Genetic Components Associated with R2 and R4 Powdery Mildew Resistance in Hop

Epidemics of powdery mildew disease in hop lead to cone spoilage, and in severe cases, crop abandonment. In order to prevent disease-associated yield losses, hops must be treated with an intensive fungicide management programme. However, the chemical toolbox available is shrinking and the horticultural industry is now increasing the uptake of sustainable biological disease control strategies, including varietal control which can offer an environmentally favourable, 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 individuals with differential resistance responses to the hop powdery mildew race-structure. Populations were both phenotyped and genotyped to enable Quantitative Trait Loci (QTL) mapping and Genome Wide Association Study (GWAS) analysis, respectively. We identified the same location on the end of chromosome 6 associated with R2 resistance in both populations. Whilst additional resistance alleles were identified, associated with R4 resistance. Notably, the focal SNPs on chromosome 6 fall on either side of a cluster of three RPM1 disease resistance genes, which form prime candidates for downstream analysis. RPM1 mediates a sensitive and effective disease response through localised cell death, a response reminiscent of the R2 phenotype. These results provide novel markers for use in both UK and international hop breeding programmes. 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.