Sudden Death Syndrome
In the North Central soybean producing areas of the United States and Ontario, Canada sudden death syndrome (SDS) is caused by the fungus Fusarium virguliforme. SDS is one of the most recognizable and important diseased in throughout the region. In some states, SDS may be present the majority of the fields and cause statewide yield reductions. In other, the disease is regional or rare. The symptoms of SDS are oftentimes similar to those caused by other soybean pathogens, making it difficult to diagnose in the field. It is known that the presence of other pathogens, such as soybean cyst nematode (SCN), can also exacerbate the disease.
The first observed SDS symptoms are often early leaf symptoms, including yellow spots between leaf veins, known as interveinal chlorosis. These lesions may expand and turn brown as the spots expand between veins, which is called interveinal necrosis. The leaf veins will remain green. As the disease progressed, leaves die and prematurely fall from the plant, while petioles remain attached to the stem. Pods and seeds may also be aborted.
Root and Stem Symptoms/Signs
SDS may cause discolored and decaying roots even if there are no noticeable foliar symptoms. The woody tissue in the taproot can become brown to grey, while the upper portion of the stem pith remains white. When soils are wet, the SDS fungus can reproduce on the root surface producing a mass of spores that are purple-blue. The foliar symptoms of SDS can appear very similar to several other soybean diseases, which make it important to closely examine the stem and root for an accurate diagnosis.
Favorable Environmental Conditions
Cool, wet conditions shortly after planting favor early root infections and disease establishment. Frequent or heavy rains midseason can favor early symptom expression. This puts irrigated fields at a higher risk for SDS. Hot and/or dry conditions in the spring or summer can halt of development of SDS. Some crop production practices may also increase the risk of disease development. Fields with poor soil nutrients, low pH, poor drainage, or moderate to severe soil compaction are at a higher risk for SDS.
Planting the most resistant varieties available is the foundation of an SDS management program. There are no soybean varieties completely resistant to SDS, but partially resistant varieties are available. However, weather and field conditions influence disease severity, even on partially resistant varieties. As there is a known link between SCN and SDS, planting SCN-resistant varieties and rotating crops may delay SDS onset and reduce disease severity. Most foliar fungicides and seed treatments are not effective in reducing SDS. However, fluopyram (ILeVO®, Bayer CropScience) has SDS severity and protected yield on susceptible varieties.
Minimizing soil movement may slow the introduction of the SDS fungus to new fields. Improving drainage and soil structure and reducing compaction can also help to alleviate SDS symptom severity. Tilling in known compacted areas may reduce disease symptoms by allowing the soil to warm more quickly during the spring. Rotating to corn does not typically reduce SDS in subsequent soybean crops. In fields with a severe history of the disease, delayed planting reduced conditions that favor the disease. Proper soil fertility levels is important in reducing plant stress and maintaining overall plant health, which may help reduce the occurrence of soybean diseases such as SDS. High soil moisture levels within a few days after planting and /or during the mid-late soybean reproductive stages may increase the risk of infection by the SDS pathogen.