Hop Handbook 2009

8/18/2019 Hop Handbook 2009

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Acknowledgements

Funding for this handbook was made by possibleby a grant from theU.S. EnvironmentalProtection Agency PesticideEnvironmental StewardshipProgram. Financial supportalso was provided by OregonHop Commission, OregonState University, Universityof Idaho, U.S. Departmentof Agriculture AgriculturalResearch Service, WashingtonHop Commission,and Washington StateUniversity. Te editorsgratefully acknowledgethe many reviewers andauthors who contributedto this publication. Wealso recognize the U.S. hop

industry for its continuedsupport of research,extension, integratedpest management, andenvironmental stewardship.

Reference in this publicationto a trademark, proprietaryproduct, or company nameby personnel of the U.S.Department of Agricultureor anyone else is intendedfor explicit description only

and does not imply approvalor recommendation to theexclusion of others that maybe suitable.

All rights reserved. Noportion of this bookmay be reproduced inany form, includingphotocopy, microfilm,information storage andretrieval system, computerdatabase, or software, orby any means, including

electronic or mechanical, without written permissionfrom the Washington HopCommission.

Copyright is not claimedin any portion of this work written by U.S. governmentemployees as a part of theirofficial duties.

© 2009 Washington HopCommission

Contributors American Phytopathological Society, St. Paul, Minnesota

C. Baird, Southwest Idaho Research and Extension Center, University of Idaho, Parma

Dez J. Barbara, Horticulture Research International, Warwick, United Kingdom

James D. Barbour, Southwest Idaho Research and Extension Center,University of Idaho, Parma

Ron A. Beatson, HortResearch, Motueka, New Zealand

John C. Bienapfl, University of California, Davis

Center for Invasive Species and Ecosystem Health (formerly Bugwood Network),University of Georgia, ifton

Amy J. Dreves, Oregon State University, Corvallis

Ken C. Eastwell, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Bernhard Engelhard, Bavarian State Research Center for Agriculture, Wolnzach, Germany

Glenn C. Fisher, Oregon State University, Corvallis

David H. Gent, U.S. Department of Agriculture - Agricultural Research Service,Oregon State University, Corvallis

Ken Gray Image Collection, Oregon State University, Corvallis

Gary G. Grove, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Frank S. Hay, University of asmania, Burnie, asmania, Australia

David G. James, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Dennis A. Johnson, Washington State University, Pullman

Walter F. Mahaffee, U.S. Department of Agriculture, Agricultural Research Service,Corvallis, Oregon

rish McGee, Sustainability Victoria, Melbourne, Victoria, Australia Mark E. Nelson, Irrigated Agriculture Research and Extension Center,

Washington State University, Prosser

Cynthia M. Ocamb, Oregon State University, Corvallis

Robert Parker, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Wilson S. Peng, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Sarah J. Pethybridge, Botanical Resources Australia, Ulverstone, asmania, Australia

Johann Portner, Bavarian State Research Center for Agriculture, Wolnzach, Germany

Cal B. Skotland, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Darrell R. Smith, Busch Agricultural Resources, Inc., Yakima, Washington

. J. Smith, HortResearch, Motueka, New Zealand

Douglas B. Walsh, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser

Florian Weihrauch, Bavarian State Research Center for Agriculture, Wolnzach, Germany

Joanna L. Woods, Oregon State University, Corvallis

Larry C. Wright, Irrigated Agriculture Research and Extension Center, Washington State University, Prosser


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Use pesticides with care. Apply them only to plants, animals, or siteslisted on the label. When mixing and applying pesticides, follow all label

precautions to protect yourself and others around you. It is a violation of thelaw to disregard label directions. If pesticides are spilled on skin or clothing,remove clothing and wash skin thoroughly. Store pesticides in their originalcontainers and keep them out of the reach of children, pets, and livestock.


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IntroductionPest Management, Crop Loss, and IPMDavid H. Gent

and weeds affecting hops in the PacificNorthwest. Principles of IPM, farm IPMplanning, pesticide toxicology, and nutrientmanagement are presented so that thegrower or pest manager can better utilizethis information in the context of an entirefarming system. Correct identification of pestproblems is the first step in IPM, and colorimages have been included as diagnostic aids

wherever possible. Information is presentedon the life cycle and biology of the primarypests of hops in the Pacific Northwest toprovide key concepts underlying managementrecommendations.

