In Vitro and In-Soil Suppression of Soybean Cyst Nematode by Fungi Isolated from Cyst Mycobiome

Soybean cyst nematode (SCN; Heterodera glycines) is recognized as the most destructive soybean pathogen. Biological control of SCN using fungi associated with cyst nematodes (Heteroderidae) could be a promising strategy. In this study, we screened 75 fungal isolates representing 55 species associated with different cyst nematodes to assess antagonistic effects on SCN. A rapid 24-well plate in vitro assay was used to evaluate fungal cell-free filtrates for their effect on SCN egg viability and hatch. Filtrates of 14 isolates significantly reduced egg viability and hatch by inducing "vacuole-like" structural aberrations in SCN eggs. The 14 fungal isolates were further tested for their effect on SCN second-stage juvenile (J2) viability and direct egg parasitism. Filtrates of Alternaria tenuissima, Fusarium solani, Purpureocillium lilacinum, Debaryomyces hansenii, Plectosphaerella cucumerina, Arthopyrenia salicis, and Cosmospora lavitskiae resulted in the most significant reduction in egg viability. Filtrates of C. lavitskiae, D. hansenii, F. acaciae-mearnsii, and F. solani were the most toxic to SCN J2s in the mortality assay. Trichoderma virens, Alternaria tenuissima, F. acaciae-mearnsii, and Pochonia chlamydosporia had the highest degree of SCN egg parasitism. Based on in vitro assays, 10 promising fungal isolates were selected for evaluation in a soil-based experiment. Trichoderma virens, Purpureocillium lilacinum, and Pochonia chlamydosporia significantly reduced SCN cyst and egg densities, and Talaromyces ramulosus, Plectosphaerella cucumerina, and C. lavitskiae provided moderate SCN suppression. These findings highlight the potential of nematophagous fungi isolated from cyst nematodes for SCN management and offer a screening approach for identifying potent fungal biocontrol agents.