Evidence of a Major QTL for Fire Blight Resistance in the Apple Wild Species Malus fusca

Fire blight is a common and very destructive disease affecting apple (Malus xdomestica) and pear (Pyrus communis) productions. Caused by enterobacterium Erwinia amylovora, its devastating economic effects ranges from losses in yield to severe damage or death of trees in the orchard, and in extreme cases loss of a whole orchard. The most effective management measure available to producers is the use of antibiotics. However, dependence on antibiotics treatment is not sustainable given risks such as the rapid selection of antibiotic-resistant Erwinia populations and environmental hazards, which have led to their application being forbidden in many countries. Therefore the planting of fire blight-resistant cultivars seems to be the most promising alternative strategy. However until now, there is no fire blight-resistant apple cultivar produced for the global market. Host resistance to fire blight in Malus is thought to be quantitatively controlled. Thus, several quantitative trait loci for resistance have been identified in some accessions of Malus spp. In this study, we explored a segregating population derived from a cross between the apple wild species Malus fusca and the Malus xdomestica cultivar 'Idared'. F-1 progenies used for mapping were artificially inoculated with E. amylovora strain Ea222 at a concentration of 109 CFU/ml in two different years. The averages of percentage lesion length (PLL) of all replicates of each genotype were used as numerical traits for statistical analysis. A Kruskal-Wallis analysis, used to determine marker-pheno-type association, revealed two DArT markers (971000 and 970840) with the highest significance. QTL mapping revealed a strong QTL explaining above 60% of the phenotypic variation. This is the first report of a fire blight resistance QTL in M. fusca.