Identification and Management of Phytophthora Root Rot in Soybean
October 25, 2023
Phytophthora Root Rot (PRR) is a disease that can affect soybeans throughout the traditional soybean growing areas in the United States and can lead to reduced yields and stand counts. The common pathogen that infects soybean, resulting in PRR, is known as Phytophthora sojae, a water-borne mold that releases spores when sufficient moisture is reached for transmission. There are many races of P. sojae that occur. When early-season rains leave soils saturated, infection can occur in many of the plants. Infection can occur with treated seed and in resistant soybean products. The difference is that the chemistry and types of PRR resistance the soybean product has can halt spread of the pathogen within the plant or minimize symptom severity despite infection. Replanting may be necessary to overcome lost populations and reduced yield potential. While early season is certainly the most common time of infection, it can occur throughout the growing season. It is important to note that PRR can be misidentified as Stem Canker; therefore, correct identification is important to help understand the true issue in each field.
Symptomology
When understanding symptomology of PRR, it can be helpful to note that it prefers soils that have poor water drainage, such as clays or areas of heavy compaction.
Early Season:
- Begins as deterioration of roots and moves to stems.
- Roots become brown and rotted causing death from damping-off (Figure 1).2
Mid/Late Season:
- Canopy shows yellowing, then begins to wilt, but typically leaves stay attached to the plant.
- Brown lesions develop on roots.
- Inner tissues develop a tan to brown discoloration.2
- Tolerant soybean products may only exhibit root rot.1
Management
Field Tolerance or partial resistance is a natural aptitude of a soybean product for protection against Phytophthora sojae.1 This information is typically found in the product description and is observed and recorded diligently in the research and development process and commercial trials. Although this is an essential piece of fighting PRR, it is not a complete solution and must be used in conjunction with other management methods.
Resistant Genes or Rps (race specific) genes give tolerance to plants against specific races of the pathogen. Soybean products are available with resistance genes Rps1a, Rps1c, Rps1k, Rps3a, or Rps 1k+ 3a.1 While these genes provide excellent tolerance against the specific race, it is recommended to couple this with other forms of management to help assure better results.
Fungicide Seed Treatments are a great choice for areas where PRR is a recurring issue. They can provide greatly reduced incidence of damping off in seedling populations and can maximize the use of Rps genes and field tolerant soybean products. A great example of this type of seed treatment would be both Acceleron® Seed Applied Solutions Standard or Acceleron® Seed Applied Solutions Basic for coverage of early season infections of Phytophthora (learn more here).3
Soil Drainage can play a major role in managing populations of P. sojae; reducing compaction and poor water drainage can positively affect infection levels. Phytophthora zoospores are produced only when the soil is saturated; if soils are not saturated early in the season, soybean products with partial resistance can escape infection and remain PRR-free throughout the season.1
Christian McGuire
Channel Agronomist
Sources:
1Martin, D. and Dorrance, A.E. 2019. Phytophthora damping-off and root rot of soybean. PLPATH-SOY-04. Ohioline. The Ohio State University. https://ohioline.osu.edu/factsheet/plpath-soy-04.
Web site verified 9/21/23. 1110_292101
