Rhizoctonia Root Rot

By Berlin Nelson, Professor, Dept. Plant Pathology

The fungus Rhizoctonia solani causes pre- and post-emergence damping-off and root rot of young and adult plants. When soil populations of Rhizoctonia are high, pre- and post-emergence damping-off can reduce stands by 50 percent or greater. Generally, Rhizoctonia on soybeans is a seedling disease, but damage has been observed on older plants. The pathogen survives in the soil and is common in this region. The higher the amount of the pathogen in the soil, the greater the potential damage to the plants.

Symptoms

Symptoms consist of seed decay and brown to reddish lesions on seedling stems and roots just below the soil line. These reddish brown lesions may become sunken and girdle the stems and kill the plant. Plants may often appear stunted and unthrifty throughout the season or, less commonly, will die. Often the stand will appear uneven because of stunted plants. Disease is often found in patches in fields. On older plants, the pathogen causes a reddish brown dry cortical root rot that may extend into the base of the stem. Root rot can greatly reduce nodulation. Foliar symptoms may include yellowing or wilting of leaves. Plants infected with Rhizoctonia may not be able to tolerate stresses such drought or hail that occur later in the season. Infected plants may wilt on hot dry days. Observations in North Dakota suggest that Rhizoctonia root rot is more severe in plants showing iron chlorosis. Damage from Rhizoctonia is commonly observed in areas when there is a long history of soybean production with close rotations or during weather conditions not favorable for seed germination and rapid growth of seedlings. Damage is usually more common in cool, wet springs when conditions are not favorable for plant emergence.

There are various anastomosis groups (AG) of R. solani. AG-4 and AG-5 are most common on soybeans but AG-2-2 and AG-3 are occasionally found. AG-2-2 can be highly pathogenic, especially at high temperatures and is more likely found on adult plants. AG-3, generally found on potatoes, is weakly pathogenic on soybeans. AG-4 and AG-2-2 are also common on sugarbeet. Because R. solani has a wide host range that includes many broadleaf crops grown in this region, crop rotation practices may affect severity of disease.

Management.

Crop rotation to non-susceptible hosts such as small grains will reduce populations of Rhizoctonia in the soil. Avoid close rotations with sugarbeets if there is evidence of Rhizoctonia in the field. Close rotations with dry beans may also increase incidence of disease. Protective seed treatments and good seedbed preparation can reduce damping-off. Seed treatments with bacteria (biological fungicide) designed to colonize the root systems has shown some control of root rot. Cultivating soil to hill up around stems promotes lateral root growth and may lessen the effect of root rot on older plants. Cultural practices to reduce stresses on the plant will help reduce the damage from root rot.