MOLECULAR ANALYSIS OF THE dspEF LOCUS IN ERWINIA AMYLOVORA WILD STRAINS OF NORTHERN ITALY
P. Minardi, S. Mucini, U. Mazzucchi
In the years following the severe 1997 epidemic in Emilia-Romagna (northern Italy), 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 in outbreaks on apple and hawthorn. The introduction of novel bacterial strains or the modification of virulence factors in the existing clonal populations was hypothesized, as well as the involvement of the dspEF locus of clone Ea1994 under immune pressure by different hosts. The AFLP analysis of serial isolates of Ea1994 during and after 6 passages 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 dspF in 7 strains with virulence higher than Ea1994 and of the dspE gene of Ea1994. Restriction analysis revealed indistinguishable profiles, and sequence analysis showed that the sequences of the dspEF locus totally correlated (99.9- 100%) with those of Ea321, a North-American Ea 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 confirmed that the DspE effector and its chaperon DspF, are highly conserved. Thus, the increase of the fire blight cases on apple in this area 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 plantings) associated with a higher inoculum potential in the environment.