【Objective】Pear fire blight caused by Erwinia amylovora is an international quarantine bacterial disease. It is one of the top ten plant pathogenic bacteria in the world with fast transmission speed, multiple transmission routes and wide host range. Biopesticide tetramycin is a pure green biopesticide developed by Liaoning Academy of Microbial Sciences in the 1970s and realized product in the early 1990s.Tetramycin can inhibit both bacterial and fungal plant diseases. At present, there are no reports on the establishment of sensitivity baseline and the evaluation of resistance of tetramycin in Erwinia amylovora in China.To establish a sensitivity baseline for Erwinia amylovora to tetramycin in Korla and Aksu regions of Xinjiang, and monitor the resistance level of pear fire blight to tetramycin in the region.This study provided a scientific basis for the field administration of tetramycin against pear fire blight, antibiotic resistance monitoring and antibiotic resistance management. 【Methods】 To isolate E. amylovora strains, samples were collected from 2021 to 2023, in ten areas in Xinjing, including Bayinguoleng and Aksu.Bacterial strains were isolated from the infected plant materials of fragrant pears.The sensitivity of 100 strains of Erwinia amylovora isolated to tetramycin was determined by inhibition zone method. Drug-resistant mutants were acclimated by streak plate method and drug containing medium method, followed by drug resistance evaluation. The preserved strains were separated in NA medium, incubated at 28 ℃ for 48 h, and single colonies were obtained by secondary activation. The single colony was transferred to NB culture medium at 28 ℃, 180 r/min for 12 h, and the test bacterial suspension was obtained when OD600 was determined to be 1.0. After the bacterial suspension was diluted to 1×104 cfu·m L-1, 100µL was coated in NA medium until dry. Three pieces of sterilized filter paper with a diameter of 6 mm were placed on NA medium in the petri dish.6µl of different concentrations of drug drops were absorbed on the filter paper sheet, with 5 treatment concentrations for each drug solution and 2 dishes for each concentration, for a total of 6 repetitions, with sterile water as the control. After being incubated at 28 ℃ for 36 h, the diameter of the antibacterial zone was measured by the cross method, and the antibacterial rate was calculated according to formula. The correlation coefficient and EC50 value were calculated according to the bacteriostasis rate. Based on the average EC50 values of all tested strains, the sensitivity baseline of tetramycin in Xinjiang was established. 【Results】 The results showed that all the tested strains, 74 sensitive strains were detected, accounting for 74%, and 26 resistant strains were detected, accounting for 26%,The distribution of EC50 values of 100 strains of Erwinia amylovora ranged from 0.199-4.84 µg/mL, with the maximum EC50 is 24.32 times the minimum EC50, the average EC50 being 1.59±1.029 µg/mL, and the 95% confidence interval being 1.3892-1.7974 µg/mL. The distribution of tetramycin to Erwinia amylovora strains showed a unimodal curve, which was similar to the normal distribution, and no Erwinia amylovora strains with significantly decreased sensitivity were found. Therefore, the mean EC50 value of 1.59±1.029 µg/mL could be used as the baseline for the sensitivity of Erwinia amylovora strains to tetramycin in southern Xinjiang. There were some differences in the sensitivity of 100 strains of Erwinia amylovora in from 4 regions, strains from Aksu were more sensitive, with mean EC50 value of 0.780 ± 0.436 µg /mL, which was significantly different from strains from Awati Township, Halayugong Township and Tiemengguan.After the four Erwinia amylovora resistance mutants were transferred to NA medium for 10 successive generations, the resistance levels of the 11th generation resistance mutants were 0.275 µg/mL, 0.43 µg/mL, 0.145 µg/mL and 0.564 µg/mL, respectively, which all decreased from the resistance level of low antibacterial strains to the resistance level of sensitive strains.The results indicated that the 4 resistant mutants was not genetic stability. The EC50 values of the Erwinia amylovora resistance mutant against tetramycin were highly correlated with the EC50 values of 84% benziothiazolinone and zhongshengmycin, and the correlation coefficients were 0.7658 and 0.9008, respectively, and the absolute values of all correlation coefficients were higher than 0.75. It can be seen that there is a cross-resistance problem between tetramycin and 84% benziothiazolinone and zhongshengmycin, which are commonly used in field to control Erwinia amylovora, so the three fungicides should be avoided as alternate standby pesticides in the field to avoid cross-resistance problems.Thus, it was confirmed that tetramycin had cross-resistance with the 84% benziothiazolinone and zhongshengmycin.【Conclusion】The basic resistance of strains in different regions to fungicides is determined by both the agent and the pathogen itself. The sensitivity baseline value of the E. amylovora isolated strains in Xinjiang to benziothiazolinone could be established according to this study.Only a few low resistant subgroup strains were detected, and the risk of resistance to tetramycin was at low level.
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