Phytophthora Rot – Phytophthora sojae

Background Information
  • Phytophthora rot is a widespread disease. It can cause a seed and seedling blight and can kill maturing plants throughout the season. This disease can cause minor and major losses, and it is most common in heavy, slowly drained soils.
Disease Agent
  • Phytophthora sojae is a fungal-like pathogen that can survive in soil for at least 5 to 10 years in association with decomposed soybean tissue. Soybean is the only known crop host for this pathogen. The pathogen infects soybean at all stages of development, and is most active and causes most damage in wet soils. Many different races are known that defeat different resistance genes in soybeans.
Symptoms
    • Phytophthora can attack and rot seeds prior to emergence, and can cause pre- and postemergence damping-off.
    • At the primary leaf stage (VI):
  • Infected stems appear bruised and are soft.
  • Secondary roots are rotted.
  • Leaves turn yellow.
  • Plants frequently wilt and die.
  • A related pathogen, Pythium, causes similar symptoms and can be differentiated only through laboratory examination.
    • Older plants may die throughout the season:
  • Dark brown lesions form on the roots.
  • A distinct dark brown discoloration of the stem may extend from below the soil line upward into the branches.
  • Leaves typically turn yellow, wilt, and stay attached after the death of the plants.
  • In plants of tolerant varieties, symptoms may be restricted to rotting of lateral roots and browning of the taproot, resulting in hidden damage that may significantly reduce yields. In tolerant varieties, plants may be stunted and not killed.
Time of Occurence
  • All season
Conditions Favoring Disease
  • Wet weather
  • Low, poorly drained areas
  • Clay, compacted soils
  • Susceptible soybean varieties
Disease Managment
    • Use varieties with race-specific resistance (Rps genes).
  • Some varieties have high levels of tolerance to Phytophthora rot and may reduce the adult phase of the disease.
  • Improve soil drainage if possible, and avoid low areas of fields, especially if a field has a history of seeding or root rot problems.
  • Crop rotation may be of some benefit.
  • Some fungicidal seed treatments can also reduce damage to seed and young seedlings.