Fermentation optimization improves antifungal metabolite production of Letendraea sp. SX 36 against Colletotrichum...

Apple anthracnose, caused by Colletotrichum species, is a devastating fungal disease that affects apple production. In this study, 200 fungal strains were isolated from soil, and six strains with strong antagonistic activity were selected through plate confrontation assays. Among them, strain SX 36, identified as Letendraea sp., showed the highest inhibition (68.25%) of Colletotrichum nymphaeae. Fermentation conditions for SX 36 were optimized using single-factor experiments, orthogonal design, and response surface methodology, resulting in a predicted inhibition rate of 85.56%, which was experimentally validated at 86.51%, an 18.36% improvement over pre-optimization levels. SX 36 also exhibited broad-spectrum antifungal activity against 12 pathogenic fungi, including Alternaria alternata, Botrytis cinerea, and Phoma arachidicola. Low-concentration fermentation broth promoted apple growth, enhancing plant height, root length, stem diameter, biomass, and seed germination. Pot experiments confirmed its efficacy, achieving a disease control rate of 94.85%. These results revealed that SX 36 is a promising candidate for the sustainable biocontrol of apple anthracnose and establish a basis for its potential large-scale biopesticide production.