Pseudomonas syringae pv. actinidiae isolated from Actinidia chinensis Var. deliciosa in Northern Italy: genetic diversity and virulence

Bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae, is responsible for significant economic losses, both in yield and quality. A collection of P.s. pv. actinidiae isolated in Northern and Central Italy, during the first severe outbreak (2008-2010) and a few years afterwards (2014), when the pathogen became endemic and established, was gathered. The genetic diversity was evaluated with rep-PCR, RAPD-PCR, and MLSA of six housekeeping and effector genes. On the same strains, the virulence was evaluated in vitro and in vivo, showing a higher disease index for the strains isolated in 2014, compared with the strains of 2010. The molecular fingerprinting, obtained by rep and RAPD analysis, revealed a high level of variability in the population of strains of P.s. pv. actinidiae from northern Italy. All the parameters considered - Na, Ne, H, I, polymorphic loci, and AMOVA - showed a higher genetic diversity within the population of northern Italy isolated in 2014, compared to the older population. The study of the genetic diversity and virulence permitted to show an increase of virulence and genetic diversity of the strains. The fast and dramatic epidemics caused by P. s. pv. actinidiae could be an interesting model to study the changes in the genetic diversity and virulence of a bacterial pathogen.