Molecular analysis of the dspEF locus in Erwinia amylovora wild strains of Northern Italy

In the years following the severe 1997 epidemic in Emilia-Romagna region fire blight cases were mainly found on pear and the AFLP analysis showed that the E. amylovora (Ea) populations belonged to the same clone (Ea1994). From 1997 to 2001, there has been an increase of the cases on apple and hawthorn. The introduction of novel strains or the modification of virulence factors in the existing clonal populations was assumed, as well as that the dspEF locus of the Ea1994 clone under immune pressure by different hosts might be involved. The AFLP analysis of serial isolates of Ea1994 during and after 6 passages in parallel on pear, apple and hawthorn showed the same genomic profiles. Modifications of the dspEF locus were investigated by means of dspE gene restriction analysis and of gene sequence analysis of the dspF in 7 strains with virulence higher than Ea1994 and of the dspE gene of Ea1994. The restriction analysis revealed indistinguishable profiles, and sequence analysis showed that the sequences of the dspEF locus correlated 99.9-100% with those of Ea321, a North-American strain. In the Po Valley the existence of more virulent strains and the increase of fire blight cases in apple do not seem attributable to changes in the dspEF locus. The results confirm that the DspE effector and its chaperon, DspF, are highly conserved. In the Po Valley the increase of the fire blight cases on apple might be the result of agronomic factors (i.e. higher plant density, new training forms, new cultivars more susceptible to fire blight, higher presence of 1-7 year implants) associated with a greater amount of the pathogen in the environment.