Synergistic Control Effect of Bacillus pumilus AR03 and Fungicides Against Tobacco Bacterial Wilt

To explore the potential of the control synergism of B. pumilus AR03 combined with seven tested fungicides against tobacco bacterial wilt, the toxicity of seven fungicides and AR03 strain against Ralstonia.solanacearum and the bio-compatibility of fungicides with AR03 were measured by the improved inhibition zone method and the plate colony-counting method, and the synergistic effect was determined by the Horsfall method. The results of in vitro toxicity test indicated that seven fungicides and AR03 all exhibited a good inhibitory effect on the growth of R. solanacearum, and the virulence of seven fungicides were trichloroiso cyanuric acid>chloroisobromine cyanuric acid plus copper sulfate>thiodiazole copper>bromothalonil plus cuppric nonyl phenolsulfonate> Metalaxyl plus thiram> thiadiazole zinc>Zhongshengmycin, and the half-maximal inhibitor concentration (EC₅₀) ranged from 101.02-212.70 mg/L. B. pumilus AR03 also had a good inhibition rate (26.13%-73.54%) in a dose dependent manner at the concentration from 1.0×10⁵ to 1.0×10⁹ cfu/mL. Results from the bio-compatibility test showed that AR03 had good biocompatibility with thiodiazole copper, Metalaxyl plus thiram, thiadiazole zinc, while thiodiazole copper performed the best. When the test concentration of 100 mg/L, the number of colonies was greater than 1×10⁷ cfu/mL. The mixture of thiodiazole copper (EC₅₀=175.21 mg/L) and AR03 (EC₅₀=6.84×10⁶ cfu/mL) in the volume ratio of 5:5 behaved a notable inhibitory and synergistic effect against R. solanacearum and yielded an I_R value of 1.482. The results of in vivo experiment showed that the control effect of the combination was significantly higher than that of thiodiazole copper and AR03 strain treatment alone. More importantly, the usage volume of thiodiazole copper in the mixture was half of the single agent, which greatly reduced the use of chemical agents.