Information on current pesticideregistrations for hops is available in the PacificNorthwest pest management handbooks (see

sidebar), which are revised annually.Te editors also acknowledge the

significant contributions of the generalreferences at right that provided thefoundation and scaffolding for this book.

Principles of Integrated Pest Management Jim D. Barbour

Integrated pest management (IPM)is a pest management strategy formallydeveloped in the 1950s by entomologists

and other researchers in response to widespread development in agriculturalsettings of pesticide resistance in insectsand mites, outbreaks of secondary andinduced insect and mite pests resulting frompesticide use, and transfer and magnificationof pesticides in the environment. Initiallyfocusing on biological control of insectsand mites in agricultural systems, IPMover the last 60 years has assumed abroader role and meaning, encompassingmanagement of diseases and weeds as wellas insects and mites (and other arthropods)in agricultural, horticultural, and urbansettings. Broadly speaking, IPM emphasizesselecting, integrating, and implementingcomplimentary pest management tactics tomaintain pests at economically acceptablelevels while minimizing negative ecologicaland social impacts of pest managementactivities. Although the details of IPMprograms vary to meet the needs ofindividual cropping situations, all are based

on several related principles.

1

Production of high quality hopsrequires careful attention to numerousarthropod, disease, and weed pests, as well as horticultural practices that mayexacerbate or suppress these pests. Multipleplant pathogens and arthropods have beendocumented as pests of hop in the PacificNorthwestern United States, and manyplants common in the region can become weeds in hop yards in certain circumstances.Te damage these organisms may causeranges from insignificant to complete lossdue to direct reduction in quantity of yieldor diminished yield quality that can renderhops unsalable.

Te goal of the Field Guide forIntegrated Pest Management in Hops is to

provide growers, consultants, extensionpersonnel, and other pest managers withcurrent, science-based information onidentification and management of arthropodpests, beneficial organisms, diseases,

Systems-level ManagementModern IPM emphasizes the

management of agricultural systems, rather

than individual pests, so as to prevent orreduce the number and severity of pestoutbreaks. Tis is also referred to as agro-ecosystem planning or whole-farm planning. A focus on whole-farm planning is alsoa focus on prevention, which expandsmanagement efforts in time and space.In agricultural crops, this includes usingcultural methods such as crop rotations andfallow periods, tillage, and variety selection(i.e., use of pest-resistant or tolerant varietiesand pest-free rootstock), and legal methods

such as quarantines. Of these, crop rotationmay be the most difficult to implement inhop because the perennial nature of the cropand the trellis system limit the productionof alternative crops in hop yards. Includedin prevention is the conscious selection ofagronomic procedures such as irrigation andfertilizer management that optimize plantproduction and reduce plant susceptibilityto pests. Prevention can be very effective andcost-efficient and presents little or no risk to

people or the environment.

PacicNorthwestPestManagementHandbooks

Pacific NorthwestPlant DiseaseManagementHandbook, http://plant-disease.ippc.orst.edu/

Pacific NorthwestInsect ManagementHandbook, http://pnwpest.org/pnw/insects

Pacific Northwest Weed ManagementHandbook, http://pnwpest/pnw/weeds

GeneralReferences

Burgess, A. H.1964. Hops: Botany,Cultivation andUtilization. WorldCrop Books,IntersciencePublication, NY.

Mahaffee, W. F.,Pethybridge, S. J.,and Gent, D. H., eds.

2009. Compendiumof Hop Diseases andPests. AmericanPhytopathologicalSociety Press, St. Paul,MN.

Neve, R. A. 1991.Hops . Chapman andHall, London.


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Pest and Natural Enemy Identication2

Te ability to accurately identify pests orpest damage is central to IPM, as is the abilityto recognize and accurately identify a pest’simportant natural enemies. Many plants andother organisms live in agricultural fields andmost of these are innocuous or even beneficial.

Accurate identification is needed to determine ifpests are present and to obtain information on

pest biology and life history that may be critical

to effective monitoring and control efforts. Forexample, damage to hop caused by the Californiaprionus beetle, Verticillium wilt, and Fusariumcanker can be superficially similar in appearance,but the first is a root feeding insect and the othertwo are caused by pathogenic fungi. Managementoptions for these pests are very different, thereforepositive identification is required to select effective

treatment options.

