Jing-Xue Luo

Fusarium wilt, caused by Fusarium oxysporum f. sp. niveum (Fon), severely restricts the sustainable development of the global watermelon industry. While conventional chemical fungicides of this disease have triggered prominent ecological issues, Bacillus-based microbial biocontrol, which combines inherent environmental compatibility with stable control efficacy, has emerged as a key green alternative to chemical management. However, the biocontrol potential of Bacillus sonorensis against this disease has not yet been fully investigated. In this study, we isolated 56 bacterial strains from healthy watermelon rhizosphere soil, and obtained a Fon-antagonistic strain A-5 with the strongest activity (70.15% mycelial inhibition rate), which was identified as B. sonorensis via polyphasic taxonomic analysis. In vitro assays showed that the sterile fermentation filtrate of strain A-5 had a maximum 81.05% inhibition rate against Fon, and its volatile organic compounds also significantly suppressed Fon growth, with broad-spectrum antifungal activity against four common phytopathogenic fungi. Functional tests confirmed that strain A-5 could secrete cell wall-degrading enzymes, produce siderophores and synthesize indole-3-acetic acid, and 17 antimicrobial secondary metabolite biosynthetic gene clusters were identified in its genome. Pot experiments verified that strain A-5 had a 78.04% relative control efficacy against watermelon Fusarium wilt, which significantly reduced seedling disease incidence and upregulated defense-related antioxidant enzyme activities in watermelon leaves. In general, B. sonorensis A-5 is a promising novel biocontrol agent for green management of watermelon Fusarium wilt.

Rice sheath blight caused by Rhizoctonia solani is a devastating global rice disease. This study aimed to isolate biocontrol bacteria from the medicinal plant Bletilla striata for managing the disease. Strain A1 demonstrated the strongest antagonistic activity, with a 91.92% inhibition rate against R. solani in vitro. It also exhibited a broad antifungal spectrum against ten plant pathogenic fungi. Morphological and molecular (16S rRNA and recA genes) analysis identified strain A1 as Pseudomonas koreensis. In detached leaf assays, lesion length was significantly reduced. Pot and field trials showed control efficacies of 65.54% and 72.53%, respectively, comparable to the chemical agent Jinggangmycin. Strain A1 secreted extracellular enzymes (protease, β-1,3-glucanase), siderophores, and auxin (IAA), and possessed phosphate-solubilizing and nitrogen-fixing capabilities. The strain significantly enhanced the activities of key defense enzymes (POD, PAL, PPO, CAT, SOD) in rice. Furthermore, both its sterile culture filtrate and the corresponding crude ethyl acetate extract exhibited strong, direct suppression of R. solani growth. LC-MS analysis identified potential antifungal compounds, including Pseudomonic Acid, Artemisinin, and Tetradecane, in the extract. In conclusion, P. koreensis A1 is a promising biocontrol and plant growth-promoting candidate for sustainable management of rice sheath blight.