Phytophthora Root and Stem Rot in Soybeans

Phytophthora root and stem rot (PRR) is a root and stem disease specific to soybeans that can infect at any growth stage. Part of PRR’s life cycle depends on soil moisture for the disease to swim to infect plant roots. With more soil moisture this growing season, we expect to find more PRR symptoms in the field this season.

• Symptoms appear as brown lesions that extend upwards from the soil line, and roots are discolored and unhealthy. Plants will be wilted and leaves remain attached to the plant. Infection occurs in patches or as randomly-infected plants, often in headlands.
• One look-alike to PRR is Northern Stem Canker where plants will appear wilted and there will be a brown lesion along the stem. But with stem canker, roots will appear healthy and the lesion won’t extend upwards from the soil line. Stem canker often also infects following stem damage from hail or storms.
• Genetic resistance is available to help manage PRR, and there are two types:
• Major gene or pathotype-specific resistance: an Rps (Resistance Phytophthora sojae) gene is completely resistant to specific pathotypes of PRR in the field’s soil. Multiple pathotypes of PRR can occur in a field, so knowing which are present in the soil is crucial to use this variety resistance.
• Last year, roughly 70 soybean fields were soil sampled for pathotype identification through the soybean disease survey. Of those fields, 83% had Phytophthora sojae present in the soil. Most commonly, pathotypes present were able to defeat Rps 1c and 1k, while Rps 3a and 6 offered the most protection versus the pathotypes present in the soil.
• There is now a commercial soil test available to identify the pathotypes of PRR in a field’s soil from AYOS technologies.
• Field tolerance or partial resistance: several genes resist infection within the plant and the plant is better able to tolerate the PRR infection without dying. This resistance is only expressed after the first true leaves are open, so it needs to be used in combination with an effective seed treatment.