Deciphering the volatilome of Bacillus siamensis NKIT9: identification of potent antifungal volatile organic compounds against Rhizoctonia solani.

The soil-borne fungal pathogen Rhizoctonia solani remains a primary constraint to global crop productivity, causing significant damping-off and fruit rot across diverse cropping environments. In response to the ecological demand for sustainable biopesticides, this study explores the potential of bacterial Volatile Organic Compounds (VOCs) as a contact-independent, fumigant-based biocontrol approach. B. siamensis strain NKIT9 was evaluated for its antagonistic activity against R. solani. In vitro sealed-plate bioassays demonstrated that NKIT9-derived volatiles effectively suppress mycelial growth by > 60%. Using Solid-phase Microextraction-Gas Chromatography-Mass Spectrometry (SPME-GC-MS), we characterized a complex volatilome consisting of 45 detected metabolites, of which 29 were identified as bacterium-derived compounds, including ketones, alkanes, and terpenes. The profile contains established antifungal agents such as 6-methyl-2-heptanone, heptadecane, and β-fenchol, the ketone 2-undecanone which were identified as the most potent bioactive constituent. These results suggest that B. siamensis NKIT9 is a promising candidate for further investigation as a source of volatile metabolites with potential application in bio-fumigation strategies for integrated disease management, pending validation under field conditions.