Identification of Resistance to Powdery Mildew in Publicly Available Male Hop Germplasm

Abstract

Powdery mildew (caused by Podosphaera macularis) is one of the most important diseases of hop in the western United States. Strains of the fungus virulent on cultivars possessing the resistance factor termed R6 and the cultivar Cascade have become widespread in the Pacific Northwestern United States, the primary hop producing region in the country, rendering most cultivars grown susceptible to the disease at some level. In an effort to identify potential sources of resistance in extant germplasm, 136 male accessions of hop contained in the U.S. Department of Agriculture collection were screened under controlled conditions. Iterative inoculations with three isolates of P. macularis with varying race identified 23 (16.9%) accessions with apparent resistance to all known races of the pathogen present in the Pacific Northwest. Of the 23 accessions, 12 were resistant when inoculated with three additional isolates obtained from Europe that possess novel virulences. The nature of resistance in these individuals is unclear but does not appear to be based on known R genes. Identification of possible novel sources of resistance to powdery mildew will be useful to hop breeding programs in the western United States and elsewhere.

Registration of seven powdery mildew‐resistant wild hop germplasm lines

“…Furthermore, limited studies have investigated the use of wild hop germplasm for the identification of powdery mildew resistance (Seigner et al, 2006). Similar to these previous studies, we identified a limited proportion of hop plants possessing resistance to one or more of the pathogen races tested and a larger proportion of them of primarily Eurasian origin (Seigner et al, 2006;Gent et al, 2018). Previous reports using wild hop germplasm used a similar sampling structure (half-sibling or bulked seed) when testing for powdery mildew resistance, and similar to their results, ours suggest the possibility of multiple different R genes, different combinations of R genes, or novel R genes present in these materials, though inheritance and segregation studies are warranted to fully decipher the genetic basis for this trait.…”

“…Specifically, there are an abundance of male breeding lines putatively possessing R1/R2 ‐based resistance and containing native North American ancestry (predominantly H. lupulus var. neomexicanus ), whereas those evaluated as possessing resistance to all of the pathogen isolates tested are mostly of European ancestry (Gent et al., 2018). These results are consistent with our own, where H. lupulus var.…”

“…A fourth pathogen isolate, HPM‐202 ( VB , V3 , V4 , V6 , VWH18 ) (Gent et al., 2018) was used on a set of germplasm exhibiting no disease development following inoculation with HPM‐204. This pathogen isolate is presumed to overcome the uncharacterized powdery mildew resistance found in WH18, a wild hop collected from Thuringia, Germany.…”

“…Following the methods of Gent et al (2018), random subsets of 576 plants were screened during each time point along with a replicated set of control plants including Northern Brewer, Yeoman, Zenith, Target, Challenger, Serebrianka, Early Choice, and Nugget to ensure infectivity of inoculum and to detect possible changes in virulence patterns (Gent et al, 2017;Seigner et al 2006). Each group of plants were inoculated with a single isolate of P. macularis originating from Minnesota (MN-001, race VB, V3, V5), until the entire collection was screened once.…”

“…Wild H. lupulus may be a rich resource of genetic diversity for disease resistance or other traits (Gent et al, 2018;Patzak et al, 2010;Smith, 2005), but the underlying genetic basis of these traits remains unclear in this germplasm. Humulus lupulus can be subdivided into five botanical varieties, which include H. lupulus var.…”