New research suggests that the critical stage for the SDS pathogen to infect soybean plants is before emergence of the germinated seed. Soybeans planted early in cool soil are susceptible to SDS infection when slow germination and emergence prolongs the contact period between pathogen and soybean. Although infection has occured, no symptoms are generally seen until the early reproductive stages.
Drawing credit: Westphal, A., T.S. Abney, L.J. Xing and G.E. Shaner. Sudden Death Syndrome of Soybean. 2008. The Plant Health Instructor. American Phytopathological Society.
The SDS pathogen is thought to survive between soybean crops as chlamydospores in crop residue or in the soil. Photo: A. Westphal et al. The Plant Health Instructor. APS
The SDS pathogen
Sudden death syndrome is caused by the soil fungus, Fusarium virguliforme. The fungus is closely related to another soybean pathogen, Fusarium solani form B, that causes seedling disease and root rot.
The SDS pathogen produces a toxin that is translocated throughout the plant and interferes with the vascular system of the plant. Sudden yellowing and death of the leaves occurs as the water supply is cut off.
Fusarium virguliforme has a blue pigmentation, and a blue coloration is sometimes found on the tap roots of plants that are severely infected with the pathogen. The coloration is due to the large number of conidia (spores) produced on the root surface.
The fungus produces thick-walled survival structures called chlamydospores in soybean root tissue. Chlamydospores can survive for several years in the soil.Chlamydospores can withstand wide fluctuations in soil temperature and moisture. As soil warms in the spring, chlamydospores near soybean roots are stimulated to germinate, and then infect soybean roots.