Effects of an Isolate of Bacillus subtilis on Southern Blight Severity of Aconitum carmichaelii Debeaux and Its Rhizosphere Microbial Community

Southern blight caused by Sclerotium rolfsii is a devastating soil-borne disease of Aconitum carmichaelii, a medicinal plant widely cultivated in China. Biological control using beneficial bacteria offers a sustainable alternative to chemical fungicides. This study investigated the influence of Bacillus subtilis FZ-7 on rhizosphere microbiome assembly and disease suppression in A. carmichaelii under field conditions. Four treatments were established: uninoculated control (C group), single inoculation with B. subtilis (B group), single inoculation with S. rolfsii (S group), and co-inoculation of S. rolfsii and B. subtilis (BS group). Plant biomass, soil physicochemical properties, and rhizosphere bacterial and fungal communities were analyzed using high-throughput sequencing of 16S rRNA and ITS genes, along with co-occurrence network analysis. Results showed that B. subtilis inoculation suppressed this growth suppression. B. subtilis alone enhanced both bacterial and fungal diversity. PCoA and LEfSe analyses revealed distinct microbial community structures among treatments, with biomarkers such as Lactobacillus, Candidatus Nitrosotalea, Rhodanobacter, and Sphingomonas for bacteria, and Trichocladium, Conocybe, Plectosphaerella, and Athelia for fungi. Co-occurrence network analysis indicated that co-inoculation of S. rolfsii and B. subtilis partially restored cooperative microbial associations. Soil organic matter, pH, and cation exchange capacity were the main environmental factors shaping microbial community composition. Besides, BS group significantly reduced the disease index from 72.35 to 28.46. Shoot fresh weight in BS recovered to 2.98 kg and rhizome fresh weight to 3.72 kg, representing 73.0% and 41.2% increases over the S treatment. Additionally, the disease index was reduced by 55.26 in BS compared with the control group. Nay more, Bacillus, Chujaibacter, and Rhodanobacter taxa were significantly enriched in BS group. In conclusion, B. subtilis FZ-7 effectively mitigates southern blight in A. carmichaelii by modulating rhizosphere microbial assembly, enriching beneficial taxa, and stabilizing microbial co-occurrence networks, highlighting its potential as a biocontrol agent for sustainable medicinal plant production.