Extended Version | Download fact sheet
Downy Mildew of Soybean
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)
- Overwinters as oospores in leaves as crop debris or on seeds.
- Sporangia germinate to infect plants, or infected seeds will produce systemically infected plants.
- Sporangia are then disseminated by wind and rain.
- Sporangia can then infected healthy plants within the same growing season, germinating within 12 hours in times of high moisture.
- Older leaves become resistant to infection.
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.