Over 60 years ago, the existence of different races of Venturia inaequalis was demonstrated using inoculation experiments with monospore isolates of the pathogen on the red-fleshed scab-resistant apple cultivar ‘Geneva’. The findings suggested the presence of at least three scab resistance genes in this host. More recently, our genetic studies with five monospore isolates of V. inaequalis have indicated the presence of at least four genes. Three of these, two dominant genes and one recessive gene, have been mapped to a 5 cM region on linkage group 4, which corresponds to a 2 Mbp region containing nine candidate NBS-LRR resistance genes on the physical map of ‘Golden Delicious’. Primers specific to the candidate genes were designed for the fine-mapping of the resistance genes and used to construct a detailed genetic map defining the loci. The consequences of the complexity of the ‘Geneva’ scab resistance for breeding and the selection of a differential host (3) for V. inaequalis pathotype monitoring (www.vinquest.ch) will be discussed.
