Genetic diversity and virulence of strains of Pseudomonas syringae pv. actinidiae isolated from Actinidia deliciosa in Piedmont

Pseudomonas syringae pv. sctinidiae (Psa), the causal agent bacterial canker of kiwifruit, is responsible for significant economic losses, both in yield and quality. During the first severe outbreak (2008-2010) and few years afterwards 40 strains of Psa were isolated. To analyse the genetic diversity of the pathogen REP, RAPD-PCR, and MLST of six housekeeping and effector genes was performed. RAPD technique, compared to REP-PCR showed an increased level of resolution in evaluating the genetic diversity within the pathovar actinidiae of P. syringae, with the set of primers used in this study. The molecular fingerprinting, Na, Ne, H, I, polymorphic loci, and AMOVA showed a high level of variability and genetic diversity in the population of Psa isolated in 2014 compared to the population of 2010. MLST showed SNPs in two of the effector genes analysed, while no mutation was found in the housekeeping genes. Furthermore, in vitro and in vivo virulence was evaluated, displaying a higher disease index for the strains isolated in 2014, compared to the older population. This study shows the evolution of genetic diversity and differences in the degree of virulence of Psa in the same geographical area, from the first epidemic outbreak to the endemic and established population. The high degree of the intraspecific variability found in Psa could be an interesting model to study the evolution of bacterial pathogen diversity.