Further insights into Malus fusca fire blight resistance
The apple wild species accession Malus fusca MAL0045 had been found to be resistant to fire blight in artificial inoculation trials with Erwinia amylovora strain Ea222_JKI. Consequently, using a population derived by crossing MAL0045 with Idared, the corresponding fire blight resistance locus of M. fusca (Mfu10) was mapped on chromosome 10 explaining up to 66% of phenotypic variation at a logarithm of the odd (LOD) ratio of 31.0 with the strain Ea222_JKI. Moreover, the very aggressive strain Ea3049 only minimally affected MAL0045 but significantly affected the population although could not overcome the resistance of Mfu10. To further understand the resistance mechanism of M. fusca, we evaluated resistance of the original mapping population, comprised of 134 individuals, to E. amylovora strain ZYRKD3-1 which causes the breakdown of the resistance of M.×robusta 5 (Mr5) and the Mr5 fire blight resistance QTL on LG3. Our results showed that the major QTL of M. fusca on LG10 could still be detected at the same exact position with a higher effect on fire blight resistance, indicating that ZYRKD3-1 has no effect on Mfu10, although the mean percent lesion length of the population was almost doubled compared with Ea222_JKI.