Genetic and phenotypic diversity of Mediterranean populations of the olive knot pathogen, Pseudomonas savastanoi pv. savastanoi

Pseudomonas savastanoi pv. savastanoi (Psav) is a member of P. syringae sensu lato, and causes olive knot disease, a disease first reported over 2000 years ago. Analysing 124 isolates of Psav from 15 countries by rep-PCR, the population genetic structure of Psav was investigated. A total of 113 distinct fingerprints were detected. Cluster analysis revealed the existence of two clusters and four sub-clusters. These clusters were associated with the geographic origin of strains, which in turn correspond to historic human migration events and trade routes across the Mediterranean Sea. In contrast, multilocus sequence typing (MLST) of 2788bp of the gapA, gltA, gyrB and rpoD genes found only 1 variable site among 77 representative strains. Virulence variation was observed within the Psav population with the most virulent strains generating knots that had a weight that was 10 fold greater than those generated by the least virulent strains. Taken together, these data suggest that today's Psav population is the result of clonal expansion of a single strain, that moderate migration of the pathogen occurred between countries, and that changes in virulence arose during its evolution.