Downy Mildew


  • The initial symptoms are small light green spots (not water-soaked) with raised, centers on the upper and lower surfaces of leaves.
  • Tan to gray tufts of fungal growth often develop on lower leaf surfaces, especially
    under wet and humid conditions.
  • It can cause defoliation and reduced yields, but yield losses are typically minimal.
  • The lesions can grow together into large irregular brown areas. Pod and seed infection can also occur. Seeds may become covered with a whitish coating.

Pathogen Involved:

  • Peronospora manshurica (a fungus).
  • Common snap bean is another host.
  • It overwinters on infected leaves and seeds, and can be transmitted by seed.

Time of Occurrence:

  • All season; young leaves are most susceptible.

Conditions Favoring Disease:

  • Dew and high humidity.
  • Warm temperatures.
  • Infested residue on soil surface.
  • Susceptible varieties.
  • Infested seeds.

Disease Management:

  • Select resistant soybean varieties.
  • Use appropriate seed treatments.
  • Bury infested residue where feasible and where disease has been severe.
  • Rotate with a non-bean crop.