Phytophthora Root Rot (GROWMARK, Inc.)
- Phytophthora root and stem rot in soybeans is favored by warm, wet soil conditions.
- Infection often occurs after planting but can occur season long
- Soil drainage, resistant soybean lines, and seed treatments are all used to help manage Phytophthora root rot.
Phytophthora root and stem rot is generally favored with saturated soils and warm weather. The Phytophthora sojae organism is a fungus-like pathogen (Oomycete) that is fairly host-specific. Recent rainfall and warm conditions have resulted in more reports of this disease. This is because this pathogen can infect and cause issues throughout the season.
What does P. sojae do?
Spores in the soil swim in free water and infect roots. There they cause root decay and may continue to grow and decay the lower stem, resulting in a characteristic leasion that runs from the soil line to the first 1-3 nodes (see figure). The central portion of the stem will often be rotted and brown (see figure). This activity inhibits water and nutrient uptake by the plant. Rapid blighting death of the plant can occur, especially if a period of dry, hot weather occurs after infections have taken place. Positive confirmation, especially if characteristic symptoms are not present, can be obtained by sending samples to your state plant diagnostic clinic. There they will run a anytibody test or specific PCR test to quickly and reliably detect the pathogen, if present.
Points to Remember
A field contains many distinct "races" of P. sojae. Different races are able to avoid detection by specific RPS genes in the plant. Rps 1k, Rps 3a, and Rps 6 resistance formerly provided the best protection against Phytophthora, but new races that overcome varietal resistance have shown up in many areas. This means that if a field contains 50% of individuals of a race that can overcome 1k, and 1k was the RPS gene in your cultivar, that resistance only protects against 50% of that field's P. sojae population. RPS resistance is active as soon as the seed goes into the ground. If you have issues with Phytophthora, switch to a cultivar with a different RPS gene, or multiple genes, the next time you plant soybeans in that field.
Field tolerance is a general form of resistance that will slow down all races of P. sojae to some extent. The caveat is that this type of resistance does not start to be expressed until around V1/2. This means that after planting, soybeans are not benefitting from this resistance until 3 weeks or so after emergence. If you have issues with P. sojae, make sure to select cultivars that have excellent field tolerance to Phytophthora.
Seed treatments containing Oxythiopiprolin, Ethaboxam, Mefanoxam, or Metalaxyl are efficactious against P. sojae, but remember that these are only providing enough gas to get that seedling to that first leaf stage. After that point most of the active ingredient has been locked up by organic matter, degraded by microbes or the environment, and been diluted in the plant and root zone. Do not expect more than 3 weeks of protection from P. sojae when using a seed treatment. Remember, these are not fumigating the soil, and P. sojae survives for many years in the soil as resistant structures. Fungicides will not impact this pathogen while it is dormant. If you have issues with P. sojae, make sure you are using a seed treatment with active ingredients mentioned above. For more information, contact your local FS Crop Specialist.