Prevalence of copper-resistant Pseudomonas syringae pv. actinidiae biovar 3 in Jiangxi Province, China

Bacterial canker of the kiwifruit caused by the etiological agent Pseudomonas syringae pv. actinidiae (Psa) is the most severe disease in kiwifruit production. Since 2008, a hypervirulent Psa biovar 3 has spread rapidly worldwide. This study presents the first systematic characterization of Psa in Jiangxi Province, a major kiwifruit-growing region of China. Among 42 bacterial isolates collected from six production areas, all were identified as the highly pathogenic Psa biovar 3 through morphological, PCR-based, and multilocus sequence analyses. In vitro bioassays revealed that tetramycin exhibited the strongest inhibitory activity against a representative strain (F7), with an EC 50 of approximately 0.002 mg/L, followed by bronopol, ethylicin, and benziothiazolinone. Notably, no copper-sensitive strains were detected, and all isolates showed a high level of copper resistance, with minimum inhibitory concentrations (MICs) values of 1.80–2.60 mM of copper sulfate, far above the conventional resistance threshold. These results demonstrate that Psa biovar 3 is the dominant pathogen in Jiangxi and that local populations pose a significant risk of copper resistance, highlighting the urgent need for integrated management strategies that reduce reliance on copper-based bactericides.