Pest and Natural Enemy Biology and Life History An understanding of the biologies and life

histories of pests and their natural enemies, as wellas an understanding of the environmental condi-tions affecting growth and reproduction, providevaluable information for pest management.Knowing which development stage of a particularpest causes damage; knowing when and where thepest is located within or near the crop when thisdevelopment stage occurs; knowing which pest

stage is susceptible to particular management tac-

Economic Injury Levels and Economic (Action) Thresholds

tics; and knowing what host plant(s) and climaticconditions are favorable (or unfavorable) to pestdevelopment—all of these help determine when,

where, and how to control the pests of interest.Te continuing trend toward more biologically-based pest management systems requires detailedinformation on the life cycles of pests, their natu-ral enemies, unintended consequences of applyingcertain control measures, and the complex interac-

tion of these factors with the environment.

Ideally, an EIL is a scientifically determinedratio based on results of replicated research trials overa range of environments. In practice, economic injurylevels tend to be less rigorously defined, but insteadare nominal or empirical thresholds based on growerexperience or generalized pest-crop response datafrom research trials. Although not truly comprehen-sive, such informal EILs in combination with regularmonitoring efforts and knowledge of pest biology

and life history provide valuable tools for plan-ning and implementing an effective IPM program.Economic injury levels are dynamic, changing withcrop value (decreasing as crop value increases) andmanagement costs (increasing as management costsincrease). In theory, economic injury levels can varyfrom year to year or even from field to field within ayear depending on crop variety, market conditions,and available management options.

Te economic threshold (sometimes calledan action threshold) is the pest density at whichcontrol efforts are triggered so as to prevent pestpopulations from reaching the economic injurylevel. Economic thresholds are probably morefamiliar to growers and field personnel than eco-nomic injury levels. Te economic threshold maybe close to or the same as the economic injury levelfor quick-acting management tactics, such as somepesticides, or much lower than the economic injurylevel for slower-acting tactics such as some bio-logical control tactics. Planning for any lag periodbetween application of a management tactic andits impact on pest numbers is an important part ofutilizing economic injury levels and economic (ac-

tion) thresholds an IPM program.

PHOTOS ABOVE:

D. G. James

FIGURE BELOW:

Graphic illustration of the use

of an economic injury level and

economic threshold for pest

management decision making.

The economic injury level isthe break-even point where

management costs equal the

damage caused by a pest.

The economic threshold is the

pest density at which control

efforts are triggered so as

to prevent pest populations

from reaching the economic

injury level. The short brown

arrows illustrate times when a

treatment should be applied

because the economic

threshold was exceeded.

Economic Injury Level

Economic Threshold

P e s t P o p u l a t i o n

Time

TREATMENT

TREATMENT

In most situations it is not necessary, desir-able, or even possible to eradicate a pest from anarea. Te presence of an acceptable level of pests ina field can help to slow or prevent development ofpesticide resistance and maintain populations ofnatural enemies that slow or prevent pest popula-tion build-up. In IPM, acceptable pest levels aredefined in terms of economic injury levels (EIL):the pest density (per leaf, cone, or plant, for ex-

ample) that causes yield loss equal to the cost oftactics used to manage the pest. Te economicinjury level provides an objective basis for makingpest management decisions. At densities below thislevel, management costs exceed the cost of dam-age caused by the pest and additional efforts tomanage the pest do not make economic sense andare not recommended. At densities above the eco-nomic injury level, losses in yield exceed the cost ofmanagement and avoidable economic losses havealready occurred: management efforts should havebeen used earlier.


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Monitoring for Pests, Damage, and Treatment Success

Te concepts of acceptable pest levels, eco-nomic injury levels, and economic thresholds implya need to monitor for levels of pests or pest damagein relation to these levels. Monitoring is funda-mental to IPM because it is used to objectivelydetermine the need for control and also to assessthe effectiveness of control after action has beentaken. Sampling and monitoring requires the abil-

ity to identify pests, pest damage, and key naturalenemies of pests, as well as knowledge of pest andnatural enemy biology and life history. In monitor-ing, the grower or a scout takes representative sam-ples to assess the growth status and general healthof the crop, the presence and intensity of currentpest infestations or infections, and the potential fordevelopment of future pest problems. Monitoringmay take many forms such as presence/absence orcounts of pests from visual inspection of plants orplant parts or traps placed in or around fields (e.g.,sticky traps, pheromone traps, spore traps). Sam-pling should be conducted to provide a representa-

tive assessment of the pest population in all areas tobe similarly treated, such as part of a field, a single

field, or adjacent fields. Various sampling schemeshave been developed to assist in monitoring efforts.

