Biocontrol activity of Bacillus velezensis D4 against apple Valsa canker

Abstract Apple Valsa canker, caused by Valsa mali, is one of the most serious fungal diseases in apple-producing regions in east Asia. One endophyte strain, designated as D4 and subsequently identified as Bacillus velezensis based on morphological and phylogenetic analyses, was studied as a potential biocontrol agent. Challenge inoculation assay showed the strain D4 suppressed the mycelial growth of V. mali on dual culture plates and caused hyphal deformities, wrinkles, and ruptures. Furthermore, the strain D4 was able to promote plant growth by producing siderophores and solubilizing phosphorus. Under laboratory and field conditions, the strain D4 exhibited strong antifungal activities on detached twigs and intact plants. Re-isolation of endophytes confirmed the strain D4 could completely colonize the plants. Comparative transcriptome analysis showed downregulation of the genes associated with the fungal cell wall’s synthesis and composition, cell membrane, and transport and energy metabolism during V. mali treatment with the D4 strain. Furthermore, the global gene expression patterns revealed that host genes mainly involved in salicylic acid, ethylene, jasmonic acid signaling and brassinosteroid synthesis pathways were upregulated during apple treatment with the strain D4. qRT-PCR conducted on a subset of 13 genes was consistent with the results of RNA-seq data. The present study explores a potential biocontrol agent for the control of apple Valsa canker and sheds light on the control mechanisms of how the strain D4 impairs the pathogen and triggers the plant’s defense.