No. 14/June 27, 1997
Rhizoctonia and Fusarium Root Rots of Soybeans
We have received a number of calls concerning a generalized rotting of the lower taproot area of young soybean plants. The root system is necrotic (dead) below the infection point and typically turns a reddish to brownish color.
The most common causes of this root rot are two fungi: either Rhizoctonia or Fusarium, with Rhizoctonia being much more common. Both can infect during hot weather and cause a root rot that affects much of the lower root system and can also involve the lower stem along the taproot.
Rhizoctonia infections are favored by moist weather at planting, which allows the seed to germinate but which then turns hot and dry at emergence, delaying the development of a satisfactory root system. The worst infections I have seen were in a field that took almost one full month to develop a full stand. The soil was warm and moist at planting, but conditions changed about 7 to 10 days after planting to very hot and dry. The seedlings struggled for the next 3 weeks, developed very small and poorly functioning root systems, and were an easy target of opportunity for the Rhizoctonia fungus. Rhizoctonia solani, the causal fungus, is not a particularly aggressive pathogen and causes most damage in fields where other factors affect early season growth and development. Weather conditions, in particular, which stress seedlings increase the likelihood of Rhizoctonia infections.
Although Rhizoctonia infections are most easily identified by small reddish to purple lesions along one side of the stem at the soil line, a more generalized rotting can occur in the lower taproot area. One particular feature of infection by this fungus is the attempt by the plant to add additional roots above the infection point. When plants are examined in the field, pay particular attention to the appearance of new roots. If soil moisture is adequate to support the remaining root system, then the new roots should help bring the plant back into productivity. If, however, hot and dry conditions remain and the roots cannot support the existing top growth, then some plant loss is to be expected.
Fusarium infections usually cause a more generalized discoloration of the root system. Roots turn dark brown to black throughout the root mass rather than producing the lesions found with Rhizoctonia. Symptoms include a blackening or browning of the vascular system in roots and the destruction of the lower taproot area. Leaves may become chlorotic (yellow) and droop or drop off.
Unfortunately, little can be done to reduce the impact of either of these diseases at this time. Seed treatments are helpful but are designed to last only about 2 weeks under good conditions. With the prolonged emergence period, much of the effectiveness of the seed treatments is gone by the time plants emerge. Some farmers report success in using cultivation to add soil around the base of plants to encourage more roots to form.
H. Walker Kirby, Extension Plant Pathology, (217)333-8414