Biocontrol Potential and Functional Characteristics of Bacillus sonorensis A-5 Against Watermelon Fusarium Wilt

Fusarium wilt, caused by Fusarium oxysporum f. sp. niveum (Fon), severely restricts the sustainable development of the global watermelon industry. While conventional chemical fungicides of this disease have triggered prominent ecological issues, Bacillus-based microbial biocontrol, which combines inherent environmental compatibility with stable control efficacy, has emerged as a key green alternative to chemical management. However, the biocontrol potential of Bacillus sonorensis against this disease has not yet been fully investigated. In this study, we isolated 56 bacterial strains from healthy watermelon rhizosphere soil, and obtained a Fon-antagonistic strain A-5 with the strongest activity (70.15% mycelial inhibition rate), which was identified as B. sonorensis via polyphasic taxonomic analysis. In vitro assays showed that the sterile fermentation filtrate of strain A-5 had a maximum 81.05% inhibition rate against Fon, and its volatile organic compounds also significantly suppressed Fon growth, with broad-spectrum antifungal activity against four common phytopathogenic fungi. Functional tests confirmed that strain A-5 could secrete cell wall-degrading enzymes, produce siderophores and synthesize indole-3-acetic acid, and 17 antimicrobial secondary metabolite biosynthetic gene clusters were identified in its genome. Pot experiments verified that strain A-5 had a 78.04% relative control efficacy against watermelon Fusarium wilt, which significantly reduced seedling disease incidence and upregulated defense-related antioxidant enzyme activities in watermelon leaves. In general, B. sonorensis A-5 is a promising novel biocontrol agent for green management of watermelon Fusarium wilt.