Early Summer Root Rots

June 25, 1999
The most frequent disease concern this week has been about root rots in soybeans. Primarily, Rhizoctonia and Fusarium root rots have been the culprits. These two rots cause a dry-type canker at the soil line, usually extending down the entire root system. I say dry just to distinguish these types of root rots from wet rots such as Pythium or Phytopthora, in which the roots are typically slimy; roots infected by Fusarium or Rhizoctonia may still be a little mushy and not completely desiccated.

Rhizoctonia root rot. The soilborne fungus Rhizoctonia solani causes Rhizoctonia root rot. Infection by Rhizoctonia alone typically will cause reddish or purple lesions at the soil line. A general rot of the taproot may also occur. Plants infected by Rhizoctonia usually make an attempt to re-root, so if soil moisture is adequate, the plant may recover.

Typically Rhizoctonia is favored by moist conditions at planting followed by some stressful condition that slows down the growth of the root system. Dry weather as the plants begin to emerge and grow is probably the most common environmental factor leading to infection by Rhizoctonia. Other stresses include compaction or herbicide injury.

The picture becomes a little muddled at this point, especially if the stress is from a factor other than weather; yes, you may have root rot in the headlands where compaction is likely, but the disease probably wouldn't have gotten a good hold on the root system if the compaction wasn't there and the plants were actively growing. It is the same sort of situation if herbicide damage has slowed the growth of the plant.

Fusarium root rot.Fusarium root rot caused by Fusarium solanidoesn't cause the same diagnostic lesions as Rhizoctonia. It typically causes a more generalized dry rot of the root system. You would be hard pressed, though, to tell them apart in the field. This situation is usually compounded by the observation that Fusarium and Rhizoctonia often occur together. The same conditions that favor infection and disease development of Rhizoctonia also favor Fusarium. When infection by both pathogens occurs together, the condition is called Fusarium root rot complex.

Stand reduction can occur from infection by these root rots, but significant losses typically occur only when stress to the crop is present. If you used a fungicide seed treatment active against these root rots, why might you still have infection? The explanation for this is that the fungicide does not remain active in the seed zone for a prolonged period of time. So if environmental conditions develop or are already present that slow growth approximately 10 days after planting, depending on the compound, the diseases may become active. Management options are pretty limited at this point in the season. If the plants are showing some root regrowth, cultivation may put some soil up around the base of the stem and further encourage root growth; this is not a feasible option in drilled or no-till soybeans, though.

Soybean cyst nematode. Start looking for symptoms and signs of soybean cyst nematode (SCN). Aboveground symptoms of leaf yellowing and some stunting are just starting to show up. At a scouting clinic in Piatt County, I was able to find white female cysts on soybean roots. If you see areas of the field that appear to have nutrient deficiency symptoms, don't forget to gently dig up the roots and have a look; SCN may be the problem.--Suzanne Bissonnette

Author: Suzanne Bissonnette