Powdery mildew (Podosphaera macularis Braun & Takamatsu) resistance in wild hop genetic resources

Abstract

Hop powdery mildew, caused by Podosphaera macularis Braun & Takamatsu (formerly Sphaerotheca macularis (Wallr. :Fr.) Lind, syn. S. humuli (DC.)Burrill) was not observed in Pacific Northwest hop yards until 1997, when it was discovered in Washington. Within one year, it had spread to Oregon and Idaho. This emerging disease caused severe economic losses to hop growers, due in part to the lack of resistance incorporated into commercial varieties. Despite several collection trips that increased the amount of wild hop (Humulus lupulus L.) germplasm available to researchers, there was no knowledge of its potential as a resource for resistance to P. macularis. The objectives of this study were: 1) to collect previously unrepresented native hop seed from the Southwestern United States, 2) to evaluate wild hop germplasm for resistance to P. macularis, and 3) to microscopically examine the progression of powdery mildew on different hop genotypes. Plant collecting expeditions were taken in September of 2002 and 2003 to obtain wild American hop germplasm (H. l. var. neomexicanus) from Colorado, Arizona, and New Mexico. During these expeditions, 60 seedlots and 28 plant accessions were obtained. This germplasm was deposited at the United States Department of Agriculture (USDA), Agricultural Research Service (ARS), National Clonal Germplasm Repository (NCGR), located in Corvallis, Oregon. More than 2,100 wild (1563 North American and 107 Kazakhstani) hop seedlings, from 54 seedlots were evaluated for powdery mildew resistance. The number of resistant seedlings observed when greenhouse temperatures were >30°C was 218. The number of resistant seedlings decreased to 62 when greenhouse temperatures were <30°C. Selected resistant genotypes were evaluated for resistance under field conditions. Of these, 31 maintained resistance to foliar infections. Cones from 13 female and monoecious genotypes were evaluated for the presence of mildew; 5 genotypes from Manitoba and North Dakota and 1 genotype from Kazakhstan exhibited high cone resistance. Additional studies examined the effect of temperature on the resistance of the host. The results demonstrated that the resistance of susceptible to tolerant genotypes increased when the plant was exposed to high temperatures (>30°C) prior to inoculation. The development of P. macularis on several native North American and Kazakhstani hop genotypes was observed using direct light microscopy. The disease progressed slower on native genotypes than on the susceptible H. l. 'Symphony'. Conidia on the resistant cultivar 'Nugget' produced only the primary germ tube prior to shriveling. On 3 infected native Kazakhstani genotypes, epidermal cells at the center of the colony began to collapse at 120 h, forming a lesion that continued to radiate outward. Fungal hyphae then shriveled in response to epidermal collapse. As a result of this work, new sources of resistance to powdery mildew were identified. A high percentage of resistant seedlings were observed in specific lots from Manitoba, Saskatchewan, and North Dakota. Resistant genotypes from North America and Kazakhstan may lead to germplasm releases for use in hop breeding programs. The partial resistance observed in wild hops suggests that multiple genes are involved. This type of resistance would be more durable than single gene resistance, which is easily overcome by the evolution of more virulent pathotypes.