Field Guide for Integrated Pest Management in Hops

32Other Viruses, Viroids, and Virus-like AgentsKenneth C. Eastwell and Dez J. BarbaraAt-A-GlanceOther Viruses,Viroids, andVirus-likeAgents◆◆These virusesand viroids donot merit controlat this time, butgrowers shouldbe aware ofsymptoms.◆◆Use of virusandviroid-freeplanting stockis a first line ofdefense.◆◆Some of theseviruses areproblematic inEurope and/orother countries,but are notcurrently an issuein the U.S.Several virus and viroids are knownto occur in hops that are not addressedby current management practices in thewestern United States. However, growersshould continue to be vigilant for theappearance of symptoms that may indicatethe presence of one of these agents.Hop latent viroidThe group of sub-viral hop pathogensthat contains Hop stunt viroid also includesHop latent viroid. The presence of Hoplatent viroid has been confirmed in mosthop-producing regions of the worldincluding the United States; wherever it isknown to occur, it is widely distributed.Hop latent viroid has a very limited naturalhost range so the primary source of newinfections is the use of infected propagationmaterial or mechanical transmission fromother hop plants. Infection by Hop latentviroid does not cause overt symptoms onmost varieties, but it can reduce alphaacid production up to 20% in the limitednumber of symptomless varieties that havebeen studied. The variety Omega is sensitiveto Hop latent viroid (Fig. 51) and infectedplants of this variety express obvioussymptoms including general chlorosis, poorgrowth, and retarded development of lateralbranches. Total alpha acid production ininfected Omega plants can be reduced by50 to 60%. The epidemiology of Hop latentviroid is still not totally clear but controlmeasures adopted elsewhere have centeredon producing viroid-free hops and plantingaway from sources of infection such as olderplantings.Apple fruit crinkle viroidAnother sub-virus pathogen, Applefruit crinkle viroid (AFCVd) was firstreported to occur in hops in Japan in 2004.This viroid is not known to occur in NorthAmerica in either its hop or fruit tree hosts.Very little additional information is availableabout this viroid in hops. Symptoms arereported to be very similar to those inducedby Hop stunt viroid and appropriate controlmeasures are similar (see Hop stunt viroid,preceding two pages).Figure 51. Yellowing of leaves and weakgrowth of Omega variety caused by Hoplatent viroid. The pathogen is widespreadin hop yards in the U.S. but symptomsare rarely produced on varieties currentlygrown in the U.S. (D. Barbara)Arabis mosaic virusReports of the hop strain of Arabismosaic virus appeared in early literature ofthe U.S. hop industry. However, recentattempts to identify infected plants failedto detect the presence of this virus incontemporary hop production in the UnitedStates. Arabis mosaic virus is transmitted bya nematode, Xiphinema diversicaudatum,which has a very limited distribution in theUnited States. The absence of the nematodevector and the adoption of new varietiesbred in the United States have contributedto the apparent elimination of Arabismosaic virus from current U.S. productionareas. In the United Kingdom, where thenematode vector is indigenous, infectionby Arabis mosaic virus is reported to reduceyield by 23% to 50%. Arabis mosaic virusis also transmitted by introducing sap frominfected plants into mechanical wounds,but this is thought to be an insignificantroute of virus spread. Plants infected withArabis mosaic virus can display a diversity ofsymptoms depending on variety, weather,