Mapping the powdery mildew resistance locus R1

ISHS Secretariat
Mapping the powdery mildew resistance locus R1

Joshua Havill is a Ph.D. candidate in the Department of Agronomy and Plant Genetics at the University of Minnesota, USA. He previously completed his BSc in Horticultural Science in 2014, and his MSc in Plant Pathology in 2017, which were both also obtained from the University of Minnesota. Hop (Humulus lupulus L.) cultivation has rapidly expanded across North America, returning to historical production areas that were focal points of production. This expansion has also led to the subsequent return and dispersal of various hop pathogens such as hop powdery mildew, caused by Podosphaera macularis (Wallr.) U. Braun & S. Takam. Hop powdery mildew is the likely reason for the collapse of the northeastern US hop industry during the early 20th century. To combat these issues, disease resistant cultivars are commonly grown in combination with chemical and cultural practices. Hop breeding, like other perennial cropping systems, is laborious and time-consuming. The advent of marker-assisted selection has the capacity to increase throughput and decrease the evaluation period required to release cultivars. Given the laborious nature of hop breeding, combining and selecting on multiple phenotypes is an important breeding effort that can be improved via marker-assisted selection. Joshua’s dissertation research aims to 1) dissect the genetic and evolutionary relationships of natural populations of H. lupulus and 2) to identify the genomic regions that confer resistance to the hop powdery mildew pathogen. A bi-parental population was developed between the resistant female ‘Zenith’ with the susceptible male breeding line USDA 21058M to address the second aim of his dissertation research. Progeny from this population were evaluated for their disease response following inoculation with a well-characterized P. macularis isolate to describe inheritance of the resistant phenotype. Subsequently, genotyping-by-sequencing was performed on both parents and progeny to identify single nucleotide polymorphisms (SNPs) useful for building a linkage map and downstream quantitative trait loci (QTL) analysis to isolate genomic regions controlling variation for this agronomically-important trait. Following QTL analysis, validation work was carried out by developing a competitive allele-specific polymerase chain reaction (PCR) (KASP) assay useful for high-throughput marker-assisted selection, specifically for powdery mildew resistance conferred by ‘Zenith’.

Joshua Havill won the ISHS Prof. Jens Wünsche Young Minds Award for the best poster at the V International Humulus Symposium, which was held virtually in Germany in March 2021.

Mr. Joshua Havill, Department of Agronomy and Plant Genetics, University of Minnesota, 411 Borlaug Hall, 1991 Upper Buford Circle, St. Paul, MN 55108, USA, e-mail:

The article is available in Chronica Horticulturae

powdery mildew resistance locus R1
Young Minds Award Winners