The disease is Mycoleptodiscus root rot, and can be easily misidentified as Rhizoctonia root rot. It is caused by the fungus Mycoleptodiscus terrestris. First reported in 1954 and found in southern and central Illinois in the 1970s, the disease had not received much attention from plant pathologists until last year. Plant pathologists in the region saw many diseased soybean plants with symptoms similar to Rhizoctonia root rot, but many plants were not found to be infected by Rhizoctonia solani. The ISU Plant Disease Clinic also received numerous soybean samples with reddish-brown root rot symptoms. Isolation results indicated that many samples were Rhizoctonia root rot, but many others were not.

Reddish-brown lesions common to two kinds of root rot.

The Mycoleptodiscus terrestris is quite different from Rhizoctonia solani and is not easy to identify after isolation. The fungus grows best in warm temperatures of 80-85° F, and, therefore, diseased plants most likely are found in summer. Observations made in southern Illinois indicate that the fungus also attacks soybean seedlings.

A typical symptom of the disease is reddish-brown lesions on basal stems. Root rot by this disease can result in reduction of root weight, according to one greenhouse study. The fungus also causes crown and root rot in alfalfa and red clover. An early study showed that strains of this fungus from red clover also can attack soybean.

Currently, very little information is available on this disease in terms of economic importance and management. It also is unknown why the disease has increased from a minor disease only in southern Illinois to a disease of regional proportions. With so many changes in farming practices, it is not surprising to see changes in plant disease pictures, however. Further, keep in mind that Rhizoctonia root rot has symptoms similar to this disease, but the importance of Rhizoctonia is far less than that of other diseases.

Some people may ask why we should worry whether the root rot fungus is Rhizoctonia or Mycoleptodiscus because symptoms of the two fungi appear the same. However, the information we collect can improve effective disease management. As we know, seedling blight caused by Rhizoctonia can be managed with a chemical treatment and damage of Rhizoctonia root rot beyond seedling stages can be reduced by mid-season cultivation. Cultivation mounds the soil around the base of soybean plants, which promotes new growth of roots. If the root rot is caused by Mycoleptodiscus, management designed for Rhizoctonia may not be effective.

If you see reddish-brown lesions associated with root rot in soybean, do not assume it is Rhizoctonia root rot. Send samples to the ISU Plant Disease Clinic for isolation and identification.