Sensitivity baseline and resistance risk assessment of Erwinia amylovora to zhongshengmycin

Abstract Pear fire blight, caused by Erwinia amylovora , leads to significant economic losses in pear production. Zhongshengmycin, a recommended bactericide in Xinjiang with over 30 years of application history for bacterial disease control, was evaluated for its resistance risk against E. amylovora . This study established the sensitivity baseline of E. amylovora to zhongshengmycin, monitored resistance emergence, and characterized resistant mutants. Using the broth microdilution method, 131 E. amylovora isolates were tested. Results indicated 80 sensitive and 51 low-resistance strains, with a mean EC 50 of 8.34 ± 3.88 µg/mL established as the sensitivity baseline. Four resistant mutants were obtained through stepwise zhongshengmycin domestication, showing resistance factors ranging from 61.92 to 7,611.20. Cross-resistance assays revealed no cross-resistance between zhongshengmycin and kasugamycin or copper oxychloride (|r| < 0.75). The resistant mutants exhibited significantly reduced survival fitness compared to parental strains. The defined baseline and low frequency of resistant isolates suggested a currently low resistance risk in the field, supporting the continued use of zhongshengmycin for fire blight management. However, implementing science-based application strategies, including rotation with non-cross-resistant agents, was recommended to preserve its long-term efficacy.