Soybean fungal root rot is commonly seen in summer. Soybean plants with root rot are noticeable in fields because of uneven growth. Affected plants may appear yellow, stunted, and wilted. The plants may be scattered or found in large patches; they often occur in fields or areas with poor drainage. Cool weather promotes these root rot problems because cooler than normal temperatures slow soybean root development and are ideal for some soilborne fungi.
Besides Phytophthora, Rhizoctonia and Fusarium are two fungi causing the root rot problems. Infected plants may have poor root systems and poor nodulation with reddish brown or dark brown lesions on taproots. For the root rot caused by Rhizoctonia, reddish brown lesions are seen on basal stems. Symptoms of Fusarium root rot include dark discoloration, although some Fusarium species can cause reddish brown discoloration on soybean roots. Fusarium root rot often is found in low spots in a field.
In general, plants can grow out root rot problems and symptoms disappear as the season progresses into late July and August. With severe infection, which is rare, infected plants wilt and die in patches. Root rot damage caused by Rhizoctonia and Fusarium can be reduced by cultivation. Cultivation warms soil and improves other soil conditions that promote new growth of roots.
|Fungal root rot of soybean.|
For management of the next soybean crop, it helps to take good notes on fields with severe root rot. Consider using a seed treatment with the next soybean crop to reduce seedling disease because root rot is a continuation of seedling rot. There are no resistant varieties available to Rhizcotonia root rot and Fusarium root rot.
Keep in mind that factors other than fungi can negatively affect plants and also interact with root rot fungi. Some common factors include iron chlorosis, herbicide stress, and potassium deficiency and these factors can weaken plants, making them more susceptible to pathogenic soilborne fungi. If the root rot is associated with a stress factor, elimination of the stress is important to root health. See the July 2, 2001, issue of ICM for an article  on iron chlorosis management.
This article originally appeared on page 140 of the IC-486(17) -- July 9, 2001 issue.