Biocontrol of Apple Valsa Canker by Bacillus sp. H12 and modulation of the apple seedlings microbiome

Apple Valsa Canker (AVC), caused by the fungal pathogen Cytospora mali, is the most devastating stem disease in East Asia apple production systems. As an eco-friendly alternative to chemical control, biological control strategies have attained prominence for mitigating environmental pollution and preventing pathogen resistance. In this study, a soil-derived bacterial strain significantly inhibited AVC and promoted plant growth. The strain was identified as Bacillus amyloliquefaciens using combined morphological and molecular characterization and designated as H12. The bacterium directly induces abnormal branching of C. mali hyphae, cytoplasmic leakage, and cell wall rupture, ultimately leading to mycelia death. H12 exhibits strong control efficacy against AVC on in vitro branches and the leaves of tissue-cultured seedlings. Notably, H12 upregulates immune-related gene expression in apple, enhances reactive oxygen species (ROS) accumulation, and increases callose deposition. Microbiome sequencing indicates that H12 modulates the structure of the apple phyllosphere bacterial community, enriches beneficial microorganisms, and enhances host resistance. Furthermore, the strain achieves a 55.56% disease control efficacy in field trials, demonstrating its potential for practical application as a promising microbial agent. In summary, H12 has dual functions, directly inhibiting pathogenic fungi and inducing host resistance, providing theoretical and practical support for the green management of AVC.