Investigation on fire blight resistance in the cross population 'Idared' x Malus xrobusta 5 with different Erwinia amylovora strains

Fire blight, which is caused by the bacterial pathogen Erwinia amylovora, is one of the most devastating diseases in apple production. Breeding for fire blight resistance is therefore one of the major aims in nearly all apple breeding programs. In this study a QTL mapping approach for the cross population ‘Idared’ × Malus ×robusta 5 (Mr5) was performed after inoculation with E. amylovora strains differing in their virulence to Mr5 to detect major and minor QTLs. The major QTL on LG3 of Mr5 was stable for all inoculations with strains inducing resistance of Mr5 whereas inoculation with all strains breaking resistance of Mr5 resulted in a breakdown of the QTL on LG3. The results gave further evidence for the existence of a gene-for-gene relationship and demonstrated clearly that the fire blight resistance of Mr5 is strain specific. Several minor QTLs were detected on different linkage groups of Mr5. Additionally, there are indications that Malus baccata ‘Jackii’ acts in a similar manner like Mr5 regarding resistance to fire blight.