Registration of Downy Mildew – Resistant Male Hop Germplasm USDA 21087

USDA 21087M (Reg. No. GP-36, PI 558607) male hop (Humulus lupulus L. var. lupulus) germplasm was released to the public on 6 Dec. 2011. This germplasm has been used by the USDA–ARS for hop cultivar development for several decades. Nonagency interest in this germplasm was expressed, and release of this material is considered beneficial for the US public. USDA 21087M was obtained from the former Yugoslavia by the USDA–ARS in 1974 as Selection 3/3. The defining characteristics of USDA 21087M are its high resistance to downy mildew in the US Pacific Northwest, potential resistance to non-v4/v6 strains of powdery mildew, and potential good storage ability in hop offspring. It is expected that hop breeders will use this germplasm as a parent for development of downy mildew– resistant hop cultivars. The release of USDA 21087M will give both public and private breeders new germplasm for cultivar and germplasm development.

H op (Humulus lupulus L. var.lupulus) is a diploid, dioecious (2n = 2X = 18+XX/XY), perennial crop produced in the Northern and Southern Hemispheres between the 35th and 60th parallels in latitude.The harvested portion of hop is the female floral structure called the strobiles-commonly called hop cones.Male hop plants are used solely for breeding purposes.Hop cones are used primarily for bittering and flavoring in beer brewing, although several other uses have been identified and implemented in recent years (Pollach et al., 1996;Stevens and Page, 2004;Cornelison et al., 2006;Zanoli and Zavatti, 2008).
Two countries-the United States and Germany-contain the largest hop-producing regions of the world.Three US states-Washington, Oregon and Idaho-produce the majority of hops on a per acre and total production basis, with new production regions found in Michigan, Minnesota, New York, and to a lesser extent, other states in the northeastern United States.Agronomic issues faced by producers are low yields of mainland European "noble" hop cultivars (such as 'Saazer', 'Mittelfruh', and 'Tettnanger')  Of the two fungal diseases in hop, downy mildew (DM) is the most devastating because of its potential for systemic growth (Neve, 1996).Cooler, moist growing regions such as in Oregon, Michigan, and New York require regular prophylactic fungicidal sprays to prevent disease outbreaks; in some cases, susceptible cultivars cannot be produced economically because the costs associated with spraying supersede farm gate receipts (Johnson et al., 2009).In addition, whole crop loss is possible in susceptible cultivars if not chemically treated.Because of these issues, it is in the public's interest that cultivar development uses parental accessions possessing resistance or at least tolerance to DM. Hop breeding would greatly benefit from having access to this male germplasm as well as information concerning its phenotypic characteristics for development of new diseaseresistant cultivars.

Methods
USDA 21087M (Reg.No. GP-36, PI 558607) was obtained from former Yugoslavia by the USDA-ARS in 1974 as Selection 3/3.This accession was collected on 30 Sept. 1970 from Cesnjica, Slovenia (598 m elevation), located in the region of Upper Carniola, Slovenia-a mountainous region northwest of Ljubljana.The Institute of Hop Research (Zalec, Slovenia) reported that this accession demonstrated resistance to DM, and for this purpose it was integrated into the USDA germplasm collection.USDA 21087M is thought to be a wild landrace of the Balkans in south-central Europe, as shown by genetic similarity studies (Townsend and Henning, 2005).Stajner et al. (2008) showed similarities to old English cultivars but not with mainland European landraces.As such, little genetic similarity exists with noble hop lines such as Mittelfruh, Tettnanger, or Saazer.USDA 21087M was first grown at the USDA-ARS hop research facility near Corvallis, OR, in 1974 and was given the USDA accession number 21087M in 1974.It was evaluated for phenotypic characteristics for several years before being saved as a male germplasm.
Extraction and collection of male lupulin glands was performed as outlined by Nickerson et al. (1988).Determination of alpha and beta acids, along with hop storage index (HSI), was performed using American Society of Brewing Chemists (1969,1992) standard methods.Downy mildew ratings using qualitative bins (1 = resistant; 5 = highly susceptible) were collected on naturally inoculated field-grown hop plants (M.S. Townsend, unpublished data, 2009) as well as inoculated field plants using infected shoot counts as a quantitative measurement (Parker, 2007).Powdery mildew (PM) ratings were obtained by inoculation of known PM races and subsequent scoring of leaf infection under greenhouse conditions (D.H. Gent, unpublished data, 2016) using inoculation techniques as reported by Henning et al. (2011)

