Rhizoctonia Root Rot in Soybean | CropWatch

By Dylan Mangel, Extension Plant Pathologist

Pathogen

Rhizoctonia seedling blight and root rot is caused by the soilborne fungus Rhizoctonia solani. This fungus can be found in most soils and survives as sclerotia (durable fungal survival structures) in soil. Yield losses have been reported to be up to 48%, but we rarely see this level of disease in Nebraska at a field scale.

Disease Symptoms

Both preemergent and postemergence seedling death can occur with this disease. Preemergence symptoms are seed decay and are often not visible in the field, presenting only as skips in the row. Postemergence symptoms on seedlings will be the appearance of brown to reddish lesions on 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 throughout the season or, less commonly, will die. Often the stand will appear uneven because of stunted plants.

On older plants, the pathogen causes a reddish-brown dry cortical root rot that may extend into the base of the stem. Later in the season, infections at the base of the plant (cortical rot) may result in plants snapping off during high winds. Root rot can greatly reduce nodulation. Foliar symptoms may include yellowing or wilting of leaves.

Damage from Rhizoctonia is commonly observed in areas where 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.

Favorable Environmental Conditions

Damage is usually more common in warm, moist sandy soils, often on the south-facing hillsides. This disease is favored by well-drained soils and stress to the plant.

Stresses that have been observed to favor disease development include herbicide injury, soil insect damage, hail, sandblasting, and soybean cyst nematode feeding.

Management

Cultural Practices

Avoid any stress to the crop as much as possible. Use appropriate management actions in fields with soybean cyst nematode. Mid-season cultivation has been shown to promote new root growth which can reduce damage.

Chemical/Biological Control

Several seed treatments offer good control of Rhizoctonia. Data and information on these treatments can be found in the Crop Protection Network publication, "Fungicide Efficacy for Management of Soybean Seedling Diseases." These products now include chemistries from FRAC groups 3 (DMI Triazoles), 7 (SDHI), and 11 (QoI Strobilurins).

Additional Resources

Updated from a previous article by Loren J. Giesler, former extension plant pathologist

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