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Phytophthora Root Rot


Phytophthora Root Rot and Stem Rot

Phytophthora Symptoms

Note dark brown lesion extending upward from soil line (2 photos below)
dark brown lesions of phytophthora root rot
phytophthora root rot on soybean stem

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

healthy soybean stem vs. stem infected with phytophthora


Disease Facts

  • Caused by the soilborne fungus Phytophthora sojae (also known as Phytophthora megasperma f. sp. glycinea)
  • Pathogen has many races, and multiple races occur in each field
  • Disease is favored by extended wet field conditions
  • May attack soybeans at any time during the growing season
  • Displays seed rot, seedling blight and root/stem rot phases
  • Above-ground symptoms may not be evident for several weeks after initial infection
soybeans wilting due to phytophthora root rot
Soybean plants in foreground wilting due to Phytophthora Root Rot.

Conditions Favoring Disease Development

  • 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 80 F
  • 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 Cycle of Phytophthora Root Rot

  • 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
Plants wilting among healthy plants is often a symptom of Phytophthora
(2 photos below)
Soybean plants wilting among healthy plants is often a symptom of Phytophthora

soybean stand reduction may result in replanting or yield loss where phytophthora exists

Impact on Crop

  • 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


Variety Selection - most effective means of managing Phytophthora

  • DuPont Pioneer researchers are developing varieties with resistance genes and field tolerance to Phytophthora
    • Rps 1C and Rps 1K are the most common race-specific resistance genes used today
    • Race-specific resistance is most effective during the seed and seedling growth stages
    • Field tolerance is effective against all races of Phytophthora, more enduring than race-specific genes
    • Field tolerance is not as effective in the seed and seedling growth stages
    • Varieties containing both genetic resistance and field tolerance have 2 mechanisms of protection
  • DuPont Pioneer rates its varieties for tolerance and provides ratings to customers -- ratings range from 4 to 6 (9 = tolerant)

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

Seed Treatments – use seed-applied fungicides in fields with a history of Phytophthora damage

  • Pioneer Premium Seed Treatment (PPST) contains metalaxyl, which has specific activity against early season Phytophthora and Pythium diseases.
    • Provides protection for up to 3 weeks
    • Especially useful when cool, wet soil conditions develop after planting.
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   Pioneer Premium Seed Treatment for soybeans is applied at a DuPont Pioneer production facility or by an independent sales representative of Pioneer. Not all sales representatives offer treatment services, and costs and other charges may vary. See your Pioneer sales representative for details. Seed treatment offering is exclusive to DuPont Pioneer and its affiliates.