Sudden death syndrome (SDS) is caused by a soil borne fungus (Fusarium virguliforme) that infects soybean roots and vascular tissue. Foliar symptoms caused by toxins the fungus produces usually appear during reproductive growth stages and can get progressively worse. Symptoms first appear as small yellow spots and progress to interveinal chlorosis (yellowing) and necrosis (brown, dead tissue). The leaves may fall off prematurely, leaving the petioles attached. The foliar symptoms may resemble those of stem canker, charcoal rot, and chemical injury.
Roots of infected plants are usually rotted, and plants can be easily pulled out of the soil. The pith tissue will remain white, while the water-conducting tissue (xylem) will have a gray-to-brown color. In some cases the pith may not be used for diagnosis if Dectes stem borer is present which will have caused a dark- brown pith. SDS symptoms will be more severe in the presence of the soybean cyst nematode, in low, wet areas of fields, and may be worse after corn, which can have stalk rot from the same pathogen. SDS is favored by more wet, cool conditions than hot and dry conditions during the growing season.