Rhizoctonia Root Rot in Soybean

By Loren J. Giesler, Former Extension Plant Pathologist

Pathogen

Rhizoctonia root rot is caused by the soilborne fungus Rhizoctonia solani. This fungus can be found in most soils and survives as sclerotia (very resistant fungal survival structures) in soil. Yield losses have been reported to be up to 48% in theU.S., but we typically do not see this level of disease in Nebraska.

Disease Symptoms

Both pre-emergent and post-emergence seedling death can occur with this disease. Pre-emergence symptoms are seed decay and are often not visible in the field. Post-emergence 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 and unthrifty 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 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.

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 which 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. Midseason cultivation has been shown to promote new root growth which can reduce damage.

NebGuide

Soybean Cyst Nematode: Identification and Management (G1383)

Chemical/Biological Control

Seed treatments containing a strobilurin fungicide can be used to manage this disease. Check the active ingredients in the seed treatments you are purchasing on company treated seed as many do not contain a strobilurin.