Biocontrol potential and mechanism of an endophyticBacillus subtilisstrain KS1 against fire blight

Fire blight, caused by Erwinia amylovora, is one of the serious challenges in the production of pears and apples. In this study, we isolated and analyzed the biocontrol efficiency and underlying mechanisms of strain KS1, an antagonistic bacterium of E. amylovora obtained from the "Huoba" pear cultivar of Pyrus pyrifolia Nakai. Strain KS1 was identified as Bacillus subtilis based on multi-locus phylogenetic analysis using 16S rRNA and rpoB gene sequences, together with average nucleotide identity (ANI) analysis of the KS1 genome against representative Bacillus species. Genomic analysis revealed that KS1 harbors multiple biosynthetic gene clusters associated with antimicrobial compound production, and surfactin and bacillibactin were detected by LC-MS analysis. Laser confocal microscopy analysis demonstrated that strain KS1 can colonize the roots, leaves, and rhizosphere soil of Pyrus betulifolia Bunge. KS1 significantly reduced the fire blight disease index on leaves, twigs, and immature fruits of Pyrus sinkiangensis Yü. In addition, KS1 promoted the growth of P. betulifolia Bunge seedlings. Field trials during the flowering period demonstrated that strain KS1 achieved control efficacies of 84.00% and 85.39% against fire blight on individual flowers and flower clusters, respectively. These results suggest that B. subtilis KS1 effectively inhibitsE. amylovora and has strong potential as a biological control agent for fire blight. This study provides insights into the antibacterial mechanisms of KS1 and supports its future development for sustainable management of fire blight.IMPORTANCEFire blight, caused by Erwinia amylovora, is a devastating disease that threatens pear output in several countries. Currently, fire blight is controlled primarily with chemical pesticides, which are causing additional issues, such as disease resistance, environmental contamination, and pesticide residues on fruit. As a result, there is a rising emphasis on creating ecologically acceptable, safe, and effective biocontrol strategies for combating fire blight in pear farming. This study explores the discovery and mechanisms of Bacillus subtilis KS1, which provides a novel biocontrol resource for managing pear fire blight. Strain KS1 has efficient and ecologically friendly biocontrol capabilities through a variety of mechanisms, including the production of antimicrobial lipopeptides (e.g., surfactin and bacillibactin), competition for colonization, induction of plant resistance, and direct promotion of growth. Therefore, this work demonstrates the great potential of B. subtilis KS1 as an alternative to chemical pesticides in sustainable pear production.