The Malus fusca fire blight resistance locus: validation of Mfu10 and genetic resolution of the region containing the locus

The apple wild species accession Malus fusca MAL0045 was found to be resistant in artificial inoculation trials with fire blight. Erwinia amylovora strain Ea222_JKI, usually used for mapping purposes in Dresden, as well as the highly virulent strain Ea3049 did not affect MAL0045. The corresponding fire blight resistance locus of M. fusca (Mfu10) was previously mapped on chromosome 10. This quantitative trait locus explaining up to 66 % of phenotypic variation at a logarithm of the odd (LOD) ratio of 31.0 was based on 134 individuals derived from a cross of M. fusca × Idared and phenotyped with Erwinia amylovora strain Ea222_JKI. To gain more insight into the resistance mechanism of M. fusca, we phenotyped the original mapping population of 134 individuals with the highly virulent E. amylovora resistance-breaking strain of M. ×robusta 5 - Ea3049, and fine mapped the region containing Mfu10. Ea3049 could not break down the resistance of Mfu10 but significantly affected it. Furthermore, the closest tightly linked SSR markers to Mfu10 were used to genotype an increased population of 1,202 individuals, leading to the identification of interesting recombinants in the region of interest. The increase in population, addition of more tightly linked SSR markers to linkage group 10 as well as the phenotypic evaluation of recombinants ensured the genetic resolution of the resistance locus.