Identification of a major quantitative trait locus for resistance to fire blight in the wild apple species Malus fusca

Fire blight, caused by the Gram-negative bacterium Erwinia amylovora, is the most important bacterial disease affecting apple (Malus x domestica) and pear (Pyrus communis) production. The use of antibiotic treatment, though effective to some degree, is forbidden or strictly regulated in many European countries, and hence an alternative means of control is essential. The planting of fire blight-resistant cultivars seems to be a highly feasible strategy. In this study, we explored a segregating population derived from a cross between the wild apple species Malus fusca and the M. x domestica cultivar Idared.F1 progenies used for mapping were artificially inoculated with Erwinia amylovora strain Ea222_JKI at a concentration of 10(9) cfu/ml in three different years. The averages of percentage lesion length of all replicates of each genotype were used as numerical traits for statistical analysis. A Kruskal-Wallis analysis was used to determine marker-phenotype association and revealed a linkage group with Diversity Arrays Technology (DArT) markers significantly linked with fire blight. After locating the positions of the DArT markers on the Golden Delicious genome, simple sequence repeat (SSR) markers were developed from chromosome 10 to replace the DArT markers and to determine the quantitative trait locus (QTL) region. Multiple QTL mapping (MQM) revealed a strong QTL (Mfu10) on linkage group 10 of M. fusca explaining about 65.6 % of the phenotypic variation. This is the first report on a fire blight resistance QTL of M. fusca.