The number of soybean plants per acre is one of the early determinants of soybean yield. Now would be a good time to assess soybean plant stand. Several factors can reduce soybean plant stand and eventually reduce soybean yield including; plant pathogens, flooding, soil crusting, and salt toxicity. Knowing the limiting factor is the first step in preventing plant stand reduction the next growing season. Soil-borne pathogens are the main cause of reduced soybean plant stand. Several soybean fields scouted this week had root and stem rots starting to develop. The most common root rot was Phytophthora root rot (Figure 1) but very few fields had Rhizoctonia root rot (Figure 2).
Figure 1. (Above) A soybean field in Turner County starting to develop Phytophthora root rot (PRR). Infected plants turn yellow and show wilting symptoms. Inset shows typical symptoms of PRR. The dark brown lesion extends above the soil line and is not sunken. Phytophthora and Rhizoctonia root rots can by distinguished by the symptoms on the stem. For Phytophthora root rot, the rotting extends beyond the soil line reaching upper nodes (Figure 1 inset). Rhizoctonia root rot is characterized by reddish brown lesions at the soil line in seedlings. The lesion expands to girdle the stem making it sunken in older plants (Figure 2 inset).
Figure 2. (Above) Rhizoctonia symptoms on soybean plants. Inset: Notice the sunken lesion girdling the stem, a typical symptom of the Rhizoctonia root rot. Phytophthora root and stem rot is caused by Phytophthora sojae. This pathogen persists in soil either on soybean residues or free in soil after the residues decompose. P. sojae commonly infects at the seedling stage, causing pre and post emergence damping off. After emergence, infected plants will be clearly visible in low areas of fields, but may also be hidden underneath the canopy of nearby plants within the row. P. sojae infection is favored by high soil moisture resulting from excessive rains, poor drainage and heavy clay soil texture. Rhizoctonia root and stem rot is caused by Rhizoctonia solani. This pathogen also survives in soil and on plant debris. It can also cause pre-emergence damping-off but the commonly observed symptom is at and after seedling stage. Unlike PRR, Rhizoctonia infection is promoted by light and sandy soils. Also injury to the plants caused by insects, soybean cyst nematode, and herbicides may increase chances of infection. 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. Most of the genes that have been incorporated into soybean for resistance to Phytophthora are vulnerable to races found in South Dakota including Rps1k. Producers should keep a good history of their fields that are prone to Phytophthora, so that they may judge the effectiveness of resistance genes in their varieties. The best strategy would be to plant varieties with Rps1k, Rps3a, Rps6 or a combination thereof (stacked). Most seed catalogues will have a field tolerance rating for Phytophthora. Integrating several management strategies (such as seed treatment and drainage) is necessary for effective management of this disease. Resistance to Rhizoctonia is not available but plants sometimes can tolerate mild Rhizoctonia infection. Residue management and seed treatment may help in reducing the chances of Rhizoctonia infection.
This article originally appeared in South Dakota State University’s iGrow newsletter and has been reposted with permission.
Emmanuel Byamukama is a plant pathologist with South Dakota State University Extension.