below)

Light green to yellow spots that, in some areas, have coalesced into larger lesions on the upper leaf surface (top left). Seeds can also be affected, giving a dusty appearance to the seed surface (top right). The underside of the leaf will have fuzzy gray-light purple areas where the pathogen is producing inoculum on the leaf surface (bottom).

Disease Cycle:

  1. Overwinters as oospores in leaves as crop debris or on seeds.
  2. Sporangia germinate to infect plants, or infected seeds will produce systemically infected plants.
  3. Sporangia are then disseminated by wind and rain.
  4. Sporangia can then infected healthy plants within the same growing season, germinating within 12 hours in times of high moisture.
  5. Older leaves become resistant to infection.

Disease Management:

Overall, downy mildew of soybean is considered a minor disease and in most situations no action is needed for management. Be sure to note when high levels of disease are observed and remove that variety from future plantings.

Cultural practices: Rotating soybean crops with a non-host for 1 or more years will help reduce inoculum in the field. Tillage can also help by burying crop debris that carries infective oospores. Using pathogen-free seed will help reduce chances of downy mildew.