Genetic components associated with R2 and R4 powdery mildew resistance in hop

Epidemics of powdery mildew disease in hop (Humulus lupulus var. lupulus) lead to cone spoilage, and in severe cases, crop abandonment. In order to prevent disease-associated yield losses, hop must be treated with an intensive fungicide management program. However, the chemical toolbox available is shrinking, and the horticultural industry is now increasing the uptake of sustainable biological disease control strategies, including genic-based resistance, which can offer a low chemical input strategy. We investigate the genetic components associated with powdery mildew resistance in (1) a hop population that segregates for R2 resistance and (2) a diversity panel containing 736 individuals with differential resistance responses to the hop powdery mildew race-structure. Both populations were phenotyped using the "Zenith" isolate (V1, V3, and Vb virulence) and genotyped to enable the association mapping of the biparental population and a genome-wide association study analysis, respectively. We identified the same location on chromosome 6 associated with R2 resistance in both populations. However, an additional resistance allele was associated with R4 resistance. Notably, the most significant single nucleotide polymorphisms on chromosome 6 fall on either side of three RPM1 disease resistance genes, which are prime candidates for downstream analysis. RPM1 mediates a localized cell death disease response reminiscent of the R2 phenotype. These results provide novel validated markers for use in international hop breeding programs. In doing so, we facilitate the pyramiding of disease resistance genes against multiple races of powdery mildew and reduce reliance upon chemical applications through providing a varietal control solution for this major hop pathogen.