Phenotypic and Genotypic Characterization of Hop (Humulus lupulus L.) Germplasm Resources for Powdery Mildew Resistance

Abstract

Common hop (Humulus lupulus L.) is a dioecious perennial plant species, cultivated primarily for the female inflorescence which is used for commercial brewing purposes and imparts bitterness, flavor, and aroma to beer. Podosphaera macularis (Wall.) Braun & Taka., which causes powdery mildew of hop, can infect both vegetative and reproductive growth phases resulting in decreased yield and quality of the harvested crop. Resistance to P. macularis has been observed to date in only a handful of genotypes with even fewer being genetically characterized, limiting the development of molecular breeding approaches. Elimination screening with four pathogen races on 6,732 hop seedlings and clones resulted in identifying seven broad-spectrum powdery mildew resistant genotypes. Using genotype-by-sequencing, we characterized the underlying population structure and genetic diversity of 2,600 wild and cultivated hop genotypes which demonstrated the presence of three subpopulations, aligning with previously described morphogeographic variation within H. lupulus. These results also showed that North American populations are more genetically-diverse than their Eurasian counterparts though further sampling may be required to better understand drivers of diversity within the compared taxa. A subset of 1,033 genotypes were phenotyped for powdery mildew response and combined with the genotype data to conducted genome-wide association mapping. We identified 13 marker-trait associations using these genome-wide association approaches and detected a previously published QTL. We built upon this work by characterizing a bi-parental population segregating for R1-based resistance derived from the hop cultivar Zenith. We identified a single genetic locus associated with powdery mildew resistance as well as biological sex. We extended this work by developing associated molecular markers for each trait, that can be applied for rapid selection at the seedling stage for either associated trait.