Soybean root rot now and then

During the past three years, soybean fungal root rot had become a significant concern by early summer. The problem this year is worse than in 1995, and at least as noticeable as it was last year. It's common to see soybean fields with uneven growth or patches of the plants that are yellowing and stunted. These plants often have poor root systems that have few lateral roots, poor nodulation, or a taproot with reddish-brown lesions.

These root rot problems can be blamed on early summer cool weather conditions. Soil temp-eratures cooler than normal slow soybean root development and are ideal for some soilborne fungi. Reduced tillage also has been blamed for the disease, however, root rot problems have been observed in both no-till and conventional tillage this year. In past years, Rhizoctonia root rot was more commonly observed in no-till fields.

Reddish-brown lesion of Rhizoctonia root rot.

In some samples sent to the ISU Plant Disease Clinic this year, I've found that fungi, as well as other factors, seem to be causing root rot problems. I've seen plants with symptoms of stress such as iron chlorosis, herbicide injury, potassium deficiency, and even minor hail injury. These stresses weaken plants, making them more susceptible to pathogenic soilborne fungi. According to a study in Minnesota, soybean roots are colonized by less aggressive fungi more often in no-till fields than in conventional tillage. In normal years, colonization by these fungi is not a concern. But when stresses occur, these fungi can cause root rot problems.

Stand reduction caused by Rhizoctonia.

If the root rot is associated with other stresses, elimination of the stress factors is important to root health. If iron chlorosis is the primary cause of root rot, consider using varieties tolerant to iron chlorosis. Plant pathologists also are experimenting with potassium fertilizers to control soybean root rot.

Fusarium root rot.

If you find the root rot is caused by Rhizoctonia, consider seed treatment. Although this only protects the plant during its early seedling stage, seed treatments ensure health seedlings tolerant to stress. Robust growth in the early stage is important in reducing root rot beyond seedling stages. Currently, pathologists in universities and the industry are studying treatments that protect soybean beyond the seedling stages.

When scouting for soybean fields, you may find many plants that have some reddish discoloration on the tap root but no visible aboveground symptoms. In this case, the discoloration may or may not be caused by pathogenic fungi. If no stunting and root rot are observed, this type of discoloration should not be a concern. Light root rot may cause only minor changes in plant vigor, and the plant may outgrow them. You can promote root growth by using cultivation to mound soil around the base of the plant.

Usually root rot problems disappear when the season progresses into August. Last year, however, we continued to receive many samples with root rot symptoms after mid-summer, typically associated with stunted plants. This year's weather pattern seems similar to last year, so we may see some of the same problems continue into the summer.

This article originally appeared on pages 140-141 of the IC-478(17) -- July 14, 1997 issue.

Updated 07/13/1997 - 1:00pm