Sept. 19, 2012

We recently received a sample of mature soybean with root rot from southern Nebraska. We visited the farm and confirmed that the disease is caused by a soil-borne fungal pathogen Rhizoctonia solani.

Symptom and Pathogen

Rhizoctonia damage is more severe on seed and young seedlings with seed rot and root rot. It is often found in patches in field (Figure 1). The roots and hypocotyl may have a shrunken, reddish brown lesion, which will be dry when decayed. This differs from the water-soaked lesion typical of a Pythium or Phytophthora infection. On older plants, the lesion is characterized by a reddish brown dry cortical root rot extending into the base of the stem (Figures 2 and 3). The foliar symptoms include leaf yellowing (Figure 1).

Diseases Cycle and Epidemiology

This fungus is saprophytic and can survive in soils or plant residues as sclerotia for many years. Soybean can become infected at any stage, but damage is more severe when it occurs at the seedling stage. Rhizoctonia also infects other legume crops, sugar beets, some vegetable, and weeds. The disease is favored by warm, wet conditions. Sometimes herbicides may cause plant stress and inhibit beneficial microbes that limit the growth of Rhizoctonia root rot of soybean. Insect injury, soybean cyst nematode damage, hail injury and mechanical damage from equipment also can create opportunistic entry points for the disease.

Management

There is no method to manage the disease at the mature stage. Planting high quality, disease-free, certified seeds is an effective way to prevent Rhizoctonia. If you suspect the seed carries the pathogen, it should be treated with a strobilurin fungicide. Additional measures to limit disease development include crop rotation, avoiding herbicide damage, avoiding soil compaction, maintaining soil fertility and pH, planting in a warm and firm seedbed, planting at the proper plant depth, and improving soil drainage.

Bo Liu
Extension Plant Pathologist, West Central REC, North Platte
Robert Klein
Extension Western Nebraska Crops Specialist, West Central REC, North Platte