CLONAL OUTBREAKS OF BACTERIAL CANKER CAUSED BY PSEUDOMONAS SYRINGAE pv. ACTINIDIAE ON ACTINIDIA CHINENSIS AND A. DELICIOSA IN ITALY

A total of 28 representative Pseudomonas syringae pv. actinidiae strains isolated from all Italian regions (Emilia-Romagna, Latium, Piedmont, Veneto) where outbreaks of bacterial canker of kiwifruit (Actinidia deliciosa) and yellow kiwifruit (A. chinensis) were observed in 2008-2010, were assessed using repetitive-sequence PCR (rep-PCR) with ERIC and BOX primer sets and multilocus sequence typing (MLST) using gapA, gltA, gyrB and rpoD genes. The 2.3 kb sequences obtained from MLST were analyzed by means of mathematical-statistical tests to infer the gene polymorphism and the genetic structure of the strains. Both primer sets used in rep-PCR indicated an overall identity among all 28 Ps. pv. actinidiae strains irrespective of the host plant and cultivar from where they were isolated, as well as of the region or year of isolation. In addition, MLST revealed a low gene polymorphism. A clonal structure and neutral selection were inferred for the P.s. pv. actinidiae strains currently causing severe epidemics on A. chinensis and A. deliciosa in Italy. This indicates that they originated, most probably, from a single or very few introductions of latently infected kiwifruit propagative material, even though the possibility cannot be ruled out that cells of the pathogen already present in Italy may have mutated.