Genetic characterization of downy mildew resistance from the hop (Humulus lupulus L.) line USDA 64035M

Abstract

Downy mildew (caused by Pseudoperonospora humuli) resistance (DMR) will facilitate the success of hop (Humulus lupulus L.) production in humid, temperate climates. However, DMR in hop has a narrow genetic base, and its genetic architecture is poorly understood. In this study, we characterized the genetic control of DMR using a biparental mapping population derived from a cross between a female, the susceptible cultivar ‘Comet,’ and a male plant, the resistant line USDA 64035M. Resistance was evaluated after inoculation with the downy mildew pathogen as disease severity and percent leaf area with sporulating lesions under greenhouse conditions. Significant (p value < 0.01) genotypic differences were observed for the traits. Broad‐sense heritability was 0.33 for disease severity and 0.72 for percent leaf area with sporulating lesions. Genome–phenome association between 4090 markers and 274 individuals identified three significant markers for disease severity and five for percent leaf area with sporulating lesions. Our study demonstrated that DMR was under polygenic control with small effect loci in this population. In addition, a pleiotropic locus on chromosome two was found to be associated with both disease severity and percent leaf area with sporulating lesions. Identifying the gene(s) underlying such a pleiotropic locus can help improve our understanding of hop response to pathogen infection.