Effect of viruses on agronomic and brewing characteristics of four hop (Humulus lupulus) cultivars in Australia

The effect of Hop latent virus (HpLV), Hop mosaic virus (HpMV), and Prunus necrotic ringspot virus [PNRSV (apple, A, and intermediate, I, serotypes)], on the survival of softwood cuttings, the vigour (height) of early season growth, cone yield, and the levels of brewing organic acids in mature plants, was assessed in four hop (Humulus lupulus) cultivars in Tasmania, Australia. Virus infections were associated with an increase in the mortality of softwood cuttings following propagation. In all cultivars, height of early growth was a poor indicator of the effect of viruses on cone yield and levels of brewing organic acids (alpha and beta acids). In cv. 'Nugget', infection by the virus combinations studied was not associated with reductions in cone yield, however plants infected by PNRSV-I, in 2000, had 11% lower alpha acids and 7% lower beta acids. In 'Opal', infection by HpLV and HpMV were the most deleterious to cone yield, however the effect of HpMV was ameliorated when in combination with PNRSV-I. Reductions in alpha and beta acid content were attributable only to mixed infections of HpLV + HpMV in combination with either serotype of PNRSV. In 'Pride of Ringwood', yield loss was mostly attributable to HpMV and to a lesser extent, HpLV. Some ameliorations in cone yield loss occurred in plants containing a mixed infection between HpMV and HpLV or either of the ilarvirus serotypes. Both of the ilarviruses and HpMV caused reductions in alpha acid content. In 'Victoria', cone yield loss was mostly attributable to combinations of viruses such as HpLV + PNRSV-I and HpLV + HpMV. The deleterious effect of HpLV + HpMV was ameliorated by PNRSV-A and to a lesser extent, PNRSV-I. Infection by the virus combinations studied did not significantly affect alpha and beta acid levels in either year. Results suggested the effect of viruses and their combinations differed between cultivars and varied between seasons. This information, when combined with knowledge of the rates of virus re-infection, can be used to recommend control strategies for the Australian hop industry.