Monitoring an area for environmentalconditions (especially temperature and relativehumidity) that are favorable or unfavorable for pestdevelopment is also important. Tis includes theuse of models (e.g., the powdery mildew risk index,degree-day for downy mildew spike emergence

and spider mites) to forecast conditions conduciveto disease or pest development, and surveying thearea for the presence of alternate hosts of hop pests(e.g., agricultural or ornamental varieties of prunethat might harbor overwintering hop aphids) andnatural enemies (e.g., flowering weeds that providehabitat for natural enemies).

Monitoring, when conducted routinely—at least weekly during the growing season—andin combination with good record keeping andawareness of model forecasts, can help determinetrends in pest and natural enemy populationgrowth over time. Tis assists in planning for

pest management decisions and assessing theeffectiveness of control actions.

Multi-tactic Management Approaches

When prevention is not effective or possibleand monitoring indicates that a pest population hasreached or exceeded an action threshold, interven-tion is required to lower pest numbers to acceptablelevels. For any given pest situation, pest/crop man-agers will need to choose one or more appropriateand compatible management tactics. Te basic

types of controls are mechanical, biological, andchemical.

Mechanical controls include simple hand-picking, erecting barriers, using traps, vacuuming,and tillage to disrupt pest growth and reproduc-tion. illage is commonly used to manage weeds inhop, and can be important in managing arthropodpests such as the garden symphylan.

Biological controls are beneficial organismsthat prey on or parasitize pests, or organisms thatdo not damage crops but compete with pests forhabitat and displace pests (e.g., Bacillus pumilus forpowdery mildew management). Some biologicalcontrol agents are commercially available for releaseinto cropping systems (i.e., fields, greenhouses) innumbers that can overwhelm pests or that supple-ment existing natural enemy populations. Addingagents to the ecosystem is referred to as augmenta-tive biocontrol; an example would be the releaseof predatory mites Galendromus occidentalis and/orNeoseiulus fallacis , which can be purchased and re-leased for management of twospotted spider mites.Natural enemy populations also can be augmentedusing commercially available chemical attractants,such as methyl salicylate. Biological control also can

be implemented by managing crops to conserveexisting natural enemies (conservation biologicalcontrol) through preserving habitat (includingalternative hosts and prey) necessary for normalnatural enemy growth and reproduction, or by us-ing management tactics (e.g., selective pesticides orpesticide uses) that have minimal negative impact

on natural enemies. In hop, biological control ismost widely practiced in the form of conservationbiological control through the use of selective pesti-cides and modified cultural practices.

Chemical controls include synthetic andnatural pesticides used to reduce pest populations.Many newer synthetic pesticides are much lessdisruptive to non-target organisms than older,broad-spectrum chemistries (e.g., organophosphate,carbamate, and pyrethroid insecticides). Insecticidesderived from naturally occurring microorganismssuch as Bacillus thuringiensis , entomopathogenicfungi and entomopathogenic nematodes, andnatural insecticides such as nicotine, pyrethrin,and spynosins are important tools in many organicfarming operations, and are playing larger roles innon-organic crop production. Selective pesticidesshould be chosen over non-selective pesticidesto preserve natural enemies and allow biologicalcontrol to play a greater role in suppressing pestoutbreaks. However, broad-spectrum pesticidesremain useful and necessary components of IPMprograms as measures of last resort when othermanagement tactics fail to maintain pests atacceptable levels.

3

Photos Above: A. J. Dreves

D. H. Gent, D. H. Gent

Check the AgWeatherNetwebsite at URLhttp://weather.wsu.edu/ foravailable diseaseand pest models.

Consult with

local experts forinformation on usesand limitationsof pest forecastmodels in IPM.


