K.E. Hummer, D.H. Gent, J.A. Henning, W.F. Mahaffee, C.M. Ocamb
A USDA-sponsored plant collecting expedition in collaboration with the Vavilov Institute of Plant Industry (VIR), St. Petersburg, Russian Federation, and the Aral Sea Experiment Station for Plant Genetic Resources, Chelkar Town, Kazakhstan, was conducted in the fall of 2000. Hop (Humulus lupulus var. lupulus) cones were collected from wild plants growing on trees in a moist area about 50 km NE of Emba, Kazakhstan. A portion of the seeds were brought to the US and donated to the US Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository in Corvallis, Oregon. Of 74 seedlings that were germinated from Kazakhstani seedlots, four seedlings from PI 635262 demonstrated resistance to powdery mildew, caused by Podosphaera macularis (Braun and Takamatsu). One of these resistant seedlings, ‘Kazak 2000’, exhibited a hypersensitive response after challenge by powdery mildew, especially when incubated at temperatures greater than 29°C post-infection. In greenhouse assays with an Oregon field population of P. macularis, macroscopic signs of powdery mildew were not observed following repeated inoculations. In laboratory assays at 18°C, ‘Kazak 2000’ did not develop powdery mildew when challenged with a P. macularis isolate capable of overcoming resistance genes Rb, R3, and R5. Infection of 55% of detached leaves was observed when challenged with characterized isolates capable of overcoming Rb, R3, R4, R5, R6 or Rb, R1, R2, R3, R5, and R6, respectively. In experimental field plots in Oregon from 2003 to 2007, only one powdery mildew colony was observed despite inoculation and exposure to natural inoculum sources each year. Infection of cones was not observed. ‘Kazak 2000’ provides a novel source of powdery mildew resistance that is effective at high temperatures against multiple races of the pathogen. Limited quantities of cuttings of ‘Kazak 2000’ are available for research or breeding. Requests should be made to the Hop Curator, USDA-ARS, National Clonal Germplasm Repository, Corvallis, OR 97333.
Hummer, K.E., Gent, D.H., Henning, J.A., Mahaffee, W.F. and Ocamb, C.M. (2009). POWDERY MILDEW RESISTANT HOP GERMPLASM RELEASE: 'KAZAK 2000'. Acta Hortic. 848, 157-164
DOI: 10.17660/ActaHortic.2009.848.17
Humulus lupulus var. lupulus, Podosphaera macularis, red mould, Sphaerotheca humuli, Sphaerotheca macularis

Acta Horticulturae