Field Guide for Integrated Pest Management in Hops

28At-A-GlanceCarlavirusComplex◆◆Use onlycertified virus-freeplanting stockwhen establishingnew yards.◆◆Insecticide usefor aphid controlis inefficientfor limiting theintroduction ofviruses, but canreduce the rate ofspread within ayard.Virus and Viroid DiseasesCarlavirus Complex: American hop latent virus,Hop latent virus, and Hop mosaic virusKenneth C. Eastwell and Dez J. BarbaraThree carlaviruses are known to infecthop plants: Hop mosaic virus, Hop latentvirus and American hop latent virus. All areknown to occur in mixed infections andall but American hop latent virus are foundworldwide. American hop latent virus isfound primarily in North America.SymptomsHop latent virus and American hoplatent virus do not cause visually obvioussymptoms on any commercial hop varieties.Of the three carlaviruses, Hop mosaic virus isthe most likely to cause both symptoms andcrop loss. On sensitive varieties, chloroticmosaic mottling can develop between majorleaf veins (Fig. 44). Severely affected plantsmay establish poorly when planted, haveweak bine growth, and often fail to attachto the string. The varieties that developthese symptoms typically are those of theGolding type or those that have Goldingparentage. However, some strains of Hopmosaic virus appear to cause infections thatmay be almost symptomless on Goldinghops. The three carlaviruses reduce growth,which is particularly critical in establishingnew plantings. Yield can be reduced byapproximately 15%, but varieties sensitiveto Hop mosaic virus can suffer losses as greatas 62% as a result of infection. Changes inbrewing characteristics induced by theseviruses are minor and appear to be analogousto over maturity of the hop cones at harvest.Disease CycleCarlaviruses are transmittedmechanically and in a non-persistentmanner by aphids.All three are transmitted by thehop aphid, Phorodon humuli, and Hopmosaic virus and Hop latent virus arealso transmitted by the potato aphid,Macrosiphum euphorbiae, and green peachaphid, Myzus persicae. Transmission byaphids is thought to be quite inefficient,however. Propagation and distribution ofvirus-infected plants is the primary modethrough which carlaviruses are spread longdistances. Root grafting and mechanicaltransmission are thought to contributeto localized spread. Carlaviruses typicallyhave narrow host ranges and for practicalpurposes the only hosts for these pathogenslikely to be near hop yards are other hopplants. Over the life of a hop planting, ahigh percentage of plants in a particular hopyard may become infected.Figure 44. Yellow mosaic pattern on Chinookdue to Hop mosaic virus. (K. C. Eastwell)ManagementSince vegetative propagation of virusinfectedplants is the principal factor invirus spread, the use of certified virus-freeplanting stock is the most practical methodof limiting any virus disease, particularlyduring the early stages of plant growth anddevelopment. Application of insecticidesto control aphids is inefficient for limitingthe introduction of virus since the viruswill be transmitted before the viruliferousaphids are killed. However, reducing aphidpopulations can reduce the rate of secondarytransmission within a hop yard.