Detection of an unstable minor QTL linked to fire blight resistance on linkage group 16 of Malus fusca

ObjectiveErwinia amylovora causes fire blight disease in Malus. A strong resistance QTL (Mfu10) was previously detected on linkage group 10 of Malus fusca accession MAL0045, using several strains of the bacterium. As no strain capable of breaking the resistance of MAL0045 has been found, it was hypothesized that a second resistance factor contributes to the fire blight resistance of MAL0045. However, to date, no minor locus has been detected with previously published strains of the bacterium. We detected a minor QTL only on a subset of a population following inoculation with strain Ea1038, which heterologously expresses an effector in a derivative of isolate Ea3049. Two genetic maps of MAL0045, one scarce, the other dense with markers, were used for QTL analyses. ResultsMfu10 was detected on LG10 with Ea1038, as was previously with Ea3049. Although no other QTLs of significant LOD was previously detected in other linkage groups with Ea3049, a QTL of significant LOD was detected on LG16 (Mfu16) after inoculation of a subset of 76 individuals with Ea1038, but only using the dense genetic map. Mfu16 improved the effect of Mfu10. However, when the number of individuals inoculated with Ea1038 was increased to 121, Mfu16 was no longer detected in the dense genetic map. We hypothesize some factors, which might be responsible for the instability of this QTL.