As soybeans continue maturing and move into pod-filling stages, growers should be on the lookout for symptoms of brown stem rot and sudden death syndrome. Understanding diseases that are plaguing your fields now is the first step toward prevention.
Sudden Death Syndrome
Sudden death syndrome of soybeans is a soil-borne fungal disease cause by Fusarium virguliforme. This fungal infection begins to affect the plant early in the growing season; however, foliar symptoms of SDS typically do not appear until beginning pod and later growth stages.
The fungal pathogen for SDS favors cool, wet soils during planting, which is when infection occurs. The incidence is often higher in compacted soils, which have reduced aeration and water percolation.
Initial symptoms include chlorotic, or yellow spots on the leaves between the veins. As the disease continues to progress, the yellow spots may expand into green parts of the leaf but stay between the veins at the leaf margin, and it is common to observe the loss of pods. Leaves may eventually drop, and leaf stems remain.
To positively identify SDS, split open a lower stem and taproot of a diseased plant. If the area appears to have tan or light brown discoloration when compared with a healthy plant, it is likely infected with SDS. Close examination of the pith will appear healthy and normal in cases of SDS.
A brown pith indicates brown stem rot. Because BSR and SDS show the same foliar symptoms, they commonly are confused until the stem is split.
There is no rescue treatment for SDS. Instead, consider these management tips for the future.
- Genetic resistance — Varieties with resistance or tolerance to SDS should be considered in fields with a history of SDS issues.
- Cultural management — Avoid early planting fields with a history of SDS. Soil temperature below 60 degrees can favor SDS infection. Reducing compaction and improving soil drainage have been shown to reduce SDS risk.
- Rotation — This fungus can survive many years in the soil without a susceptible host, so crop rotation has not shown to be an effective management tool.
- Chemical — Seed treatment fungicides have shown some efficacy in managing early season SDS infection.
Brown Stem Rot
Brown stem rot of soybeans is a soil-borne fungal disease caused by Phialophora gregata. The fungus survives in infected soybean residue left on the soil surface.
The fungus invades through the roots in spring, with symptoms not developing until after pod formation. Symptoms are very similar to SDS, aside from the presence of a brown pith above the soil line.
However, even without the presence of significant symptoms, brown stem rot may still cause yield loss. Soybean plants infected with strains that only cause stem symptoms suffer less yield loss than those infected with strains that also cause leaf symptoms. Ultimately, the degree of yield loss depends on environmental conditions, variety and fungal strain.
Like SDS, there is no rescue treatment for brown stem rot, but consider these management strategies.
- Crop rotation — Planting non-host plants, such as corn or small grains, will prevent buildup of the brown stem rot fungus.
- Genetic resistance — Consider varieties with good tolerance for problem fields.
- Tillage — Burying residue aids in residue decomposition, helping to reduce inoculum in the soil.
- Chemical and biological control — Foliar fungicides have not been found to have an effect on brown stem rot, nor will seed treatment fungicides.
Additional scouting and management information is available at www.extension.purdue.edu/extmedia/BP/BP-41-W.pdf.
Contact your local seed adviser or agronomist for help selecting the best soybean varieties for prevention of SDS and brown stem rot.