Rhizoctonia root rot is one of the most common soilborne diseases of soybeans. Diseased plants usually occur singly or in patches in the field. Disease is typically more common on the slopes of fields. Rhizoctonia infects young seedlings, causing pre- and post-emergence damping off. Infected seedlings have reddish-brown lesions on the hypocotyls at the soil line. These lesions are sunken, remain firm and dry and are limited to the outer layer of tissue. If seedlings survive the damping off phase, infections may expand to the root system, causing a root rot. The root rot phase may persist into late vegetative to early reproductive growth stages. Older infected plants may be stunted, yellow and have poor root systems.
The fungus survives on plant residue or in soils as sclerotia. When soils warm, the fungus becomes active and infection may occur soon after seed is planted. The fungus grows better in aerated soils; thus, disease is more severe on light and sandy soils. Symptoms may disappear if infected plants grow out of the root rot problems although plants may remain stunted.
Best time to scout is VE through R1. Check plants in fields with poor emergence or slowed early growth.
Variety selection: Resistance has been reported in some varieties; however, there are no varieties being developed for resistance to Rhizoctonia root rot.
Crop rotation: Unfortunately, many strains of Rhizoctonia can infect corn, alfalfa, dry bean and some cereal crops.
Stress factors: Eliminating stress factors, such as use of herbicides that cause injury to soybean roots, can help reduce root rot problems.
Seed treatments: Most fungicide seed treatments, except metalaxyl/mefenoxam, are effective against Rhizoctonia.
Photo by Daren Mueller