Genotypic and phenotypic analyses of fire blight resistance and inheritance in apple cultivars and wild Malus accessions of the Fruit Genebank of the Julius Kühn-Institut (JKI-ZO)

Apples ( Malus domestica Borkh.) are valuable fruit crops worldwide. Private breeders and public institutes, such as the Julius Kühn Institute (JKI-ZO) in Dresden-Pillnitz, Germany, have a history of breeding apple cultivars resistant to various pathogens, e.g. the fungal apple scab ( Venturia inaequalis ) and bacterial fire blight ( Erwinia amylovora ) diseases. Fire blight is a destructive apple disease, and resistance is found in wild species, some of which are also donors of resistance to scab, for example, Malus floribunda 821 (Mf821), which is used in breeding programs to introgress this resistance. Thirty apple cultivars/breeding clones were bred in different years at JKI-ZO carrying the scab resistance gene Rvi6 from Mf821. Meanwhile, Mf821 is one of three donors of fire blight resistance mapped to the distal end of linkage group 12 (LG12), with another minor locus mapped to LG15. We genotyped the 30 cultivars/breeding clones and Malus wild species accessions with markers linked to the fire blight loci on LG12 and LG15. Some genotypes were also artificially inoculated with E. amylovora strain Ea1177 and a mixture of strains in the greenhouse to ascertain their fire blight status. Phenotyping results showed that cultivars ‘Joachim Gauck’, ‘Enterprise’ and ‘Reanda’ had the lowest disease incidence amongst the cultivars. Accessions of M. baccata , M. floribunda and M. prunifolia that are highly resistant to fire blight were identified. Interestingly, none of these cultivars was found to carry the LG12 resistance haplotype, whilst the resistant wild accessions of M. floribunda did. The implications of these results are discussed.