Connie Strunk
Plant Pathology Field Specialist
Phytophthora root and stem rot is making an appearance in South Dakota soybean fields. Phytophthora continues to be one of the most damaging diseases found in South Dakota soybeans as this fungal pathogen (Phytophthora sojae) survives in the soil and in infected residue causing poor stand establishment, as well as plants that wilt and die prematurely (Figure 1). Phytophthora root and stem rot infection is favored by soils with poor drainage and soils which are also compacted.
Figure 1. A portion of a field showing poor stand establishment, wilting, and dying soybean plants due to being infected with Phytophthora root and stem rot. Photo: Emmanuel Byamukama.
Seed decay (seed rot) and pre- and post-emergence damping-off, can occur when plants are infected with Phytophthora sojae at the seedling growth stage. Plants that have emerged and escaped damping-off may still be infected and may later show symptoms of Phytophthora infection when plants are under stress such as moisture stress. Infection starts in the roots and progresses upwards on the plant stem above the soil line. A characteristic symptom of the stem rot phase of the disease is a dark brown color on the stem and lower branches (Figure 2.). Infected plants are often found in the low lying areas of the field but may also be hidden underneath the canopy of other plants.
Figure 2. Note the brown lesion extending up the soybean plant from the soil line. This is a characteristic of Phytophthora stem rot. Photo: Emmanuel Byamukama.
Managing Phytophthora root and stem rot can be difficult simply due to the fact that there are many (more than 50) physiological races (genetic forms) of this pathogen found in North America with many of these races found affecting soybeans in South Dakota. Management options for Phytophthora include selecting plant varieties which have higher levels of field tolerance, which is a general non-race specific type of resistance. Also, growers are encouraged to utilize varieties which incorporate different races such as Rps1K, Rps3a, Rps6, or a variety that is stacked (several different races incorporated into the seed). Growers are also encouraged to keep field records on fields which have had Phytophthora so that they can evaluate the effectiveness of the resistance genes in the variety that they have selected and used. Other management tools that growers can employ include improving soil drainage and avoiding soil compaction. Fungicide seed treatment is recommended for fields with a history of damping-off. Refer to the Soybean Seed Treatment section in the 2014 South Dakota Pest Management Guide for Soybeans.
Source : SDSU