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Phytophthora Infection in Soybeans and Management Options

Phytophthora sojae is a destructive soybean pathogen that can be a major cause of stand establishment problems. Seeds and seedlings can be infected and killed any time after the seed has absorbed moisture. Later in the season following periods of heavy rain, Phytophthora infects and kills soybeans by causing stem rot or chronic root rot.

Soils warmer than 60°F and flooded or saturated clay soils are optimal conditions for infection. Phytophthora infection is most common in fields with poor drainage, but it also occurs in well-drained fields if the pathogen is present and the soils are saturated for 7 to 14 days due to heavy rain or irrigation. 

Stem and root rot phase
Look for Phytophthora infection following rainy periods later in the season. If infected, you’ll see a distinct chocolate-brown lesion moving from the soil line up the stem. Phytophthora will slowly girdle its host causing wilting, yellowing and death.

Phytophthora’s root rot phase is not as easily recognizable as its stem rot phase. Infected plants can be stunted and less vigorous, but this is hard to spot unless the infected plants are near a healthy comparison. Often you will see a general severe wilting of the plant, which can look gray-green from a distance. This is known as “Shepard’s Crook” since you are seeing the undersides of the leaves as they droop. This can be confused with herbicide injury since symptoms commonly run down a row for some distance rather than appearing in a circular area, which is how many diseases express themselves within a field. 
For disease scouting, you are most likely to find the disease in the following places:
  • Low and wet spots in a field
  • Fields with high clay content
  • Fields that have been in no-till for a few years
  • Weedy areas that may be the result of stand reduction earlier in the season
  • Stem canker: The symptoms of Phytophthora can look similar to stem canker. However, one way to distinguish between them is to check whether the diseased plants have root rot. Stem canker does not cause root rot, only stem lesions. 
  • Flooding: Flooding injury can also be mistaken for Phytophthora. If damaged by flooding, the root’s outer area can easily be pulled off, leaving a “rat tails” look.
During the growing season, management options are severely limited since measures must be taken during the planting season to protect against the disease. If Phytophthora is present in your field now, begin planning your management options for next season as the disease is likely to appear again. By looking ahead to planting, you can prevent Phytophthora from damaging more of your soybeans.
  1. Plant resistant varieties: There are 2 types of genetic resistance. Single-gene resistance is very specific to particular races with the most common being Rps1a, Rps1c, Rps1k, Rps3a, Rps6 or a combination of these. Field tolerance levels will vary with all resistance genes.
  2. Use quality seed treatments: At planting, your local Golden Harvest Seed Advisor can determine specific seed treatments that fit your field’s needs. Higher usage of seed treatments can reduce Phytophthora early in the season, but they may offer little help against the stem and root rot phase of infection later in the season. 
  3. Improve soil drainage: This is one of the most important control measures since well-drained fields dramatically reduce the incidence of the disease.
  4. Rotate crops: Crop rotation will not eliminate the disease since it can live in the soil for years. However, planting soybeans continuously will increase the inoculum levels in the soil and increase the disease pressure.
Contact your local Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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