STUDY OF THE VIRULENCE IN WILD-TYPE STRAINS OF ERWINIA AMYLOVORA DEVOID OF THE PLASMID pEA29

The recent discovery of Spanish strains of Erwinia amylovora that do not harbor the plasmid pEA29, opens new possibilities to study the role of this plasmid in virulence of E. amylovora. A screening of the plasmid content of strains from international collections and isolates from Spain has been performed and eight strains lacking the pEA29 have been found so far (three from Spain, two from Serbia-Montenegro, two from Belgium and one from the United Kingdom). These strains were compared to reference strains of E. amylovora. The results showed very similar biochemical and molecular characteristics; however, four of the strains carried a plasmid of higher size (circa 70 kb), named pEI70, as demonstrated by restriction analyses and hybridization assays. The symptoms produced by these strains in nature were similar to that of strains that carry pEA29, and these results have led to study the virulence level of these strains and the possible relationship with their plasmid content because of the described effect of pEA29 on this feature. The virulence analyses after inoculation of immature pear showed the existence of differential level of virulence depending on the strain, but all the strains without the pEA29 increased their virulence after the introduction of this plasmid, confirming its effect on the virulence of E. amylovora.