Studies on the viruses of hop (Humulus lupulus L.) in New Zealand

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The incidence, spread, and effect of viruses on yield and α-acid content of hops (Humulus lupulus L.) in New Zealand was determined. In 1984, sixty plants in each of 27 Nelson hop fields, mostly of cv. Superalpha, Greenbullet and Sticklebract, were tested for viruses by ELISA. Both the intermediate (I) and apple (A) serotypes of prunus necrotic ringspot virus (PNRSV) were detected. Of 24 fields containing PNRSV, 15 had above 81 % incidence. Seventeen of 24 fields containing arabis mosaic virus (ArMV) had below 41 % incidence. Twelve of 22 fields containing hop mosaic virus (HMV), 13 of 20 fields with hop latent virus (HLV) and 9 of 13 fields containing American hop latent virus (AHL V), had below 21 % incidence. Virus spread was studied by annual testing of evenly spaced plants within three fields. In one field incidence of PNRSV (I), ArMV, HMV and HLV remained unchanged over three years (n=150). Over two years, HMV and HLV increased by 21 % and 8% respectively in the second field (n=150), while PNRSV(I) and ArMV increased by 13% and 17% respectively in the third field (n=105). The nematode vector (Xiphinema diversicaudatum) of ArMV was found in 6 out of 8 fields after examining 30 evenly spaced soil samples from each. Of the 31 samples containing the nematode, 28 were from field edges. The effect of viruses on cv. Superalpha was studied in four trials. In one trial, no difference was recorded in dryweight of cones per plant, or cone α-acid content at harvest (February), between plants with no detectable virus, or those with HLV, HMV/HLV or ArMV. In the second trial, no difference in these variables occurred between plants with no detectable virus or those with ArMV, HLV, HMV/HLV, ArMV /HLV or ArMV/HMV/HLV. In the third trial, PNRSV(I)/HLV, ArMV/HLV or PNRSV(I)/ArMV/HLV infection reduced yield by 17%, 29%, and 39% respectively, compared to HLV infection alone. In addition, cones from plants infected with PNRSV(I) contained 3-4% less α-acid. In the fourth trial, ArMV /HMV /HLV and PNRSV(I)/ArMV/HMV/HLV reduced yield by 27% and 35% respectively, compared to HMV /HLV infection. PNRSV(I)/HMV /HLV and PNRSV (1)/ ArMV /HMV /HLV infection reduced α-acid content by approximately 3% compared to HMV /HLV or ArMV /HMV /HLV. All infected plants in these trials were without symptoms. Yield reductions caused by PNRSV(I) were due to reduced individual cone weight, mainly through reduced numbers and weight of bracteoles. Reduced cone α-acid content resulted from fewer bracteoles with less glands. A virus-tested scheme for N.Z. hops could be beneficial given the extent of infection, the slow rate of spread, and the effect of viruses in reducing yield and α-acids in the cultivar Superalpha.
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