Occurrence of copper-resistant Pseudomonas syringae pv. syringae strains isolated from rain and kiwifruit orchards also infected by P-s. pv. actinidiae

During spring 2014-2015, leaves showing extensive leaf spotting were collected from Actinidia deliciosa and A. chinensis orchards located in northern, central and southern Italy where a copper-based strategy used to control bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa), was ineffective. In addition, bacterial isolates were recovered from wind-driven raindrops. Psa was isolated solely from raindrops, whereas P. s. pv. syringae was consistently isolated from both symptomatic leaves and rain. The P. s. pv. syringae isolates were molecularly typed using repetitive-sequence PCR and assessed in vitro for their resistance to copper sulphate and for the presence of the cop ABCDRS gene cluster. An ad hoc quantitative real-time PCR was performed to confirm the P. s. pv. syringae capability for Actinidia spp. leaf infection using a low dose of inoculum and co-inoculation with Psa. A copper-resistant P. s. pv. syringae strain isolated from rain effectively colonized Actinidia spp. leaves from a low dose of inoculum. When such a strain was co-inoculated with a reference Psa strain, they reached a higher density than in inoculations performed singly. Copper-resistant P. s. pv. syringae strains infecting the leaves of Actinidia spp. appear to be widespread in Italy and, in some circumstances, they could have replaced Psa in leaf colonization and disease development. Populations of this phytopathogen showing resistance to copper would appear to be dispersed in the environment by the rain. This raises questions concerning the reliability of long-term strategies to control kiwifruit bacterial canker caused by Psa solely with copper compounds.