Bacillus Thuringiensis Pb-3-1 Strain of Complex Action against PhytonomusVariabillis and Fusarium of Alfalfa and Profile Its Voc

A novel bacterial strain, PB-3-1, was isolated from the rhizosphere soil of alfalfa (Medicago sativa) and identified as Bacillus thuringiensis based on morphological, physiological, and 16S rRNA gene sequence analysis.The objective of this study was to evaluate the insecticidal, antifungal, and plant growth-promoting properties of the PB-3-1 strain and to characterize its volatile organic compounds (VOCs).In laboratory bioassays, treatment with PB-3-1 culture broth resulted in 50% mortality of Phytonomus variabillis larvae by day 10.Antifungal activity was demonstrated against phytopathogenic fungi of the genus Fusarium, with inhibition zones measuring 41.3 2.1mm for F. oxysporum and 32.0 2.0mm for F. solani.A phytotoxicity assay on alfalfa seedlings revealed that PB-3-1 was non-toxic and significantly stimulated plant growth.At a concentration of 10CFU/mL, the culture broth enhanced root and stem elongation by 42% and 68%, respectively, compared to the control.VOC profiling using GC-MS identified 15 components in the culture broth, dominated by pyrazine derivatives, including pyrazine, 2,5-dimethyl-(75.36%),and others.The total pyrazine content constituted 80.15% of the detected VOCs, suggesting their key role in the strain's biological activity.These findings indicate that B. thuringiensis PB-3-1 exhibits strong biocontrol potential against P. variabillis and Fusarium spp., while also promoting alfalfa growth, highlighting its value as a multifunctional biological agent in sustainable crop protection.