Fine mapping of the scab resistance locus Rvi12 derived from Malus baccata ‘Hansen’s baccata #2’

Apple scab is a disease caused by the fungus Venturia inequalis which leads to significant economic losses in apple production especially in temperate regions. Breeding programs are attempting to introgress scab resistance genes from wild apple varieties into commercial cultivars for the control of the disease. Most of the commercially available scab resistance varieties to date rely on the Rvi6 (Vf) resistance gene from Malus floribunda 821. The evolution of new pathotypes of V. inequalis, 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 mapped to apple linkage group 12 in the cross ‘Gala’ × ‘Hansen’s baccata #2’ in an interval between two SSR markers Hi02d05 and Hi07f01. 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 635 plants. Rvi12 was fine-mapped to an interval of 0.79 cM, spanning 882 kb of the ‘Golden Delicious’ genome sequence. The 23 heterozygous SNPs fine mapped to the Rvi12 mapping interval were screened in eight apple breeding founder lines and for 21 of the 23 SNPs, the allele linked in coupling to the Rvi12 resistance locus were found only in ‘Hansen’s baccata #2’. The SNPs identified will therefore be useful for the efficient identification of apple genotypes carrying the Rvi12 resistance locus.