Occurrence of copper‐resistant Pseudomonas syringae pv. actinidiae strains in kiwifruit orchards of Central Italy

Bacterial isolates belonging to Pseudomonas syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker, have been isolated, during 2021 and 2022, from Actinidia chinensis orchards of Central Italy planted with cultivars Hayward and Zespri G3®. A total of 23 isolates were obtained from twigs and leaves that showed the typical symptoms of bacterial canker, including leaf spotting and twig canker. All of them showed resistance to 1.2 mM copper sulphate added to mannitol–glutamate–yeast extract (MGY) medium. No other Pseudomonas syringae pv. actinidiae strains were iso- lated from the orchards. The P. s. pv. actinidiae strains that showed resistance to cop- per have the copA and copD genes that regulate the efflux of copper through the inner membrane and the copper sensor copS gene. They do not possess the cusABC com- plex that confers resistance to high concentration of copper. Representative copper- resistant strains were proven pathogenic to kiwifruit plants upon artificial inoculation. This is the first record of copper-resistant P. s. pv. actinidiae strains in Italy and, as far as we know, in Europe.