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Pesticide Toxicology and Selectivity

Pesticide Toxicity RatingsDouglas B. Walsh

4

Pesticides are essential tools inIPM when other management tacticsfail to control pests at acceptable levels. Approximately 250 to 300 pesticideactive ingredients are used in the PacificNorthwestern United States, and inevitablypesticide use involves some degree ofexposure and risk to humans, non-targetorganisms, and the environment. able 1 isprovided as a guide to the relative impactof specific pesticides registered for use onhop on non-target beneficial arthropods.Te pesticide “signal word” (column 2 oftable) indicates the potential hazard thesepesticides could pose to a mixer or applicator.Te signal word “Danger” identifies a

product as being a Category 1 pesticide,and includes products such as 2,4-D,ethoprop, and folpet. Tese products have atoxicological profile that could cause injuryor irritation to individuals exposed to lowconcentrations. Te signal word “Warning”identies a product as a Category 2 pesticide,and includes products such as clethodim,cymoxanil, and beta-cyfluthrin. Tese arematerials that will typically require theuse of fairly extensive personal protectiveequipment, but exposure levels required to

cause injury or irritation are substantiallygreater than Category 1 pesticides. Tesignal word “Caution” identifies a Category3 pesticide, and includes products such asthe biocontrol bacterium Bacillus pumilus ,carfentrazone, and various Bt formulations(e.g., Bacillus thuringiensis subsp. kurstaki ). A Category 3 pesticide is a product that cancause injury or irritation at a relatively highexposure rate. Personal protective equipmentis required, typically including safety glasses,

pants, rubber boots, gloves, and long-sleevedshirts. No signal word is required for aCategory 4 pesticide. Simple safety rulesshould be followed with these products toavoid exposure.

Pesticide impacts on humans do notnecessarily mirror the impacts those samepesticides would have on beneficial hopyard arthropods. Human physiology differsfrom arthropod physiology, and substantialdifferences exist among arthropods as well.Differences in both susceptibility and

resilience factor into a pesticide’s impacton a population of beneficial arthropods.Large predatory insects, for example, maybe able to survive greater doses (i.e., be lesssusceptible) than smaller predatory insectsand mites. However, larger insects typically will complete only one or a few generationsover the course of a growing season in thePacific Northwest, whereas a smaller insectmight complete more generations and havea greater chance of recovering its populationlevel (i.e., be more resilient). If a populationis depressed due to pesticide exposure itmay not recover in a hop yard unless thereis an immigration of new individuals fromoutside of the yard.

o standardize topical mortalitystudies, the International Organization forBiological Control (IOBC) has categorizedpesticides using a ranking of 1 to 4.Category 1 pesticides in the IOBC ratingsystem are rated as “harmless” to a candidatepopulation of beneficial arthropods if lessthan 30% of a populations dies followinga direct exposure. A Category 2 pesticidein the IOBC rating system is defined as“slightly harmful” to the beneficial. Directexposure to the pesticide will result in

mortality levels between 30 and 79%. ACategory 3 pesticide in the IOBC ratingsystem is defined as “moderately harmful”to the beneficial arthropod. Direct exposureto the pesticide will result in mortalitylevels between 79 and 99%. A Category4 pesticide in the IOBC rating systemis defined as “harmful” to the beneficial.Direct exposure to the pesticide will resultin mortality levels greater than 99%. (IOBCcategories 1-4 should not be confused

with the categories 1-4 relating to humanexposure and indicated by signal words“Danger,” “Warning,” and “Caution” asdescribed in the first column of this section.)able 1 provides information on three keybeneficial arthropods that occur on hop:predatory mites, lady beetles, and lacewinglarvae. Te rankings are summarized froman amalgam of research projects that havebeen conducted on these organisms in thePacific Northwes on crops including treefruit, hop, mint, and grape.

Photos: D. H. Gent,

W. S. Peng

J. D. Barbour,

D. G. James


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5Table 1. Signal Words and Relative Impact of Pesticides Registeredfor Use on Hop on Representative Non-target Benecial Arthropods

