Field Guide for Integrated Pest Management in Hops

Verticillium WiltDavid H. Gent and Mark E. NelsonVerticillium wilt is a potentiallydamaging disease of hop and numerousother crops including alfalfa, cherry, maple,mint, potato, as well several herbaceousplants, woody ornamentals, and commonweeds. On hop, Verticillium wilt may becaused by two related fungi, Verticilliumalbo-atrum and V. dahliae. The host rangeand severity of disease caused by thesepathogens varies. Several strains of V. alboatrumhave been described. Some may causerelatively minor wilting symptoms (nonlethalor fluctuating strains) while others cancause severe symptoms (lethal or progressivestrains) that rapidly can kill susceptiblevarieties. Non-lethal strains of V. albo-atrumare common in the Pacific Northwest andhave been reported on hop. Lethal strainsof Verticillium albo-atrum have not beenreported from the United States. Verticilliumdahliae causes a relatively minor wilt diseaseon hop. This pathogen has a broader hostrange than V. albo-atrum, and occurscommonly on hop in the United States.SymptomsDisease symptoms vary dependingon the aggressiveness of the Verticilliumpathogen that is attacking the plant. Withnon-lethal strains of V. albo-atrum, diseaseFigure 40. Upward curling and wiltingof leaves associated with Verticilliumwilt caused by a non-lethal strain ofVerticillium albo-atrum. (D. H. Gent)Figure 41. Swollen bine with wilted leavesresulting from infection by a non-lethalstrain of Verticillium albo-atrum, one of theVerticillium wilt pathogens. (D. H. Gent)symptoms often appear initially on lowerleaves as yellowing and death of tissuebetween major veins and upward curlingof leaves (Fig. 40). Affected bines becomenoticeably swollen (Fig. 41) and when thesestems are cut open the vascular tissue isdiscolored a medium to dark brown (Fig.42). These symptoms generally are firstrecognized near flowering or when plantsbecome moisture stressed. Eventually, oneor all of the bines on a hill harboring theinfection completely wilt (Fig. 43). Lethalstrains of V. albo-atrum can cause rapiddeath of leaves, side arms, and plant death.Bine swelling is less apparent with lethalstrains of V. albo-atrum, but the degree ofvascular browning is more severe than thatassociated with non-lethal strains of thepathogen. Verticillium albo-atrum has beenreported on hop more frequently in Oregonthan Idaho or Washington.Symptoms of Verticillium wiltcaused by Verticillium dahliae may varydepending on environment and variety.In some cases, such as with the varietyWillamette, plants may be infected butthe only noticeable symptom is swelling ofthe bines and a general yellowing of lowerleaves near the main bines. Some degree ofbrowning often is present when these binesare cut open. Verticillium dahliae tends tocause more severe symptoms on hop plantsin Washington than Oregon.At-A-GlanceVerticilliumWilt◆◆Plant resistantvarieties whenpossible.◆◆Clean equipmentbetween yards tominimize spreadingthe pathogen.◆◆Plant onlydisease-freerhizomes orcuttings.◆◆Do not returntrash or compostfrom yards withVerticillium wilt tohop yards.◆◆Control weedswith herbicidesand reducecultivation wherepossible.◆◆Reduce nitrogenfertilization asmuch as possible.25