Pseudomonas syringae pv. actinidiae strains isolated from past and current epidemics to Actinidia spp. reveal a diverse population structure of the pathogen

A total of 40 Pseudomonas syringae pv. actinidiae (Psa) strains isolated from past and current epidemics of kiwifruit bacterial canker worldwide were compared using repetitive-sequence PCR (rep-PCR) fingerprinting with BOX, ERIC and REP primer sets. The strains were also assessed for the presence of 23 type III secretion system effector genes, tolerance to sodium arsenite, the presence of coronatine and phaseolotoxin and for growth trends in Actinidia deliciosa leaves. Rep-PCR revealed the occurrence of 11 different Psa lineages among the strains and indicated a relevant genetic variability within the strains isolated in Japan during 1984–2011, whereas all strains obtained from the current pandemic showed the same banding pattern. All lineages showed the same growth trend when inoculated into A. deliciosa leaves. The difference between Psa strains from past and current epidemics was confirmed by the detection of different repertoires of type III effector protein genes. Not all Psa strains isolated in Japan during past epidemics of kiwifruit bacterial canker amplify genes of the argK-tox cluster of phaseolotoxin, suggesting their absence or gene mutation. The results of an arsenic tolerance assay indicated that almost all strains isolated in Italy during the current epidemic of kiwifruit bacterial canker were relatively tolerant to 0.15–0.35 mM sodium arsenite, whereas those isolated in Chile and New Zealand were very sensitive. Remarkably, the two strains from China were tolerant or very sensitive. Collectively, these data indicate a composite population structure of this pathogen, which was able to diversify in Japan during 27 years of recurrent infections to A. deliciosa. The current naming of Psa populations based on their numbering and presence/absence of phytotoxins should be reconsidered.