Comparative mapping of fire blight resistance in Malus

Fire blight, caused by enterobacterium Erwinia amylovora, is the most important bacterial disease affecting pome fruit. It can cause devastating economic losses and is reliably controlled only by the application of antibiotics, which are banned in many European countries due to environmental and sustainability issues and consumers' mistrust. One solution could be the utilization of fire blight resistant cultivars in apple production. In 2003, we started an approach at Dresden-Pillnitz to detect different mechanisms conferring resistance to fire blight aimed at their combination in new cultivars. Four segregating populations were established to map QTLs for fire blight resistance. The donors used were three wild species accessions of Malus baccata, M. fusca , M. xrobusta 5 and the Pillnitz cultivar 'Rewena'. The susceptible parent in each case was Idared. Grafted scions of each progeny were inoculated with E. amylovora strain Ea222-JKI. Average percent lesion length (PLL) of all progenies was determined. Genetic linkage maps were established using DArT-SCAR-, SNP-, and SSR-markers. Whereas in Rewena no QTL could be determined, major QTLs were detected in M. baccata on linkage group 12, in M. fusca on linkage group 10, and in M. xrobusta 5 on linkage group 3 explaining up to around 50, 85 and 85% of the phenotypic variance, respectively. The fact that all resistance QTLs are located on different linkage groups enhances the chance that different mechanisms are acting in the donors. Additionally, trees of the Idared by M. xrobusta 5 population were planted in an orchard and flowers were inoculated for phenotyping. The QTL on linkage group 3 could be confirmed after mapping.