Properties of Bacillus sp. Strains with Antifungal and Plant Growth Promoting Activities

<b>Background and Objective:</b> Onion (<i>Allium cepa</i> L.) is one of the most important commercial vegetable crops worldwide with high economic value. However, fungal diseases seriously threaten onion production by reducing yield and bulb quality. For sustainable disease management, the use of biological control agents has emerged as an environmentally friendly and effective alternative to chemical fungicides. This study aimed to select potential antifungal bacteria that can promote plant growth in onion. <b>Materials and Methods:</b> Bacterial isolates were isolated from rhizosphere soil samples, screened and characterized for antifungal activities. Strong antagonistic bacterial strains were selected. These bacterial strains were identified and characterized by their biochemical features. In addition, three selected strains were evaluated for their growth-promoting capacity by pot assay. All experiments were conducted in triplicate; data were expressed as mean±SD and analyzed using one-way ANOVA followed by Tukey's test (p<0.05) in GraphPad Prism 9.3.1. <b>Results:</b> Three bacterial. strains exhibited strong antagonistic activity against <i>Sclerotium rolfsii</i> and <i>Curvularia lunata</i> through the production of diffusible compounds and volatile organic compounds (VOCs). In addition, physiological and biochemical characterization revealed that all strains belonged to the genus <i>Bacillus.</i> Moreover, soil amendment with the culture of <i>Bacillus</i> sp. T3, T33 and T36 significantly enhanced onion growth. <b>Conclusion:</b> These findings suggest that the selected <i>Bacillus</i> sp. strains represent promising candidates for the development of biocontrol agents against fungal pathogens.