Phytophthora root and stem rot is an economically important disease of soybeans that is most severe in poorly drained soils. Diseased plants often occur singly or in patches in low-lying areas of the field that are prone to flooding. Phytophthora sojae can infect soybeans at any growth stage from seed to maturity. Early season symptoms include seed rot and pre- and post-emergence damping off. Stems of infected seedlings appear water-soaked, while leaves may become chlorotic and plants may wilt and die. On older plants, symptoms vary depending on the variety. For susceptible plants, leaves become chlorotic between the veins and plants wilt and die, with the withered leaves remaining attached. Varieties that are not fully susceptible may appear stunted, but plants are typically not killed.
The most characteristic symptom of Phytophthora root rot, however, is a dark brown lesion on the lower stem that extends up from the taproot of the plant. The lesion often reaches as high as several nodes and will girdle the stem and stunt or kill the plant.Phytophthora sojae survives on crop residue or in the soil as oospores. When soil temperatures reach 60ºF, oospores germinate to produce structures that release swimming spores, called zoospores, under saturated soil conditions. The zoospores are attracted to soybean roots. Infection occurs via the roots, and from there the pathogen colonizes the roots and stems. Disease is most common in poorly drained soils, but may occur in other soils as well.
The best time to scout is VE through R6;1-2 weeks after excessive rain. It occurs in wet, waterlogged soils and compacted soils.
Variety selection: Phytophthora root rot is best managed by planting resistant varieties. Many race-specific resistance genes (called Rps genes) to Phytophthora sojae have been identified in soybean breeding lines. Some of these genes have been incorporated in commercial soybean varieties; thus, there are soybean varieties available that have complete resistance to a specific race of Phytophthora sojae. There are numerous races (now called pathotypes) of Phytophthora sojae, and many pathotypes can exist in a single field. Furthermore, new pathotypes can develop that can infect varieties with specific Rps genes. Partial resistance is available to Phytophthora sojae. Partial resistance is effective against all races of Phytophthora sojae; however, it is only expressed after the first true leaves emerge, not in very young seedlings.
Crop rotation and tillage: Continuous soybean production may increase disease severity. But rotation to non-hosts may not reduce disease severity because oospores can survive in soil for long periods of time. Disease is more severe in no-till fields because these fields can be wetter. If tillage is considered to improve drainage, use proven conservation tillage practices to maintain soil quality.
Seed treatments: Where Phytophthora sojae is a problem, seed treatments with mefenoxam or metalaxyl as an active ingredient can provide some protection. Seed treatments are especially helpful with poor quality seed and in fields with a history of this problem.
Photo by Daren Muller