Farms.com Home   Expert Commentary

Scout For Root Rots In Soybean

Jun 22, 2017
By Emmanuel Byamukama
Assistant Professor & SDSU Extension Plant Pathologist
 
Figure 1. Wilting soybean seedlings due to a root rot disease. Notice also reduction on plant stand due to soil crusting.
 
Root Rots Developing in Soybeans
 
Root rots are starting to develop in soybeans. The most common root rot observed currently is Phytophthora root rot (PRR). Rhizoctonia root rot is showing up as well but to a lesser degree. The number of soybean plants per acre is one of the early determinants of soybean yield, yet root rots can adversely affect the plant stand establishment (Figure 1). Now would be a good time to assess soybean plants for root rots. Several factors such as plant pathogens, flooding, soil crusting, and salt toxicity can reduce soybean plant stand and eventually reduce soybean yield. Knowing the limiting factor for plant stand establishment is the first step in preventing plant stand reduction the next growing season.
 
Identification
 
Phytophthora and Rhizoctonia root rots can by distinguished by the symptoms they cause on the lower parts of the stem. For Phytophthora root rot, the rotting extends above the soil line reaching the upper nodes (Figure 2). Rhizoctonia root rot is characterized by reddish brown lesions at the seedling soil line. The lesion expands to girdle the stem making it sunken in older plants (Figure 3).
 
Figure 2. A soybean field in Turner County with Phytophthora root rot (PRR) starting to show up. Notice soil crusting, an indication of excessive moisture and poor drainage which all promote PPR development. Inset: A close-up of Phytophthora root and stem rot symptoms- the dark brown lesion expanding above soil line.
 
Figure 3. Rhizoctonia symptoms on soybean plants. Inset: Notice the sunken lesion girdling the stem, a typical symptom of the Rhizoctonia root rot.
 
Causes
 
Phytophthora Rot
 
Phytophthora root and stem rot is caused by Phytophthora sojae. P. sojae persists in the soil, and commonly infects at the seedling stage, causing pre- and post-emergence damping off. However, this disease can develop on soybeans throughout the growing season especially as plants become moisture stressed. After emergence, infected plants will be clearly visible in low areas of fields and entrance areas which are likely to have soil compaction. P. sojae infection is favored by high soil moisture resulting from excessive rains, poor drainage and heavy clay soil texture.
 
Rhizoctonia Rot
 
Rhizoctonia root and stem rot is caused by Rhizoctonia solani. This pathogen also survives in the soil and on plant debris. It can also cause pre-emergence damping-off but the commonly observed symptom (wilted girdled stems) is at and after seedling stage. Injury to the plant caused by insects, soybean cyst nematode, and herbicides may increase chances of infection.
 
Management
 
Management of Phytophthora root and stem rot is difficult in part due to the many genetic forms (physiological races) of the pathogen. Many P. sojae races are found in South Dakota. A recent soil survey in South Dakota showed the P. sojae resistance genes (Rps) 7, 1a, 1k, 1c, and 1d were frequently defeated indicating that use of these genes may not be effective (Figure 4). Producers should keep a good history of their fields prone to P. sojae, to judge the effectiveness of the resistance genes in the variety planted. Most seed catalogues will have a field tolerance rating for Phytophthora root and stem rot, making it easier to choose plant varieties with genes that are not frequently defeated by P. sojae or cultivars that have stacked resistance genes. Integrating other management strategies (such as seed treatment and improved drainage) is also necessary for effective management of this disease for fields that have a history of Phytophthora root and stem rot.
Click here to see more...