Sudden death syndrome (SDS) of soybean was first observed in Arkansas in 1971. Since then, SDS has been found in many states in the Midwest. As its name suggests, the reason for the sudden occurrence of this disease has remained a mystery. During reproductive growth, well-grown soybeans die in about 1 or 2 weeks without previous warning signs. Soybean researchers have struggled to find an answer to the SDS mystery for more than 20 years. Recently, in studies funded by the Iowa Soybean Promotion Board, plant pathologists at Iowa State University have made a significant discovery that may provide a key to solving the mystery.

Many fungi that cause plant diseases respond to temperature consistently, regardless of their developmental stage. However, ISU plant pathologists discovered that the causal fungus of SDS has two phases during the growing season, each of which responds differently to temperature: an early seedling infection phase and a late foliar phase during soybean reproduction. Temperature differentially influences the development of the pathogen during these two phases. In the early season, the SDS fungus requires low soil temperatures (59F) to infect roots of soybean seedlings. When soybeans reach the reproductive stages of development, however, the fungus causes maximum defoliation in a short time if intermediate soil temperatures (72 to 75 degrees F) occur, resulting in sudden death. Another important finding was that the level of defoliation decreases rapidly as soil moisture decreases. Thus, high levels of soil moisture are needed throughout the growing season for maximum SDS development. These new findings will soon be published in Phytopathology, a scientific journal of the American Phytopatho-logical Society.

This information will help soybean growers manage SDS. The results suggest that maintaining healthy seedlings may reduce the risk of SDS, because reducing root infection in the seedling stage can reduce sudden death during the reproductive stage. Seedling root infection can be controlled by increasing early-season soil temperature and reducing excessive soil moisture in the root zone. These effects may be achieved through management of root zone temperature and moisture levels, for example by use of ridge-till. We know that ridge-tilled soil warms earlier in the spring. Also, increased drainage from the ridges reduces moisture on the soil surface. Indeed, in 1995, many fields with SDS were found in eastern Iowa, but less or no SDS was found in ridge-tilled fields in that region. Using this information should also help reduce the spread of SDS to other regions. Experiments to explore management methods based on the new findings are the next step of SDS research at ISU. For detailed information on SDS, order the SDS fact sheet, ISU Extension publication PM-1570.