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Incidence of Five Viruses in Hop (Humulus lupulus L.) in the Pacific Northwest. R. E. Klein, Irrigated Agriculture Research and Extension Center, Washington State Univcrsity-Prosser, Prosser, 99350. S. D. Husfloen, Irrigated Agriculture Research and Extension Center, Washington State Univcrsity-Prosser, Prosser, 99350. Plant Dis. 79:425. Accepted for publication 20 February 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0425B.

Hop, a vegetatively propagated perennial, is cultivated on 18,500 ha in Ihe Pacific Northwest (PNW) states of Washington, Oregon, and Idaho. Five viruses, including Prunus necrotic ringspot (PNRV) and apple mosaic (ApMV) ilarviruses and hop mosaic (HMV), hop latent (HLV), and American hop latent (AHLV) carlaviruses, have been reported on hop in the PNW but the incidence of these viruses was not known. Consequently, an enzyme-linked immunosorbent assay-based virus survey was conducted in 1994 to determine virus incidence in the major hop cultivars grown in the PNW. Twenty samples were collected randomly from each of a total of 160 hop yards. The cultivars sampled included Galena, Cluster, Nugget, Willamette, Chinook, Cascade, and Tettnanger. These cultivars cumulatively compose 85% of the hop hectarage in the PNW. The number of hop yards and the cultivars sampled in each state was in approximate proportion to their overall hectarage. HLV, the most common virus, was detected in 91.9% of the samples. AHLV was detected in 74.2%, PNRV 55.5%, HMV 37.0%, and ApMV 1.0% of the samples. Differences in virus incidences among states and among varieties were not significant for HLV, AHLV, or ApMV. HMV incidence varied significantly among cultivars but not states, from 0% for Chinook hops to 90.6% for Cluster hops PNRV incidences were significantly lower in Washington-grown Willamette and Chinook hops than in their counterparts grown in Oregon and Idaho, respectively. However, the differences in PNRV incidence were not significant for the cultivars Cluster, Galena, or Nugget. Consequently, lower PNRV incidences in Washington could not be unambiguously attributed to the Washington PNRV-free hop rootstock program. The virus incidences reported here for the PNW contrast with survey results from the Czech Republic (1) and with data published in monographs and proceedings for Germany and the United Kingdom. In Europe, ApMV is the predominant ilarvirus and HMV is more common than HLV.

References: (1) J. Polak. Distribution of apple mosaic virus in hop in gardens of the Czech Republic. Ochr Rostl. 30:85. 1994.