Fire blight in Emilia-Romagna and Veneto regions:virulence and genomic variability of Erwinia amylovora strains

The official cases of fire blight has been remarkable increased in host plants other than pear-tree in Emilia-Romagna (Northern Italy) beginning from 1997 when the first certified cases were also reported in the close Veneto. In this region the territorial distribution of the new foci in the following years and the genomic characteristics of the Erwinia amylovora (Ea) isolates based on the AFLP analysis pointed out the spread of the pathogen from the Emilia-Romagna border to north-west. Moreover, the AFLP genomic analysis of Venetian and Emilian strains revealed that all of them belong to the same clone that had caused the primary foci in 1994 in Emilia-Romagna. Adaptation to host plants other than pear may modify genetic frequencies in Ea sub-populations with possible effects even on virulence. In this study the AFLP fingerprints of the Emilian isolates from different host plants were very similar (SD³ 0.987). The virulence analysis of the strains from the two regions revealed two homogeneous groups not correlated to the host plant or the province. The majority of the strains had a virulence slightly lower than that of the reference strain OMP-BO 1077/7, associated to the primary foci in 1994, and only 7 strains had a virulence higher not correlated to the host plant or the province.