Phytophthora Root and Stem Rot
Phytophthora root and stem rot is a common soilborne disease caused by a water mold, Phytophthora sojae. In poorly drained soils, Phytophthora can have a sever economic impact.
The most common early season symptoms of Phytophthora are areas of no stand and pre- and post- emergence damping off. Later in the season, Phytophthora symptoms on older plants include a brown lesion on the lower stem that extends from the root into the upper portions of the canopy. Symptoms may also include chlorosis of leaves, wilting, stunting, and death.
Favorable Environmental Conditions
Planting into saturated, warm (>60°F) soils with a history of Phytophthora root and stem rot poses the greatest risk for disease. Disease symptoms is most common in low-lying areas that are poorly drained and prone to saturation or flooding. Planting into no till also poses a risk for disease development due to increased water retention and higher inoculum density in the top 2 to 3 inches.
The best management tools for Phytophthora is the use of resistant varieties. There are two forms of resistance: race specific (Rps genes) and partial resistance. Rps genes will provide complete resistance to certain P. sojae populations. The most common Rps genes are Rps 1a, Rps 1c, Rps 1k, Rps 3a, and Rps 6. Partial resistance offers some resistance to all races of P. sojae. However, partial resistance is not expressed until the first true leaves have developed, leaving emerging seedlings still vulnerable. Other disease management options include some form of and/or tiling to improve drainage and seed treatments with active ingredients mefenoxam, metalaxyl, or ethaboxam.