In past years, sudden death syndrome (SDS) has appeared during the last week of July or the first week of August in Iowa. Therefore we anticipate symptoms of SDS will begin appearing in the state within the next couple of weeks. Although we do not expect SDS to be as widespread or as severe as the 2010 growing season, there have been some Iowa counties that have received higher-than-normal precipitation. We expect the risk of SDS in these counties to be higher since disease development is favored by wet conditions.
The first symptoms of the disease are usually found on more compacted and low areas of the field. First symptoms are seen on the leaves of infected plants as scattered, yellow spots between leaf veins. Large sections of leaf tissue between veins turn yellow as spots grow together. These yellow blotches soon turn brown, but the veins remain green. Eventually the leaves die and drop, but the petioles remain on the stem. Infected plants are also easily pulled from the soil because the roots are rotted. When split lengthwise with a knife, the internal tissue of the main or tap root will be gray to reddish brown, not healthy white.
There are no in-season management options for SDS, but scouting is still important for several reasons.
- First, this is a good time to evaluate soybean varieties for resistance to SDS. Growing resistant varieties, or avoiding very susceptible varieties, is the most effective way to reduce losses to SDS.
- Also, identifying fields or parts of fields with SDS can help with future management practices. These management tactics include reducing soil compaction since the disease has been associated with compacted soil; planting fields with a history of SDS towards the end of a planting schedule when soils may be warmer and drier; and testing for the presence of soybean cyst nematodes (SCN).
- SCN is usually, but not always, associated with SDS and may increase its severity, especially in varieties that are SCN-susceptible. Therefore, management practices to reduce SCN populations, including SCN-resistant variety selection and preventing the spread of soil from field to field, may delay onset and spread of SDS.
Plant pathologists and agronomists continue research to improve our understanding of the biology of the fungus that causes SDS and develop improved management options for the disease. The Iowa State University (ISU) soybean breeding program continues to develop and release germplasm with improved resistance to SDS that is available to all private soybean breeding companies. These Iowa State scientists collaborate with scientists at other universities. Most of the SDS research at Iowa State is funded by soybean checkoff dollars from state, regional and national organizations, namely the Iowa Soybean Association, the North Central Soybean Research Program and the United Soybean Board.
Key research advances from the last five years of research on SDS at Iowa State include:
- Development and release of soybean breeding lines with improved resistance to SDS that can be used by seed companies to develop resistant varieties adapted to Iowa.
- Discovery, identification and molecular characterization of a toxin produced by the SDS fungus that causes the disease and that the toxin needs to be exposed to light to cause the disease on the leaves.
- Discovery that the fungus needs to colonize the central part (or vascular system) of the roots so that the toxin can be moved up from the roots to the leaves in the cells that carry water up the plant.
- Discovery that soybean seedlings are most vulnerable to root infection in the first few days after planting, and that in cold soils the seedlings are vulnerable to infection for a longer period of time than when planting occurs in warmer soil.
- Discovery that the SDS fungus can survive in corn residue, including corn kernels dropped in field, and this may be a way the fungus overwinters from season to season.