Biocontrol Potential of Bacillus velezensis xd3 in Alleviating Root Rot of Garlic (Allium sativum L.)

Garlic root rot, mainly caused by Fusarium proliferatum, severely compromises yield and quality and causes substantial economic losses worldwide. Biocontrol using antagonistic bacteria has emerged as an efficient and sustainable strategy for managing this soil-borne disease. In this study, Bacillus velezensis xd3 was isolated and exhibited strong antagonistic activity against eight phytopathogenic fungi, with F. proliferatum as the representative pathogen. Scanning electron microscopy showed that treatment with the cell-free filtrate of B. velezensis xd3 induced prominent morphological abnormalities in F. proliferatum mycelia, including collapse, shrinkage, and distortion. Greenhouse pot assays further verified that B. velezensis xd3 significantly alleviated garlic root rot symptoms, achieving a biocontrol efficacy of 59.85%. Biochemical assays confirmed that B. velezensis xd3 is capable of secreting cellulase, amylase, protease, and siderophores. In addition, untargeted metabolomic profiling of the fermentation supernatant putatively indicated that B. velezensis xd3 may produce antifungal metabolites including lipopeptides (surfactins b and c), polyketides, and siderophores. Transcriptomic data revealed that B. velezensis xd3 exerts antifungal effects against F. proliferatum by modulating the expression of genes associated with membrane structure, transmembrane transport, and redox homeostasis. Collectively, these findings demonstrate that B. velezensis xd3 is a promising biocontrol candidate for the sustainable management of garlic root rot disease.