Phytophthora Root Rot and Stem Rot

• Associated with wet soil conditions
  • Commonly occurs on heavy, poorly drained or compacted soils
  • May occur on any soil saturated for an extended period of time
• The ideal temperature for infection is 60 to 80º F (15 to 27º C)
• Successive years of soybeans on the same fields may increase damage potential
• Application of high levels of potash, manure or municipal sludge immediately before planting may increase disease severity

• Disease-causing fungus is a water mold, or oomycete, characterized by oospores and zoospores
• Oospores act as survival mechanism of the fungus
  • May persist in soybean residue and soil for years
• Zoospores are produced when oospores germinate in the presence of a soybean crop
  • Zoospores also produced from infected soybean tissue during the growing season
  • Swim through films of water to the root
  • Fungus infects root and grows into and among the root cells of the plant
• Disease survives in soybean residue and in the soil

• The Phytophthora fungus can kill plants at all stages of growth
• Stand reduction may result in replanting or yield loss
• Replanting is common when early infection results in severe seed rot and damping off of seedlings
• In some cases, infected stands survive but are less productive than healthy stands
• Yield reductions can range from as little as 5% to more than 50% depending on severity

Seed Rot Phase – may begin at imbibition

• Infected seeds become dark brown and soft to mushy
• Complete deterioration of the seed may occur

Seedling Blight Phase – occurs at emergence or soon after

• “Damping off” – rapid decay, wilting and plant death
• Symptoms include a dark brown to black discoloration of the stem, usually beginning at the soil line
• Diseased tissues quickly become soft and water-soaked, and wilting and plant death may soon follow

Root and Stem Rot Phase – symptoms begin in the root

• Brown, discolored taproot and secondary roots and less root mass
• Nodulation is often minimal, leading to chlorotic, N-deficient plants
• Affected plants may be stunted, so fields have an uneven appearance

Root and Stem Rot Phase – symptoms may spread to the stem

• Brown discoloration develops at the soil line
• Dark-brown to red-brown lesion may progress up the stem (key diagnostic feature of the stem rot phase)
• Diseased tissues quickly become soft and water-soaked, and wilting and plant death may soon follow, especially during stress periods

• Variety Selection and Seed Treatments – most effective means of managing Phytophthora
  • Corteva Agriscience researchers are developing varieties with resistance genes and field tolerance to Phytophthora
  • Corteva Agriscience uses molecular breeding to develop varieties with race-specific resistance genes such as Rps 1C, Rps 1K, and Rps 3a
  • Race-specific resistance provides complete resistance to specific races throughout the growing season
  • Field tolerance provides partial protection against all races of Phytophthora
  • Field tolerance is not as effective in the seed and seedling growth stages, but provides benefit in-season
  • Varieties containing race-specific resistance and favorable field tolerance coupled with DuPont™ Lumisena™ fungicide seed treatment is the ideal combination because they provide multiple mechanisms to protect against Phytophthora
  • Pioneer^® brand soybean varieties are rated for tolerance and these ratings are provided to customers – ratings range from 2-9 (9=tolerant) with a majority of varieties scoring between 4 and 6
• Field Drainage and Soil Structure – improve field drainage and remediate compaction and hardpan layers if possible
• Planting Date – on heavy soils or in no-till systems, early planting may not be an option