Sudden death syndrome (SDS) is a disease reported in most soybean-growing areas of the United States and the world. In 1998, SDS was ranked second behind soybean cyst nematode as the most important disease of soybeans in Illinois. The soilborne fungus, Fusarium solani f.sp. glycines, which is the causal organism of SDS on soybeans, infects soybean roots. Under severe conditions, SDS can result in flower and pod abortion, premature defoliation, and yield losses. The use of fungicides to control this pathogen has not been effective, and crop rotation does not provide a viable control alternative. Although SDS potentially can be controlled by host-plant resistance and some progress has been made in developing resistant varieties, screening for resistance is difficult because disease expression is often environmentally controlled. Soil moisture appears to be one environmental parameter that influences the amount of SDS occurring in the field, as disease development appears to be favored by high soil moisture or rainfall and irrigation. Development of SDS in the field is more severe in compacted areas. Disease surveys conducted around the state have shown that sudden death syndrome occurs more frequently in fields under high production, in wet and compacted areas, and in fields with high populations of soybean cyst nematodes, Heterodera glycines. Research has also shown that the soybean cyst nematode (SCN) is not required for infection by the SDS pathogen. The presence of SCN appears to increase plant stress when both organisms are present, which in turn can increase the severity of foliar symptoms compared to those caused by the SDS pathogen alone.
Symptoms produced by SDS begin as chlorosis and necrosis of the interveinal tissue of leaves, which then coalesces, forming large yellow and brown areas between a green midvein and green lateral veins (Figure). Other symptoms are 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. When leaves drop off the soybean plant, they often drop from the top, leaving the petiole attached to the stem. Infected plants in the field prematurely turn yellow and then brown, whereas healthy plants remain green. The foliar symptoms of SDS seen in the field are similar to those of brown stem rot, but internal symptoms differ in that with SDS there is no pith discoloration. Some uniform reddish brown vascular discoloration can occur with SDS, but without a streaking pattern. Leaf symptoms of the stem canker disease can be confused with SDS; however, stem canker has cankers on the lower stem, and plants with SDS do not have cankers.
This disease is difficult to control for several reasons. First, there are still many unknown factors about this pathogen and how it is causing disease on soybeans. Second, there are no currently labeled resistant varieties. Currently, researchers at the University of Illinois are investigating mechanisms for resistance in several soybean varieties. Their research program is investigating whether the root system of the soybean plant may resist colonization or overcome infection using some unknown mechanism. In addition, researchers at the University of Illinois are investigating how tillage regimes and rainfall may influence disease development and root-system colonization.
In the meantime, it is best to try to manage this disease by lessening the impact of SDS.
- Learn to identify SDS in the field, as symptoms may appear similar to more common diseases such as brown stem rot or stem canker.
- Select soybean varieties that mature at different times. Use either different maturities within a maturity group or use different maturity groups. Early maturing cultivars appear to be a better choice.
- 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 planting in your area of the state.
- Use cultural practices to improve drainage in low spots, reduce cyst nematode populations, and reduce soil compaction.
- 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