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Sudden Death Syndrome--What's Up with Those Roots?

August 16, 2002
It will be interesting to see the impact sudden death syndrome (SDS) has in soybean fields this season. In many areas of the state, foliar symptoms of SDS have not yet begun to appear and may not appear in fields planted late in May or early June.

The foliar symptoms produced by SDS begin as chlorosis and necrosis of the interveinal tissue of soybean leaves, which then coalesce, forming large yellow and brown areas between the green midvein and green lateral veins.

Foliar symptoms of SDS.

Infected plants in the field prematurely turn yellow and then brown, whereas healthy plants remain green. Other symptoms include rotting of roots, necrosis of the crown, discoloration of the vascular tissue in roots and stems, premature defoliation of the soybean plant, and abortion of the flowers and pods. The effects of root infection are not as apparent and usually remain unnoticed unless the blue-colored spores of Fusarium solani f. sp. glycines, the causal organism, are observed on the roots of mature plants.

Signs of Fusarium solani f. sp. glycines on the soybean root system.

Results from our 2000 and 2001 field research on SDS show that infection of the soybean root system occurred without the presence of observable foliar symptoms. Disease pressure (foliar symptom severity) was higher in 2000 than in 2001, but colonization frequency by the fungus on the roots was greater in 2001.

Currently, foliar symptoms are not visible in this year's field study, and foliar symptoms may not develop due to the late planting date. However, the fungus has already been isolated from root systems of plants in both infested and noninfested plots collected during June and July.

In addition, our previous research shows that when severity of SDS is low to moderate, foliar symptoms are not always a good indication of yield potential. Yield of the six resistant soybean cultivars in 2000 decreased by 5% to 20% when inoculated with the causal organism, even though they had relatively low levels of foliar disease severity. In contrast, the yields of two susceptible cultivars were similar in the infested and noninfested plots, even though they showed moderate foliar symptoms in the infested plots. The implications of our results are that the fungus causing SDS colonizes the soybean root system and can potentially cause reductions in yield even though the plant is showing no foliar symptoms.

This explains, in part, why this disease is difficult to control. This pathogen still has many unknown factors about how it is causing disease on soybeans. Our research shows that the root system plays an important role in soybean plants' resistance to SDS; however, more research is needed to specifically identify that role.

In the meantime, it is best to try to manage this disease by lessening the impact of SDS:

1. Learn to identify SDS in the field, as symptoms may appear similar to more common diseases such as brown stem rot or stem canker.

2. Select soybean varieties that mature at different times. Either use either different maturities within a maturity group or use different maturity groups. Early-maturing cultivars appear better.

3. Delay planting or extend planting time so that all soybeans are not at the same growth stage at the same time. However, do not wait past the suggested time for your area of the state.

4. Use cultural practices to improve drainage in low spots, reduce cyst nematode populations, and reduce soil compaction.

5. Crop rotation is of limited value because this organism can persist in the soil for many years. However, planting continuous soybeans is not recommended because this can increase other diseases.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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