Mapping of Fire Blight Resistance QTL in the European Pear ( Pyrus communis L.) Cultivar ‘Petersbirne’

ABSTRACT The breeding of pear ( Pyrus communis ) cultivars that meet market requirements and are at the same time resistant to fire blight is a challenging task beacuse appropriate resistance donors are scarce. To identify valuable resistance sources, phenotypic screening and genetic characterization are indispensable. We established an F 1 population by crossing two moderately resistant pear cultivars, ‘Petersbirne’ and ‘Harrow Sweet’, to identify loci for resistance in ‘Petersbirne’ and to evaluate if the combination of two resistant cultivars will lead to a higher resistance level in the progeny compared with a cross with the susceptible ‘Conference’. The ‘Petersbirne’ × ‘Harrow Sweet’ population had a lower average percent lesion length (PLL), 17.8, compared with 34.3 of the ‘Harrow Sweet’ × ‘Conference’ population. Mapping of the severity of fire blight in ‘Petersbirne’ revealed one resistance QTL on linkage group (LG) 02, which shares the same resistance haplotype as determined for ‘Harrow Sweet’. Further, progenies homozygous for the resistance haplotype for the LG02 QTL had a significantly lower PLL value than the heterozygous progenies. Additionally, we identified a previously unidentified putative minor QTL on LG14 in ‘Harrow Sweet’ but could not confirm previously detected QTL on LGs other than LG02. Although a new resistance locus could not be identified in ‘Petersbirne’, the combination of two resistant cultivars seems to enhance the number of fire blight‐resistant progenies and thus breeding success.