Active IngredientSignal

WordTrade Name

Benefcial Arthropod IOBC Rankinga

Predatory

Mites

Lady

Beetles

Lacewing

Larvae

Fungicides

Bacillus pumilus Caution Sonata 1 ND ND

Boscalid Caution Pristine 1 ND ND

Copper Caution Various formulations 1 ND ND

Cymoxanil Warning Curzate 60DF ND ND ND

Dimethomorph Caution Acrobat ND ND ND

Famoxadone & cymoxanil Caution Tanos ND ND ND

Folpet Danger Folpan 80WDG ND ND ND

Fosetyl-Al Caution Aliette WDG ND ND ND

Kaolin Caution Surround 3 ND ND

Mefenoxam Caution Ridomil ND ND ND

Metalaxyl Warning MetaStar ND ND ND

Mineral oil/petroleum distillate Caution Various formulations 2 ND ND

Myclobutanil Warning Rally 40W 2 1 ND

Phosphorous acid Caution Fosphite and other formulations ND ND ND

Pyraclostrobin Caution Pristine ND ND ND

Quinoxyfen Caution Quintec 1 ND ND

Sodium borate Warning Prev-Am 2 ND NDSpiroxamine Caution Accrue ND ND ND

Sulfur Caution Various formulations 2 ND ND

Tebuconazole Caution Folicur 3.6F 1 ND ND

Trioxystrobin Caution Flint 1 ND ND

Herbicides

2,4-D Danger Weedar 64 and other formulations ND ND ND

Carfentrazone Caution Aim EC 1 ND ND

Clethodim Warning Select Max 1 ND ND

Clopyralid Caution Stinger 1 ND ND

Glyphosate Caution Roundup and other formulations 1 ND ND

Norurazon Caution Solicam ND ND ND

Paraquat Danger Gramoxone and other formulations 1 ND ND

Pelargonic acid Warning Scythe ND ND NDTriuralin Caution Trean and other formulations 2 ND ND

Insecticides/Miticides

Abamectin Warning Agri-Mek and other formulations 3 3 ND

B. thuringiensis subsp. aizawai Caution Xentari and other formulations 1 2 ND

B. thuringiensis subsp. kurstaki Caution Dipel and other formulations 1 2 ND

Beta-cyuthrin Warning Baythroid XL 4 4 4

Bifenazate Caution Acramite-50WS 1 2 ND

Bifenthrin Warning Brigade and other formulations 4 4 4

Cyuthrin Danger Baythroid 2E 4 4 4

Dicofol Caution Dicofol 1 1 ND

Ethoprop Danger Mocap 4 4 ND

Fenpyroximate Warning Fujimite 1 3 ND

Hexythiazox Caution Savey 50DF 1 1 NDImidacloprid Caution Provado and other formulations 1 3 3

Malathion Warning Various formulations 2 4 3

Naled Danger Dibrom 2 4 3

Pymetrozine Caution Fulll 1 1 1

Pyrethrin Caution Pyganic and other formulations 2 2 2

Spinosad Caution Success and other formulations 2 2 1

Spirodiclofen Caution Envidor 2 2 1

Spirotetramat Caution Movento 1 1 1

Thiamethoxam Caution Platinum Insecticide 1 1 ND

a International Organization for Biological Control (IOBC) has categorized pesticides using a ranking of 1 to 4. Rankings represent relative toxicity base

on data from studies conducted with tree fruit, hop, mint, and grape. 1 = less than 30% mortality following direct exposure to the pesticide; 2 = 30 to 79

mortality; 3 = 79 to 99% mortality; and 4 = greater than 99% mortality. ND = not determined.


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Pesticide Resistance ManagementMark E. Nelson, Robert Parker, and David H. Gent

6

Strategiesto MinimizeDevelopmentof PesticideResistance

Utilize cultural◆practices toreduce pathogen,weed, and pestpopulationswheneverpossible. Forexample, removingoverwintering agshoots or basalspikes and basal

sucker growthby mechanical orchemical methodshelps reduce theinoculum level ofpowdery mildewand downy mildew.

Limit the number◆of applications ofresistance-prone

pesticides asdirected by thelabel.

Apply pesticides◆at rates speciedon the label; do notreduce rates.

Adjust◆application volumeper acre based

on the size andvolume of the cropto attain excellentspray coverage.

Alternate or tank◆mix products withdiverse modes ofaction within andbetween seasons.

Many of the most widely usedpesticides pose an inherent risk of resistancedevelopment. Pesticide resistance is aconsequence of repeated use of an herbicide,fungicide, or insecticide/miticide withthe same mode of action, resulting in a

lack of efficacy for a particular pesticideagainst a particular pest. Resistance hasbeen documented among numerous peststhat may affect hop. Examples includeherbicide resistance in kochia and pigweed,organophosphate resistance in hop aphidand twospotted spider mite, and Ridomilresistance in the downy mildew pathogen.

