Reports have recently come in concerning Rhizoctonia infections in soybeans. Rhizoctonia root and stem rot of soybeans, caused by Rhizoctonia solani, is a common damping off or seedling disease. The pathogen can last several years in the soil and on crop residue. Rhizoctonia typically does not result in extensive yield loss, although the pathogen has been known to cause 50% stand loss in susceptible soybeans.
Infection normally takes place in wet soils but the disease may then persist under warm, dry conditions. Infection is favored under warm, moist conditions in well drained or sandy soils. Stress or injury can encourage Rhizoctonia infections. Herbicide injury, soil insect feeding, cyst nematodes, low soil fertility, hail, and sandblasting all appear to foster infections. Some herbicides cause stress to the soybean plant but may also inhibit certain microorganisms in the soil that compete with Rhizoctonia. Frequent soybean cropping can result in a buildup of the Rhizoctonia pathogen in the soil and on crop residue. (Disease Triangle – Diseases occur when there an infectious pathogen is present, a suitable host is available, the proper environment prevails, and there is sufficient time for infection to take place.)
Rhizoctonia causes sunken, reddish lesions that can vary in size. These lesions will normally show up on one side of the soybean hypocotyl or stem, near or below the soil line. The lesions are usually small and contained on the soybean stem or taproot, but lesions can occasionally penetrate into the tissue and surround, or girdle, the hypocotyl or stem. When lesions girdle the taproot, the soybean plant often responds by sending out adventitious roots above the infected area.
Infected plants may survive and continue to host the disease through the season. On older plants, the pathogen may cause a dry, reddish, cortical decay. Surviving plants may be stunted, with pale or yellowish foliage. These plants may also snap off later in the season under windy conditions. Rhizoctonia symptoms are easily confused with symptoms of Fusarium infection and with heat canker in sandy soils.
Rhizoctonia root and stem rot infection can be managed by managing stress factors and by use of seed treatments, especially those containing strobilurin fungicides. Plant soybeans in warm, well drained, well prepared, fertile seedbeds. Avoid high herbicide application rates, where possible. There are no rescue treatments for Rhizoctonia, although some growers have promoted adventitious root growth by cultivating soil up around the soybean stems.