Sudden death syndrome (SDS) of soybean was estimated to be the second most important soybean disease in the United States in 2010. The country's first report of SDS occurred in Arkansas in 1971; the disease reached Illinois by 1986, and it has now spread to most soybean-producing areas in the United States. The disease is caused by the fungus Fusarium virguliforme, which infects soybean roots, causing root and crown rot. As part of root infection, the fungus produces a toxin that is translocated through the plant, causing chlorotic mottling of the leaves that can progress into interveinal chlorosis, necrosis, and premature leaf drop .
Planting into soils that are cold, wet, and/or compacted tends to be associated with greater symptom development because these conditions increase the rate of root infection by the fungus. Studies have shown that increased tillage and later planting can reduce disease symptoms, but it is not advisable to delay planting as a way to avoid SDS because later planting is generally associated with lower yield. This association between soil moisture and SDS symptoms has been confirmed in irrigation experiments, and researchers often irrigate SDS screening tests to increase disease levels.
When SDS was first identified as a soybean disease, it was found to be associated with soybean cyst nematode (SCN). The association between SDS and SCN has been studied, though the interaction between them is still not fully understood. We do know that SCN is not required for SDS symptom expression, but the presence of SCN has been shown to increase SDS symptoms. It is believed that root wounding caused by SCN can help introduce the fungus into the roots of the plants, resulting in greater root infection and more overall SDS symptoms.
The best way for growers to combat SDS is to select varieties with resistance to the disease, especially for fields with a history of SDS. Many companies that sell soybean varieties provide SDS resistance ratings of their products. Another resource for information on variety resistance levels is the VIPS website (www.vipsoybeans.org). As part of the VIPS testing system, varieties are subjected to SDS resistance evaluation.
Resistance in soybean to SDS is partial, meaning that varieties with a good resistance rating will show some symptoms in fields with high disease pressure. However, you can expect that these partially resistant varieties will have fewer symptoms and less yield loss than varieties with poor resistance ratings. In any partially resistant variety, the resistance is caused by multiple genes. The down side of this complicated inheritance of resistance is that it increases the difficulty of developing resistant varieties, but on the up side, this type of resistance is usually durable, meaning it will not break down quickly.
Genetic studies have shown that varieties with SCN resistance typically have more field SDS resistance than varieties that are SCN susceptible. To identify sources of new genes that can increase SDS resistance in soybean varieties, plant introductions from the USDA-ARS soybean germplasm collection have been tested for resistance to SDS. From this screening, a number of plant introductions with resistance to SDS have been identified and are being used to develop new SDS-resistant varieties. Researchers at the University of Illinois and other institutions supported by the soybean checkoff have mapped genes from plant introductions and varieties that give SDS resistance, and these genes are now being used to breed varieties with improved resistance.
Professor of Crop Sciences