Characteristics
Chemical analyses of USDA 21087M lupulin glands indicate its primary use as a breeding parent for aroma-type or dual-purpose hop cultivars where higher levels of alpha acids in offspring are not necessary.Phenotypic measurements of USDA 21087M showed alpha acids averaged 39.7% (v/v), while beta acids averaged 21.4% (v/v), with alpha acids comprising 64.3% of the total bittering acids (v/v) (Table 1).This germplasm also exhibited good storage potential in offspring, with a HSI of 0.26 (Nickerson and Likens, 1979).
Replicated field studies with inoculated plants demonstrated that USDA 21087M has resistance to DM infection similar to the cultivars 'Magnum' and 'Fuggle'.Duncan's multiple range test using infected shoot counts and percentage leaf infection showed no statistical difference in DM resistance between USDA 21087M and 'Orion' 'Perle', and 'Omega' and Magnum (Table 2).As a precaution, breeders are recommended to apply prophylactic measures such as field pruning and chemical control with fungicides registered for use on hop.Genetic transfer of resistance to offspring has not been studied, and the breeding value for this trait is unknown, but observations in breeding nurseries suggest offspring possess good resistance (data not shown).
No published experiments are available on resistance levels of USDA 21087M to PM strains found in the Pacific Northwest.Greenhouse observations of inoculated plants (D.H. Gent, unpublished data, 2016) showed it as being susceptible to v4/ v6 strains of PM but resistant to other strains of PM found in the US Pacific Northwest.As PM strains possessing v4/v6 are prevalent in the Pacific Northwest, regular prophylactic spraying for this disease is recommended.No cases of Verticillium wilt (caused by Verticillium albo-atrum Reinke & Berthier) were observed in nursery plots.Growers are cautioned against growing USDA 21087M in known Verticillium-infested fields due to the potential susceptibility of this germplasm based on its parentage.No published studies have been performed on USDA 21087M susceptibility to hop aphids, nor is there any information on its susceptibility to spider mite infestation, although no -------------------- Henning and Townsend (2005).‡ Means within a column and subheading of male or female that have similar letters are not significantly different from one another based on Fisher's LSD test.

Discussion
Hop breeders from private breeding companies and new state breeding programs have requested access to USDA 21087M for development of hop cultivars with resistance to DM.The lack of publicly available male lines for developing germplasm or cultivars with DM resistance suggests the need for such lines in burgeoning hop-breeding programs across the United States.Downy mildew infections of hop occur on a regular basis in all major growing regions outside of the Yakima Valley in Washington State, and DM-susceptible cultivars in these regions require numerous prophylactic sprays of up to 8 to 10 applications during the growing season (Johnson et al., 2009).Production of DM-resistant cultivars significantly reduces management costs, resulting in higher farm gate returns for growers.The use of USDA 21087M in crossing schemes with DM-resistant female lines could potentially result in cultivars with superior resistance coupled with higher yields if parents are of sufficient genetic distance from one another (Henning et al., 2010).
Before 2012, PM races present in the US Pacific Northwest possessed non-v4/v6 virulence genes, and USDA 21087M was resistant to PM (Wolfenbarger et al., 2014)

Conclusion
The release of USDA 21087M provides a new hop germplasm exhibiting resistance to DM as well as non-v4/v6 strains of PM.As such, this germplasm represents a vital resource for hop breeding in the United States.Furthermore, the chemical composition of lupulin glands suggests the use of this male for the development of aroma or dual-purpose hop cultivars.USDA 21087M is released to the public at the request of multiple breeding companies and private individuals.
. Since 2012, v4/ v6 PM races have prevailed, thus ensuring susceptibility of 21087M.Nevertheless, combining R4/R6 genes with other resistance genes not yet overcome by current races of PM found in the US Pacific Northwest should result in more sustainable resistance, particularly if quantitative resistance genes are combined from both parental sources.