Resistance develops in a pestpopulation and not in individuals. It occurs when a pesticide is applied repeatedlyand susceptible pests are controlled but

naturally resistant individuals of the samespecies reproduce and increase in absenceof competition. Resistant strains of thepest become prevalent in a population overtime due to this selection pressure. Forexample, studies have shown that kochiais a genetically diverse weed species andin a kochia population a small number ofplants (i.e., 1 in 1,000,000 plants) may benaturally resistant to a particular herbicide.Repeatedly exposing kochia populations

to the same herbicide may result in arapid buildup of resistant weeds. Resistant weeds will then dominate over time dueto this selection pressure and previouslyeffective herbicides will fail to control thepopulation.

Resistance can be quantitative orqualitative. Quantitative resistance manifestsas a gradual loss of control that occurs as apest population becomes more tolerant to apesticide. In these situations, a product mayperform brilliantly when first used and then

over a period of years slowly deteriorate inefficacy. As a result, the compound mustbe applied at higher rates and/or shorterintervals in order to maintain control. Anexample of this quantitative resistance isfosetyl-Al (Aliette WDG) against the downymildew pathogen. Te registered labelrate for Aliette has been 2.5 lbs. per acre(and remains so in most hop productionareas), but this rate is no longer effectivefor control of the downy mildew pathogenin Oregon, where a Section 24c “Special

Local Needs” registration was sought andreceived for the higher rate of 5 lbs. per acre. Alternatively, qualitative resistance is “all ornone,” where a pesticide performs brilliantlyfor a period of time but provides no controlafter resistance develops. A good example of

qualitative resistance is metalaxyl (Ridomil)against the downy mildew pathogen. Onceuseful, this fungicide now provides nocontrol in yards where resistance is present.

Note that persistence of resistance in apest population varies among pesticides andpests. For instance, resistance to metalaxylcan still be detected in the downy mildewpathogen in hop yards that have not beentreated with this fungicide in over 10 years.Conversely, resistance to abamectin (Agri-

Pigweed. (H. F. Schwartz, Colorado State

University, Bugwood.org)

Hop aphids on leaf. (D. G. James)


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Mek) in spider mite populations appears todiminish over time when abamectin is notused over a period of years.

Te risk of resistance developmentis linked closely to the reproductive anddispersal ability of a pest. Pests that have ahigh reproductive potential (e.g., powdery

mildew and spider mites) generally have ahigher risk of resistance development thanpests with a low fecundity. Other factorsthat influence resistance development arethe fitness (relative vigor) of resistant strainsversus susceptible strains, dispersal ability ofthe pest, availability of nearby populationsof susceptible strains of the pest, thenumber of individuals needed to initiate aninfestation or infection, and reproductivemechanisms of the pest (asexual or sexualreproduction). On hop, many pesticides

used for management of powdery mildew,downy mildew, spider mites, and hop aphidhave a risk of resistance due to the highlyspecific mode of action of the pesticides andbiological characteristics of the pests.

Given that few pesticides withnovel modes of action are currently underdevelopment, it becomes readily apparentthat sustained profitability of the hopindustry requires efforts to prevent ordelay resistance development. A key point

in resistance development is that only avery small percentage of individuals in apopulation have the potential for resistanceto a given mode of action. Terefore, theoverall objectives of resistance managementare to reduce the populations of pestsexposed to a given mode of action, as wellas reduce the duration and frequency of thatexposure, thereby reducing the opportunityfor those few individuals with resistancepotential to become predominant in thepopulation. Utilizing diverse modes of

action and limiting the total number ofapplications of a particular mode of actionare fundamental to resistance management.

For downy mildew and powderymildew, resistance generally can beprevented or delayed by limiting thenumber of applications of any resistance-prone fungicide class (no more thanthree per season and no more than two

sequential applications), use of single orblock applications in alternation withfungicides from a different group, and useearly in the season before the diseases are well established. Do not alternate resistance-prone products with other products inthe same fungicide class as cross-resistancehas been documented in the DMI andstrobilurin fungicide classes. For example, arotation of Flint and Pristine would not beeffective since both fungicides have activeingredients with the same mode of action.

Torough application coverage is essential.Similar principles apply to resistance

management for spider mites and hopaphids. Limit the number of applicationsof any resistance-prone product as directedby the label (ideally not more than onceper two seasons in a given yard), use singleor block application in alternation withproducts with a different group modeof action, target applications against themost vulnerable life stage of the pest, and

integrate non-chemical control measuresbefore pests exceed economic thresholds.Selection of products with a high degreeof selectivity for beneficial arthropodscan allow biological control to reducepopulations of resistant pest strains, andthus help to delay resistance.

7

Strategies, cont.

