Phytophthora root and stem rot (PRSR) of soybean is a disease that can be a significant problem in wet years such as we are having this year. Cooler-than-normal temperatures, frequent rainfall, and poorly-drained soil favor this disease. PRSR is caused by the oomycete Phytophthora sojae, a soilborne plant pathogen that can survive for years in soil with few or no symptoms that it exists until favorable conditions occur. The pathogen can infect all stages of the soybean. In seedlings and young plants of susceptible cultivars, severe wilting and plant mortality (Figure 1) is a common outcome of PRSR. In older plants, large sunken lesions that generally girdle the lower stem, extend up from the soil line (Figure 2). Optimum temperature for infection is from 77 F to about 82 F. Disease incidence may be random in a field, with affected plants interspersed with apparently healthy plants, or severe wilting of almost all the plants in an area may be associated with low spots, heavy clay soils, or areas with poor internal drainage.
PRSR is controlled primarily with the use of resistant cultivars. There are two general types of resistance, single-gene (R-gene) resistance, which provides a high level of resistance to some pathotypes of the pathogen while being ineffective against other pathotypes, and partial resistance that provides a lower level of protection to the plant. Partial resistance is provided by the action of several genes, and while the level of resistance may be somewhat less spectacular than with R-gene resistance, partial resistance is effective across a much broader range of pathotypes. Generally the most effective control of PRSR via genetic resistance is in cultivars that contain both R-gene and partial resistance.
Improving soil drainage in fields can help minimize PRSR to some degree. In addition, certain seed treatment fungicides (products containing metalaxyl or mefenoxam) can also be useful in minimizing the disease, particularly in combination with resistant cultivars.