The hop plant is subject to a number of virus diseases, which are mainly of concern to growers because of their effects on production, but the question of their effects on the brewing quality of cones harvested from diseased plants has also been raised. An investigation is described in which hop plants of the variety Chinook were artificially infected with one of the four viruses found in the hop growing regions of the northwestern USA, namely American hop latent virus (AHLV), common hop latent virus (HLV), hop mosaic virus (HMV) and Prunus necrotic ringspot virus (PNRV) and the effects on plant growth, yield, bitter substances content and essential oil content of infection with each virus were determined by comparison with healthy plants. A summary of the results is presented in tabular form. All four viruses caused some impairment of growth (fewer nodes, fewer leaves, shorter laterals and, in the case of AHLV and HMV, reduced plant height) and reduction in yield (most severe in the case of HMV with a 62% drop in yield, while losses caused by the other three ranged from 13 to 15%). The alpha acid, beta acid and total essential oil contents of hops from plants with HLV, PNRV and HMV were actually slightly higher than those from virus free plants, but this increase was insufficient to cancel out the yield reductions, so that the production figures for hop oil and bitter substances (quantity per unit of land area) were lower. The essential oil composition of hops from infected plants was also altered. These changes in composition are similar to those which occur in disease free hops harvested later than normal. As it is not uncommon for a certain proportion of the hop crop to be slightly overripe when harvested, this is unlikely to affect the brewing process or beer quality, but the yield losses are a much more serious matter and amply justify the stringent precautions taken to prevent virus infection of commercially grown hop plants.
Keywords : composition disease hops quality virus yield