Phytophthora root and stem rot is showing up in soybeans in South Dakota again this year as evidenced by portions of fields with poor stand establishment as well as large numbers of wilting and dying plants. The main cause of wilted plants around this time (flowering) is Phytophthora root and stem rot. Phytophthora root and stem rot is one of the most damaging diseases of soybeans in South Dakota. This disease is caused by a fungal pathogen, Phytophthora sojae, a fungus like microorganism that survives in the soil and in infected residue. Infection is favored by poor drainage especially in heavy clay soils and compacted soils. Low wet spots in the field entrance areas have a higher percentage of infected plants (Fig. 1).
P. sojae and closely related fungal like microorganisms commonly infect at the seedling stage, causing pre-emergence damping off, seed rot as well as post-emergence damping off and seedling blight, pre-damping or post-damping off can occur. Infected plants at this phase will be clearly visible in low areas of fields, but may also be hidden underneath the canopy of nearby plants within the row. (Additional information on seedling diseases may be obtained in the iGrow article: Scout For Seedling Diseases)
Plants that have emerged and escaped damping-off may still be infected and will later show root and stem rot. Infection starts in the roots and progresses upwards on stem above the soil line. A characteristic symptom of the stem rot phase is a dark brown color on the stem and lower branches (Fig. 2). By this time, roots will be rotted and the infected plants can easily be pulled from the soil. Often plants with the stem rot phase of the disease will still have a functional tap and major lateral roots and cannot be easily pulled from the ground. Symptom progression up the stem will depend on susceptibility of the variety as well as time of infection, soil type, and weather conditions. (Fig. 3).
Management of Phytophthora root and stem rot is difficult in part due to many genetic forms (physiological races) of the pathogen. Over 50 races of this fungus occur in North America. Many of these races are found in South Dakota. Most of the genes that have been incorporated into soybean for resistance to Phytophthora are vulnerable to races found in South Dakota, including Rps1k. Variety selection based on knowledge of races in a particular field would be helpful, but this is not possible for most South Dakota farmers. Farmers should keep a good history of their fields prone to Phytophthora, so that they may judge the effectiveness of resistance genes in their varieties.
In the absence of specific information for fields with recurring Phytophthora issues, current strategy would be to plant varieties with Rps1k, Rps3a, Rps6 or a combination thereof (stacked). Farmers should also try to select varieties with higher levels of field tolerance, which is a general non-race specific type of resistance. Most seed catalogues will have a field tolerance rating for Phytophthora. Since resistance genes for Phytophthora are vulnerable to defeat from new emerging races, farmers should not depend solely on major gene resistance. Therefore integrating several management strategies is necessary for effective management of this disease.
Improving drainage (avoiding soil compaction and draining wet spots) lessens chances of Phytophthora infection. Fungicide seed treatment is recommended for fields with a known history of damping-off (see the Soybean Seed Treatments section of the 2013 South Dakota Soybean Crop Protection Guide publication). Use of fungicides as an in-furrow or banding application, although effective, has not generally been incorporated into soybean production because of product costs.