Suppression of Heterodera glycines in soils from fields with long-term soybean monoculture

Experiments were conducted in a greenhouse to evaluate soils for their degree of suppressiveness against the soybean cyst nematode, Heterodera glycines. The soils were collected in 2003 and 2004 from two fields with long-term soybean monoculture and one field with corn/soybean annual rotation. The soils were autoclaved, treated with formalin, or untreated. Both formalin and autoclave treatments removed suppressive factors and increased the nematode egg population density as compared with untreated soil. The mixture of 10% untreated soil with 90% treated soil resulted in similar nematode population densities as in the untreated soils collected from the two monoculture fields, indicating the suppressive factors were biological and can be transferred. The soils from the monoculture fields appeared to be more suppressive than the soil from the corn/soybean rotation. Hirsutella rhossiliensis was observed in all three soils, and parasitised a large percentage of second-stage juveniles of H. glycines in the monoculture soils in 2004. The fungus may be a major factor for the suppression of the nematode population, but in this study other biological factors cannot be ruled out.