The Dual Role of 2,4-Diacetylphloroglucinol in Pseudomonas bijieensis Y5 against Apple Valsa Canker: Antifungal Activity and Immune Response

Abstract Apple Valsa canker is a destructive fungal disease limiting sustainable apple production. Biological control using antagonistic microorganisms is therefore of great importance. This study evaluated Pseudomonas bijieensis Y5 as a biocontrol agent and clarified the role of its key metabolite, 2,4-diacetylphloroglucinol (DAPG). Strain Y5 significantly suppressed disease development in branches and leaves and, in greenhouse assays, promoted plant growth. It exhibited stable, long-term colonization in plant tissues, enabling sustained protection. Genomic and metabolomic analyses linked their efficacy to DAPG production. DAPG directly inhibited the pathogen by disrupting cellular structures and also activated plant immunity, increasing reactive oxygen species, callose deposition, and defense gene expression. Mutants lacking the phl gene cluster could not produce DAPG, which impaired both their antifungal activity and their ability to trigger plant immunity. Overall, Y5 controls apple Valsa canker through DAPG-mediated antimicrobial activity and immune induction, supporting its potential for sustainable disease management.