| Bacterial blight of rice (BB) and bacterial leaf streak of rice (BLS) were caused by Xanthomonas oryzae pv. oryzae and X. oryzae pv. oryzicola, respectively. BB is one of the most serious diseases of rice, and BLS is emerging in importance. Application of bactericides is an indispensable complementary tool, especially in regions where susceptible cultivars are frequently planted. Bismerthiazol and streptomycin are mainly bactericides wich have been used for the control of bacterial plant disease, but bismerthiazol-resistant stains of X. oryzae pv. oryzae and streptomycin-resistant strains of X. oryzae pv. oryzicola have been found. In our study, monitoring of bismerthiazol resistance in X. oryzae pv. oryzae has been done in vivo, and the results showed that bismerthiazol-resistant stains of X. oryzae pv. oryzae are common in rice fields. Monitoring of streptomycin resistance in X. oryzae pv. oryzae and X. oryzae pv. oryzicola has been done in vitro and resistance risk in vitro has been evaluated.In 2007 to 2009,505 single-colony strains of X. oryzae pv. oryzae were obtained from diseased rice leaves which were collected from provinces of China including Yunnan, Jiangsu, Anhui, Hubei, Guangdong, Hainan and Hunan Provinces. The sensitivity of these strains to bismerthiazol was tested by evaluating the blight severity after inoculation on the treated rice plants.The resistance criterion for the in-vivo test was defined by the distribution frequency of relative inhibition of field isolates treated by 300μg/ml bismerthiazol, in which the tested isolate was declared resistant when the inhibition of it was less than 70% of the reference strain ZJ173. According to this discriminatory criterion,62 isolates (12.28% of tested population) of X. oryzae pv. oryzae appeared resistant to bismerthiazol. Resistance frequencies in 2007,2008 and 2009 in the pathogen population were 11.21%,20.00% and 10.42%, respectively. However, there were no significant differences in the sensitivity of the seven arbitrarily selected bismerthiazol-resisitant and three bismerthiazol-sensitive strains, which were identified on the treated rice plants in vitro (according to the EC50 values).In 2008,148 single-colony strains of X. oryzae pv. oryzae and 344 single-colony strains of X. oryzae pv. oryzicola were obtained from diseased rice leaves which were collected from five provinces of China including Yunnan, Sichuan, Jiangsu, Anhui and Hunan Provinces. In 2009,96 single-colony strains of X. oryzae pv. oryzae and 43 single-colony strains of X. oryzae pv. oryzicola were obtained from diseased rice leaves which were collected from four provinces of China including Jiangsu, Guangdong, Hunan and Jiangxi Provinces.The sensitivity of these strains to streptomycin was tested by the distinguishing dose method. In our survey of streptomycin resistance in X. oryzae pv. oryzae and X. oryzae pv. oryzicola, four streptomycin-resistant isolates of X. oryzae pv. oryzae were found in one county of Yunnan Province in 2007. So, the overall resistance frequency was very low.Lab streptomycin-resistance was induced by both ultraviolet illumination and streptomycin tame, and four resistance mutants of X. oryzae pv. oryzicola were obtained. MIC, EC50, cross-resistance and pathogenicity were compared between the sensitive strains and streptomycin-resistants. The MIC values of streptomycin against the four sensitive strains were less than 1.6μg/ml, however, the MIC values of streptomycin to the mutants were more than 800μg/ml. The EC5o values of streptomycin to the sensitive strains RS105 were 0.20μg/ml, but the EC5o values of streptomycin to the mutants were ranged from 9.72-115.66μg/ml, the resistance factor (RF) was ranged from 48 to 569. The resistance was stable after the mutants were cultured in the streptomycin-free media, and there was no difference in pathogenicity between sensitive strains and streptomycin-resistance mutants. The results suggested that streptomycin application in the field may result in the resistance risk. And the cross-resistance test showed that there was no cross-resistance between streptomycin and amicarthiazol or tetracycline in X. oryzae pv. orvzicola. |
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