Fine mapping of the Rvi5 (Vm) apple scab resistance locus in the 'Murray' apple genotype

Apple scab, caused by the fungal pathogen Venturia inaequalis, is the most devastating pathogen in the apple-growing industry. In the last two decades, many studies have been initiated to identify new resistances to apple scab and to introgress them into new cultivars through breeding. The Rvi6 gene from Malus floribunda 821 has been the one most intensively used in breeding programmes worldwide, but the identification of new pathogen strains that are virulent to Rvi6 has increased the need for pyramiding of more than one resistance gene to obtain cultivars with durable resistance. Here, we report on the fine mapping of the Rvi5 apple scab resistance locus using two large segregating populations. A region of about 1 cM at the distal end of LG17 carrying the Rvi5 resistance gene was detailed by developing and mapping 10 molecular markers, including SCAR, SSR and SNP markers. The Rvi5 locus was restricted to a region of 228 kb on the 'Golden Delicious' reference genome between the two flanking SSR markers FMACH_Vm4 and FMACH_Vm2. Three co-segregating molecular markers were also developed (SSR FMACH_Vm3, Vm-SCAR1 and Vm-SNP5). All these markers will facilitate the development of marker-assisted selection protocols for this gene using both low-cost methods and high-throughput systems. The findings of this study will thus be useful for further investigation of the Rvi5 resistance locus of 'Murray', aimed at candidate gene identification and the physical isolation of the resistance gene.