SUDDEN-DEATH SYNDROME OF SOYBEAN - ETIOLOGY, SYMPTOMATOLOGY, AND EFFECTS OF IRRIGATION AND HETERODERA-GLYCINES ON INCIDENCE AND SEVERITY UNDER FIELD CONDITIONS

Soybeans were grown in field microplots in sterile, noninfested soil versus soil infested with either Fusarium solani (form A), Heterodera glycines (the soybean cyst nematode), or both. Symptoms of sudden death syndrome occurred on plants in soil containing F. solani or F. solani plus H. glycines. Signs (macroconidia) sometimes occurred on symptomatic roots. Fusarium solani was reisolated from symptomatic plants but not from asymptomatic ones. Histological data further confirmed F. solani as the causal agent and documented the presence of F. solani chlamydospores in infected roots and cysts. Fusarium solani was not isolated from surface-disinfested seeds of infected plants. Irrigation increased disease incidence and severity. Heterodera glycines was not necessary for infection of plants by F. solani; however, when combined with F. solani, leaf symptoms occurred earlier and were more severe. Inoculation with F. solani plus H. glycines increased the incidence of tip dieback of pods, a disorder of uncertain cause. Fusarium solani was isolated in high frequency from roots of symptomatic plants sampled in the South and Midwest. However, F. solani form B was the most common isolate from roots. A significant positive correlation occurred between incidence of the two F. solani forms in roots of symptomatic plants.