Complete Elimination of Agrobacterium by Bacillus velezensis RTS-M۱۱ in Co-Culture: A Novel Biocontrol Strategy for Rose Crown Gall Disease

Agrobacterium is a phytopathogenic bacterium responsible for crown gall disease, a significant agricultural problem affecting economically important crops, including roses. The emergence of antibiotic-resistant strains has complicated disease management, necessitating alternative control strategies. This study investigated the antagonistic activity of Bacillus velezensis RTS-M۱۱ against two Agrobacterium isolates (designated Agro A and Agro B) obtained from crown gall-infected Dutch roses cultivated in Shiraz, Iran. Co-culture experiments were performed in liquid medium at three initial ratios (۱:۱, ۱:۲, ۱:۳). Serial dilution and plating on Nutrient Agar were used to quantify surviving colonies. Antibiotic susceptibility testing was performed on both Agro A and Agro B using the disk diffusion method against ۱۲ clinically relevant antibiotics. B. velezensis completely eliminated both Agrobacterium isolates in every co-culture condition, with ۰ CFU detected across all dilutions and ratios. In contrast, Agrobacterium controls showed normal growth (>۱۰⁴ CFU), confirming that elimination was specifically due to the presence of Bacillus. The Agrobacterium isolates were confirmed as pathogenic by a positive carrot disc assay and characterized biochemically as catalase+, oxidase+, urease+, citrate+, with growth at ۳۵°C, consistent with the Agrobacterium tumefaciens complex. Antibiotic susceptibility testing revealed that both isolates were resistant to cephalosporins (cefazolin, cefuroxime) and colistin, while remaining susceptible to ciprofloxacin, imipenem, trimethoprim-sulfamethoxazole, and doxycycline. These findings demonstrate that B. velezensis RTS-M۱۱ possesses potent, strain-independent antagonistic activity against rose infecting Agrobacterium, including multidrug-resistant isolates. This supports its development as a promising biocontrol agent for sustainable rose cultivation.

کلیدواژه ها:

Bacillus velezensis ، Agrobacterium ، biocontrol ، rose crown gall ، antibiotic resistance ، Iran