Fine-mapping of the apple scab resistance locus Rvi12 (Vb) derived from ‘Hansen’s baccata #2’

Apple scab is a disease caused by the fungus Venturia inaequalis, which leads to significant economic losses in apple production especially in temperate regions. Breeding programmes are attempting to introgress scab resistance genes from wild apple into commercial cultivars to control the disease. Most of the commercially available scab-resistant varieties to date rely on the Rvi6 (Vf) resistance gene from Malus floribunda 821. The evolution of new pathotypes of V. inaequalis, which have caused the breakdown of Rvi6-based resistance, at least in northern Europe, highlights the need for the characterisation and pyramiding of scab resistance genes from different sources for durable disease resistance. In this study, the scab resistance gene Rvi12 from Malus baccata ‘Hansen’s baccata #2’ was confirmed as mapping to apple linkage group 12 in the cross ‘Gala’ 9 ‘Hansen’s baccata #2’ in an interval between SSR markers Hi02d05 and CH02h11b. Using the ‘Golden Delicious’ genome sequence, novel SSR markers and SNPs were identified in the Rvi12 mapping interval and mapped in an extended mapping population of 1,285 plants. Rvi12 was fine-mapped to an interval spanning 958 kb of the ‘Golden Delicious’ genome sequence. The 18 SNPs fine-mapped to the Rvi12 interval were screened in eight apple breeding founders, and for 16 of the 18 SNPs, the alleles linked in coupling with the Rvi12 resistance locus were found only in ‘Hansen’s baccata #2’. The SNPs identified will thus be useful for the efficient identification of apple genotypes carrying the Rvi12 resistance locus.