Virulence and genomic variability of Erwinia amylovora strains in Northeastern Italy

During 1997, in occasion of the severe epidemic caused by Erwinia amylovora (Ea) on pear-tree in Emilia-Romagna, the first certified cases of fire blight were also reported in Veneto. Of the 14 ascertained foci, 11 were found in Rovigo province, ie. in areas facing the northern border of Emilia-Romagna and 2 were found in Padua and Verona provinces. In the following two years, the territorial distribution of the new foci in Verona, Padua and Venice provinces clearly pointed out that from Venetian areas close to the border with Emilia-Romagna the Ea was spreading mostly towards the north-west. In this study, the AFLP genomic analysis of 14 isolates from pear-tree showed that all Venetian isolates were referred to the same clone originally coming from Emilia-Romagna. After 1997 in Emilia-Romagna there has been a substantial increase in the official cases of fire blight on host plants other than pear-tree. From the clonal lineage of Ea, introduced in 1994, may originate secondary lines different for pathogenic traits selected by the new host plant or a new clonal lineage could have occurred. The AFLP of the Ea strains isolated from host plants other than pear-tree was carried out in order to study the genetic variability between the Emilian strains which were found to belong to the original clone. Moreover, to test the hypothesis of continuity of the original clonal lineage, the changes of the virulence grade among the strains was studied. The pathogenicity test on immature pear fruitlets was performed measuring the minimum/maximum axis of the lesion induced by each strain (14 Venetian and 47 Emilian strains). The strain OMP-BO1077/7, associated to the primary foci in Emilia-Romagna on pear during 1994, was used as a reference strain. For each strain the virulence index was calculated and statistical analysis was used to determine the significant difference with the reference strain