Overview: Rhizoctonia root rot is one of the most common soilborne diseases of soybean. It is worldwide in distribution. Regional epidemics are often sporadic over years.
Symptoms: Preemergence seed rot and postemergence damping-off are most common. Infected plants have reddish brown lesions on the hypocotyl at the soil line. As the season progresses, root symptoms can be found in plants at advanced vegetative to reproductive growth stages. These plants may have become infected as seedlings with lesions on these plants continuing to enlarge down girdle the stem. Infected plants are stunted, yellowing, and have poor root systems as lateral roots decay often with only the tap root and secondary roots remaining.
Causal Organisms: Rhizoctonia solani (teleomorph Thanatephorus cucumeris) differs from other Rhizoctonia species in hyphae morphology. The fungus is characterized by dolipore septa and hyphal branches at right angles that are constricted at their point of origin. Hyphae are brown and have multinucleate cells. Sclerotia do not have a rind and are the primary survival structure of the fungus. The fungus is readily isolated on water agar from freshly infected tissues.
Disease Cycle: The fungus is saprophytic and survives in soils in the absence of host plants as sclerotia or as mycelium colonizing plant debris. The food base in soils also affects parasitism and inoculum potential. Populations decline sharply when soils are flooded for a long period of time or are irrigated. Survival rate is reduced by high soil temperature, solarization, and fallow for a year. Infection begins soon after seeds are planted. After emergence, the primary point of invasion may be in the hypocotyl area at the soil line. If seedlings survive the damping-off phase, infections may continue to develop into root rot when lesions expand down to the root system.
Management: Resistance to R. solani is not available, but variation in tolerance has been reported. Reduce other stresses, such as herbicide injury, poor soil, insect damage, and soybean cyst nematode feeding.