New insights about the complexity of Pseudomonas syringae pv. actinidiae across the world

Abstract

The bacterial canker of kiwifruit caused by Pseudomonas syringae pv. actinidiae (Psa), is an emblematic example of a catastrophic disease of fruit crops. In 2008, after decades of limited damages, a new, extremely virulent form emerged and devastated thousands of hectares of Actinidia spp. orchards all over the world with impressive economic losses. To elucidate the diffusion of different Psa types on world scale and their presumptive movements, it is necessary to recognize genetic diversities within its populations. We analyzed an exhaustive collection of 142 Psa strains, that included a conspicuous number isolated in the Asiatic area of origin of Actinidia spp. MLVA has been chosen as the investigative technique for its efficacy and reproducibility, as well as its simplicity and low cost. A panel of 13 VNTR loci was identified and used to study the Psa complex. Results indicate that this MLVA assay can reveal a much wider diversity among Psa populations throughout the world than previously assessed. The congruence of results with those of other molecular approaches was demonstrated and the VNTRs loci were divided in two different panels having an increasing discriminatory power. Ten main clonal complexes were depicted that correlate repeatedly with geographic origin of strains, but also with their virulence and isolation time. The recently originated hypervirulent type of Psa, in which, beside the known strains from Europe, New Zealand and Chile, only few strains from Shaanxi Province in China are also included, confirmed to be genetically very homogeneous. China proved to be the area where the broadest genetic variability resides but, interestingly, different Psa types are retrievable in both Japan and Korea. The low virulent strains group together and are deeply different from any other Psa, so that they can no longer be considered as a real Psa variant. The effectiveness of MLVA in resolving differences between Psa strains and related implications on pathogen spread are discussed.