Bacillus velezensis LW-66: A Broad-Spectrum Biocontrol Agent Against Apple Tree Canker and Other Plant Fungal...

Plant fungal diseases, such as apple tree canker caused by Valsa mali, have caused severe losses in agricultural production. Traditional chemical fungicides induce drug resistance in pathogens and cause environmental pollution. Therefore, it is of substantial importance to screen efficient and environmentally friendly bacterial strains as potential biocontrol agents. The tea rhizosphere harbors abundant microbial resources, and previous research has identified microorganisms with antifungal activity existing in this environment. Therefore, in this study, we isolated antagonistic bacteria with broad-spectrum biocontrol potential from tea rhizosphere soil. In this study, a strain with strong antagonistic activity against V. mali was isolated from tea rhizosphere soil. Based on morphological characteristics, 16S rRNA gene sequencing, and whole-genome analysis, the isolated strain was identified as Bacillus velezensis and designated as LW-66. This strain demonstrated broad-spectrum antifungal activity against various plant pathogenic fungi, including Valsa mali, Fusarium graminearum, Bipolaris sorokinianum, Alternaria solani, and Exserohilum turcicum. The active extract of B. velezensis maintained strong stability across a wide range of temperatures (25-90 °C) and pH values (2-8), with stability decreasing only when the temperature reached 100 °C or pH ≥ 10. In a preventive assay using detached apple branches inoculated with V. mali, the control efficacy of LW-66 against apple tree canker reached more than 90%. Additionally, in a therapeutic assay using V. mali-infected potted apple seedlings, the LW-66 bone-glue bacterial agent achieved a survival rate of up to 90%. Whole-genome analysis revealed that the genome of LW-66 contains 13 predicted secondary metabolite biosynthetic gene clusters, seven of which showed high homology (≥92% similarity) with known antimicrobial gene clusters, including surfactin, bacillaene, macrolactin H, fengycin, difficidin, bacillibactin, and bacilysin. These gene clusters may be connected to the broad-spectrum antifungal activity of B. velezensis, as well as its ability to disrupt hyphal morphology. The volatile organic compounds produced by LW-66 inhibited V. mali growth by 91.70%. Collectively, these findings demonstrate that B. velezensis LW-66 has a wide antimicrobial range and strong antagonistic effects against multiple plant pathogenic fungi. Therefore, B. velezensis shows promise as a biocontrol agent for managing fungal diseases in plants, providing a basis for developing LW-66-derived biocontrol products aimed at controlling diseases such as apple tree canker.