Characterization of Genetic Resistance to Apple Scab: Apple scab, caused by the fungus Venturia inequalis, is one of the oldest diseases of apples and on average has the highest economic impact on apple production globally. In the US, 18 fungicide sprays may be required in one growing season. Almost 20 major apple scab resistance loci conferring race-specific and broad spectrum resistance, originating from different genetic sources, have been identified so far. Some of these loci have been fine mapped and are being used in breeding programs. The majority of scab resistant cultivars rely on one resistance gene (Vf  or Rvi6), originating from M. floribunda 821. However, new scab fungus isolates are emerging that can overcome some of these resistance loci. A network of orchards with a set of differential host cultivars as indicator plants has been proposed to better define gene-for-gene relationships in the V. inaequalis-Malus pathosystem (Vinquest). The differential set accounts for both race-specific and race-nonspecific interactions and is better suited for comparing results from genetic studies across experiments, to monitor apple scab virulence in the field and for identification of genes. We are in the process of obtaining a differential host set to be established in multiple locations in the US as well as well-characterized reference isolates that will be used to monitor fungal isolates in the US and identify novel isolate-specific and broad spectrum resistance alleles using QTL mapping and GWAS. We are also developing genome editing and marker-assisted strategies to pyramid multiple alleles conferring isolate-specific resistance to create durable resistance.