Effectiveness of Hirsutella minnesotensis and H. rhossiliensis in control of the soybean cyst nematode in four soils with various pH,texture, and organic matter

The parasitism of soybean cyst nematode, Heterodera glycines, by the fungi Hirsutella rhossiliensis and Hirsutella minnesotensis and their biocontrol effectiveness against the nematode were investigated in four soils with various pH, texture, and organic matter. Fungal parasitism was assayed in the soils in 25 mL vials. As expected, percentage of H. glycines second-stage juveniles (J2) parasitized by either fungus increased with increasing number of fungus-colonized J2 initially added into the soils. Parasitism of J2 by the fungi was negatively related with soil pH. Both positive and negative relationships with fungal parasitism were observed for soil sandiness and organic matter. In greenhouse study, both fungi at 0.2–0.8 g fresh mycelium of liquid culture per 0.3 L pot and 1% corn-grits culture effectively reduced nematode population density. The relationship between biocontrol effectiveness and the soil factors depended on fungal species and inoculation levels. In general, percentage reduction of egg population density in the soil was negatively correlated with soil pH and positively correlated with sandiness. There was no or weak correlation between egg reduction and organic matter. The percentage of J2 parasitized by the fungi 2 months after planting did not correlate with the soil factors. Plant growth was better in the two soils with intermediate pH and sand than the soil with high pH and low sand or with low pH and high sand. It appeared that soil pH and/or texture are important in influencing biocontrol effectiveness, but further studies are needed to determine the effect of individual factors because they are correlated.