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United States Department of Agriculture

Agricultural Research Service

Research Project: Reducing the Impact of Diseases on Hop Production

Location: Forage Seed and Cereal Research

2013 Annual Report


1a.Objectives (from AD-416):
Objective 1: Identify new genetic markers for selecting resistant hop germplasm to downy and powdery mildews.

Objective 2: Develop new disease resistant germplasm for public release.

Objective 3: Construct optimized integrated management approaches for powdery mildew susceptible cultivars. Sub-objective 3.A: Describe the ontology of crown bud development, susceptibility of crown buds to powdery mildew in different developmental stages, and dynamics of flag shoot emergence. Sub-objective 3.B: Develop a PCR assay to rapidly identify mating type in P. macularis; determine prevalence of mating types among isolates of P. macularis in the Pacific Northwest.

Objective 4: Develop and apply genotyping approaches to assess the diversity, geneticdifferentiation, and sexual recombination in the downy mildew and powdery mildew pathogens. Sub-objective 4.A: Identify and develop simple sequence repeat markers in Pseudoperonospora humuli and elucidate the degree of diversity, selfing, and population differentiation within and among population at multiple hierarchical scales. Sub-objective 4.B: Identify polymorphic loci among isolates of Podosphaera macularis and characterize the genetic diversity, population structure, and relatedness of the population in the Pacific Northwest U.S. to other populations of the pathogen in the world.


1b.Approach (from AD-416):
Objective 1: Identify molecular markers associated with plant resistance to P. humuli and P. macularis. Progeny developed from crosses of resistant and susceptible parents will be screened by pathogen challenge, and single nucleotide polymorphic (SNP) marker identification and genotyping-by-sequencing will be performed.

Objective 2: Development of multiple pathogen resistant germplasm or varieties. Progeny of crosses of parental material that possess relative resistance to powdery and downy mildews will be successively challenged with each disease to identify germplasm with enhanced resistance to both. Resistant germplasm that possess excellent agronomic and brewing characteristics will be released.

Objective 3: Construct optimized integrated management approaches for powdery mildew susceptible cultivars. Hypothesis 3.A: Successful perennation of the powdery mildew fungus occurs via infection of juvenile crown buds and such crown buds develop asynchronously. Management factors that reduce late season severity of powdery mildew will reduce overwintering survival of the pathogen. Crown bud development phenology will be determined and plants will be challenged with powdery mildew at selected stages. Treatment at different stages will be evaluated. Hypothesis 3.B: The absence of the ascigerious stage of Podosphaera macularis in the Pacific Northwestern U.S. is due to the absence of one of requisite mating types of the fungus. PCR amplification of conserved regions in MAT1-1 and MAT1-2 loci of P. macularis will be optimized and pathogen isolates collected from a variety of cultivars and hop yards in the Pacific Northwest will be tested to determine the frequency of each mating type.

Objective 4: Develop and apply genotyping approaches. Hypothesis 4.A: P. humuli is heterothallic, possessing a high degree of genetic diversity in the Pacific Northwestern U.S, and the population is structured at the scale of individual fields. P. humuli isolates will be obtained from three hop yards in western Oregon. After suitable SSR loci and primers are developed from pyrosequences, genotyping will be performed by capillary sequencing of 7 to 10 SSR loci per isolate. Hypothesis 4.B: The population of Podosphaera macularis in the Pacific Northwestern U.S. exhibits a low degree of genetic diversity or structure based on geography or cultivar host. Multiple nuclear loci will be identified, PCR-amplified, and sequenced from Pacific Northwest, northeastern U.S., and European isolates. Population genetic parameters will be calculated and differentiation among geographic populations will be estimated.


3.Progress Report:
This is a new project replacing expired project, 5358-21000-040-00D, Genomics, Germplasm development and IPM of Hop" in June of 2013. This project continues the research from the previous project and the objectives in this new project relate to research completed in the previous 5-year project period. The new project was operational for less than four months. The progress report for this research is documented in the annual report for 5358-21000-040-00D. New research was launched to develop basic epidemiological information on strains of the powdery mildew pathogen capable of overcoming a widely deployed host resistance gene. Germplasm containing multiple sources of resistance to powdery mildew were developed and have been advanced to multiple hill observations.


Review Publications
Gent, D.H., Mahaffee, W.F., Mcroberts, N., Pfender, W.F. 2013. The use and role of predictive systems in disease management. Annual Review of Phytopathology. 51:267-89.

Last Modified: 7/30/2014
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