Harnessing rhizosphere bacteria for fungal disease management in Paeonia lactiflora

Paeonia lactiflora Pall., a traditional Chinese ornamental flower, holds significant aesthetic and economic value but remains highly susceptible to fungal diseases. In this study, eight candidate biocontrol bacteria (SY1-SY8) were isolated and identified from the rhizosphere soil of P. lactiflora, and their antagonistic effects against four major fungal pathogens were evaluated using plate confrontation assays, fermentation broth antagonism tests, and pot experiments. Among these isolates, Bacillus subtilis SY8 exhibited the most significant inhibitory effect against all four pathogens. Treatment with SY8 significantly reduced the incidence of black spot disease caused by Alternaria alternata FSY1 and enhanced the activities of phenylalanine ammonia-lyase and polyphenol oxidase in P. lactiflora leaves. Additionally, SY8 treatment reduced the abundance of pathogenic fungi in both the phyllosphere and endophytic compartments, while increasing the diversity and evenness of the phyllosphere bacterial community, thereby promoting more complex microbial interactions. Metabolomic analysis revealed that SY8 substantially altered key metabolic pathways in P. lactiflora leaves, including carbon metabolism, amino acid biosynthesis, and the citric acid cycle. These metabolic shifts collectively enhance the antioxidant capacity, cellular repair mechanisms, and disease resistance of plants. Furthermore, metabolomic profiling of the SY8 fermentation broth identified specific bioactive metabolites associated with its antagonistic activity against FSY1. SY8 significantly enhanced the resistance of P. lactiflora to black spot disease in by increasing the activities of defense-related enzymes, modifying the rhizosphere microbial community, and regulating metabolic pathways. These findings underscore the potential of SY8 as a promising biocontrol agent for managing fungal diseases in P. lactiflora.