Baseline sensitivity of natural populations and resistance of mutants of Xanthomonas oryzae pv. oryzae to a novel bactericide, zinc thiazole



During 2009–2010, a total of 323 isolates of Xanthomonas oryzae pv. oryzae were obtained from rice with symptoms of bacterial leaf blight (BLB) in four provinces (Zhejiang, Jiangsu, Anhui and Hubei) in China. These isolates were tested for baseline sensitivity to zinc thiazole, a novel bactericide with strong antibacterial activity against Xanthomonas. The sampled pathogenic population had similar sensitivity to zinc thiazole (0·1–16·8 mg L−1) in all four regions and over the whole two-year study period. The baseline sensitivity was distributed as a unimodal curve with a mean EC50 value of 6·79 ± 1·61 mg L−1. The risk of mutation to resistance of zinc thiazole in X. oryzae pv. oryzae was further evaluated in vitro and in vivo. Twelve zinc thiazole-resistant mutants were obtained through ultraviolet (UV) irradiation, culturing on zinc thiazole-amended nutrient agar (NA) plates, and culturing on zinc thiazole-treated rice plants. These zinc thiazole-resistant mutants had resistance factors (RF = EC50 value of a mutant / EC50 value of the wildtype parent of this mutant) of 12·4 to 186·1 with a mean RF value of 44·1. Mutants obtained via UV irradiation, culturing on NA plates and culturing on rice plants had mean RF values of 51·8, 24·5 and 14·4, respectively. All mutants showed decreases in resistance to zinc thiazole after 20 successive transfers on bactericide-free media or 10 successive inoculation–reisolations on bactericide-free rice plants. No significant difference was found in bacterial growth and sensitivity to bismerthiazol between zinc thiazole-resistant mutants and their parents. However, a significant decrease was observed in the pathogenicity of zinc thiazole-resistant mutants compared with their parents, especially for mutants obtained via UV irradiation.