NPIPM:Rhizoctonia root rot on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, and Dean Malvick
Causal Organism
Rhizoctonia root rot is caused by a soilborne fungal pathogen Rhizoctonia solani. There are 12 distinct groupings of R. solani, known as anastomosis groups (AGs). The causal agents of root rot on soybean are primarily members of G 2-2 and AG-4 although isolates from other AGs are known to also cause Rhizoctonia root rot.
Symptoms and Signs
Soybean seedlings are most vulnerable before the first trifoliate leaf develops. Infected plants develop red-brown lesion on the hypocotyls (the stem of germinating seed) near the soil line. The lesions may develop and girdle the germinating seedlings altogether, causing post-emergence damping-off. In cases where the seedlings survive, root rot may develop between late vegetative and reproductive development stages or plants may recover completely.
Infected plants may appear stunted with yellowing leaves. Plant wilt may follow under heat and drought pressures. However, if soil moisture is adequate, infected plants may not show these above ground symptoms.
Infected plants may be scattered individually or in groups across the field.
Disease Impacts
In a severe infection where most plants in a field were infected, a yield reduction of 48% was recorded in Iowa.
Life Cycle and Epidemiology
Rhizoctonia solani can survive in the soil as sclerotia (a survival organ) or mycelium in the soil or plant debris. This fungal pathogen can live as saprophyte in the soil, and thus can live without a host plant for long periods of time. Surviving fungal structures act as primary inoculum early in the season. Rhizoctonia solani infect the plants' hypocotyls at or near the soil line. Infected plants may die depending on the pathogen virulence and environmental conditions. In surviving plants, the pathogen can spread to the root system and cause root rot of the lateral roots. As the lateral roots decay, the plants become vulnerable to excess heat and drought.
Warm soil with high humidity is favorable for R. solani development on host plant. Usually, temperatures ranging between 78.8 and 89.6°F (26-32°C), soil moisture between 30-60% and soil pH above 6.6 are considered conducive for the development of Rhizoctonia root rot. The impact of the disease is exacerbated by nutrient and water deficiency.
Management Approaches
Cultural Methods
Tillage may lead to better soil drainage which in turns reduces favorable conditions for disease development. Injuries by nematodes and herbicides may increase plant susceptibility to rhizoctonia root rot and, thus, should be minimized.
Host Plant Resistance
Commercial soybean varieties vary in their response to R. solani. In a greenhouse study, some soybean cultivars showed partial resistance against R. solani. Other cultivars may also show similar reduced severity rating when infected with R. solani.
Chemical and Biological Control
Seed treatments with strobilurin fungicide or biological control agents are available to assist in managing R. solani.
