Soybean root development has been slower than normal this season due to the cool soil temperatures and extreme moisture. These conditions, however, are ideal for the development of some pathogenic fungi. The ISU Plant Disease Clinic has received soybean samples with root rot problems and we expect to receive more samples for identification in late June. Root rot problems can be mistaken as herbicide injury and, worse, soilborne fungi interact with herbicide stress.
What is root rot? In Iowa, Fusarium, Rhizoctonia, and Phytophthora cause most of the soybean root rot. Soybean plants having fungal root rot are noticeable in fields with uneven growth. Infected 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. The infected plants may have poor root systems and poor nodulation with reddish brown or dark brown lesions on taproots. The distribution of plants injured by herbicides may follow the spray pattern uniformly across a field or a portion of a field. Soybeans injured by herbicides also may have reddish brown discoloration on taproots.
Stress and root rot. Stress factors are often associated with severe fungal root rot. Soybean samples submitted to ISU for diagnosis often have fungal infections and symptoms caused by other abiotic factors, such as iron chlorosis, potassium deficiency, and herbicide injury. In normal years, fungi do not cause significant damage to soybeans. When other stresses occur and conditions are favorable for disease development, fungi can cause severe root rot problems. Soybean seedlings become more susceptible to pathogenic soilborne fungi when weakened by other stresses.
Management of the problems. Because there is no resistance to Rhizoctonia or Fusarium root rot, agronomic practices that enhance plant vigor are most effective in preventing root rot. There are no remedial strategies to cure root rot after it is established. If plants only have reddish brown discoloration in the taproot without root rot, this discoloration should not be a concern. If plants with this reddish brown discoloration have light root rot, they often can recover from the light fungal infection if environmental conditions are favorable and other stresses are not a factor. Use cultivation to mound soil around the plant base and promote root growth.
When severe root rot is associated with other stresses, elimination of the stress factors is important to root health. If root rot is prevalent in a field, it may be appropriate to delay herbicide application until plants are more vigorous. However, the size of weeds, the severity of the weed infestation, and the size of the soybeans must be considered; follow label recommendations. There is no remedial action that can be taken if herbicide stress enhances the root rot problems. If iron chlorosis is the primary reason for root rot, consider the use of iron- chlorosis-tolerant varieties.
The use of herbicide-resistant soybean may be an approach to reduce some herbicide-stress-induced root disease problems commonly observed in soybeans. ISU plant pathologists, agronomists, and weed scientists are working on ways that herbicide-tolerant soybean could be used to manage root rot diseases.
This article originally appeared on pages 118-119 of the IC-480(15) -- June 22, 1998 issue.