Fire blight resistance of Malus xarnoldiana is controlled by a quantitative trait locus located at the distal end of linkage group 12

Erwinia amylovora is the pathogen which incites fire blight - the most devastating bacterial disease affecting pome fruit production worldwide. So far, only one functionally proven gene for resistance to this disease has been reported in Malus, even though several quantitative trait loci (QTLs) have been identified. Since some highly aggressive strains are capable of overcoming described resistances in Malus, including that of the only isolated and functionally-proven resistance gene, it is important to identify more resistance donors useful for the establishment of pyramided and durable resistance. Quantitative trait locus analyses were performed on F-1 progenies derived from a cross between the Malus xdomestica cultivar, Idared, and the wild apple species M. xarnoldiana. Progenies were artificially inoculated with two strains of E. amylovora - the moderately aggressive strain Ea222_JKI, and the highly aggressive Canadian strain, Ea3049. A QTL was identified at the distal end of linkage group (LG) 12 of M. xarnoldiana explaining over 50% of the phenotypic variation among the progenies inoculated with Ea222_JKI. A significant QTL effect was also identified on LG12 using the phenotypic data of the progenies inoculated with Ea3049. The fire blight resistance QTL of M. xarnoldiana (FB_Mar12) appeared to be located at a similar position as those of Evereste and/or M. floribunda clone 821 below the SSR marker Hi07f01 on LG12 of the apple genome. The results presented here suggest that this QTL could be allelic with the other reported fire blight QTLs on LG12. Nevertheless, it is imperative to perform more studies aimed at further elucidating the resistance mechanism of this wild apple species since it displays a significant resistance effect to Ea3049.