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

Soybean cyst nematode (SCN; <i>Heterodera glycines</i>) is recognized as the most destructive soybean pathogen. Biological control of SCN using fungi associated with cyst nematodes (<i>Heteroderidae</i>) 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 <i>Alternaria tenuissima</i>, <i>Fusarium solani</i>, <i>Purpureocillium lilacinum</i>, <i>Debaryomyces hansenii</i>, <i>Plectosphaerella cucumerina</i>, <i>Arthopyrenia salicis</i>, and <i>Cosmospora lavitskiae</i> resulted in the most significant reduction in egg viability. Filtrates of <i>C. lavitskiae</i>, <i>D. hansenii</i>, <i>F. acaciae-mearnsii</i>, and <i>F. solani</i> were the most toxic to SCN J2s in the mortality assay. <i>Trichoderma virens</i>, <i>Alternaria tenuissima</i>, <i>F. acaciae-mearnsii</i>, and <i>Pochonia chlamydosporia</i> 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. <i>Trichoderma virens</i>, <i>Purpureocillium lilacinum</i>, and <i>Pochonia chlamydosporia</i> significantly reduced SCN cyst and egg densities, and <i>Talaromyces ramulosus</i>, <i>Plectosphaerella cucumerina</i>, and <i>C. lavitskiae</i> 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.