Screening and Optimization of Solid-State Fermentation for Esteya vermicola, an Entomopathogenic Fungus Against the Major Forest Pest Pine Wood Nematode

Pine wilt disease (PWD), caused by the pine wood nematode (PWN,Bursaphelenchus xylophilus), is one of the most serious threats to pine forests worldwide. The fungusEsteya vermicola, with its lunate conidia capable of parasitizing the PWN, has shown promise as an efficient biological control agent against PWD. Solid-state fermentation (SSF) is preferred for large-scale applications in the field, as it facilitates microbial agent transport and ensures a long shelf life. However, research on enhancing the yield of lunate conidia fromE. vermicolathrough SSF is limited. In this study, we initially achieved a yield of 3.04 × 108conidia/g using a basic SSF medium composed of wheat bran, corn flour, and soybean flour. To improve this yield, we employed an orthogonal experimental design (OED) to identify the optimal medium composition, which required a wheat bran-to-corn flour-to soybean flour ratio of 7:2:1 (w/w/w), a substrate-to-water ratio of 1:0.7 (w/v), and the addition of 1.33% (w/w) glucose, 1.33% (w/w) yeast extract fermentation, and 1.33% (w/w) MgSO4. Using the response surface methodology (RSM), we calculated the optimal fermentation conditions, which were 24.9 °C, 78.0% relative humidity (RH), an inoculation volume of 16.3% (v/w), and a fermentation time of 7.1 days. Under these conditions, the yield of lunate conidia reached a maximum of 16.58 × 108conidia/g, a 4.45-fold increase after optimization. This study improved the yield ofE. vermicolalunate conidia and provides insights for developing biopesticides based on this strain.