Include low-◆resistance-riskcompounds inspray programsas much as and

whenever possible.Do not rely onresistance-pronecompoundsto attempt tocontrol severepest outbreaks.For example withpowdery mildew,petroleum oils andcarbonates are thebest eradicativefungicides.

Select miticides◆and insecticideswith a high degreeof selectivityfor benecialarthropods to allowbiological control toreduce populationsof resistant pest

strains.

Utilize synthetic◆fungicides proneto resistancedevelopmentprotectively beforepowdery mildewor downy mildewhas become aproblem. Avoid

making more thantwo consecutiveapplicationsof synthetic

fungicides (e.g.,DMI, quinoline,strobilurin,carboxamideor morpholine

classes).

Twospotted spider mites. (D. G. James)

Powdery mildew. (D. H. Gent)


13/90

Disease ManagementFungal & Bacterial Diseases

Alternaria Cone DisorderDavid H. Gent

cone disorder is primarily a disease of conesdamaged by mechanical injury. Severeoutbreaks often are associated with windinjury accompanied with high humidityor extended periods of dew. Te pathogensurvives between seasons on decaying plantmaterial, organic matter, and/or as a weak

pathogen on other plants.

ManagementManagement of Alternaria cone

disorder requires accurate diagnosis ofthe disease, which is confounded by itssymptomatic resemblance to powderymildew or downy mildew. Simply recoveringthe fungus from discolored cones does notnecessarily indicate that it was the causeof the browning since the pathogen alsois found on healthy cones. Te disease

can be minimized by reducing damage toburrs and cones caused by strong winds,pesticide applications, and other pests andpathogens; promoting air circulation in thecanopy; and timing irrigations to reduceperiods of wetness on cones. No fungicidesare registered for control of Alternaria conedisorder. However, certain fungicides (e.g.,Flint and Pristine) applied for control ofpowdery and downy mildew likely providesome suppression of Alternaria cone disorder

when applied later in the season.

At-A-Glance:

AlternariaCone Disorder

Symptoms◆

easily confusedwith powdery and/or downy mildew.

Promote air◆circulation in thecanopy.

Time irrigations◆to reduce periodsof wetness on

cones.

Some powdery◆and downy mildewfungicides likelyprovide somesuppression of

Alternaria conedisorder whenapplied later in theseason.

Conrm◆cone browningis caused by

Alternaria conedisorder beforeimplementing anycontrol measures.

8

Figure 2. Further discoloration of cones affected

by Alternaria cone disorder. (S. J. Pethybridge)

Figure 1. Reddish-brown discoloration of the

tips of bracts and bracteoles of a cone affected

by Alternaria cone disorder. (D. H. Gent)

Alternaria cone disorder is causedby the fungus Alternaria alternata , whichis widespread in hop yards and other

agricultural systems worldwide. Strains of Alternaria fungus are known to attack morethan 100 other plants, including crops suchas apple, potato, sunflower, and wheat.

While the presence of the fungus is widespread, the disease is not known tobe associated with direct yield losses in theU.K. and Australia and is thought to be ofminor importance in the United States. Tedisease can occasionally damage cones andreduce crop quality. It is reported to occurmost commonly on late-maturing varieties

exposed to wind injury, humid conditions,and extended periods of wetness oncones. Cone browning caused by powderymildew and downy mildew is commonlymisdiagnosed as Alternaria cone disorder.

Symptoms Alternaria cone disorder symptoms

vary depending on the degree of mechanicalinjury to cones; they may be limited toone or a few bracts and bracteoles or in

severe cases entire cones may becomediscolored. Symptoms appear first onthe tips of bracteoles as a light, reddish-brown discoloration (Fig. 1). Bracts mayremain green, which gives cones a stripedappearance. When cones have been damagedby wind, disease symptoms may appearon both bracteoles and bracts as a moregeneralized browning that can cover entirecones (Fig. 2). e disease can progressrapidly; the killed tissue becomes dark brownand is easily confused with damage caused by

powdery or downy mildew. Affected bractsand bracteoles may display a slight distortionor shriveling of the diseased tissues.

Disease Cycle Alternaria alternata generally is a

weak pathogen that invades wounds createdby insect feeding, mechanical injury, orlesions created by other pathogens. Otherstrains of the fungus may survive as a decayorganism on textiles, dead plants, leather, or

other organic materials. On hops, Alternaria


14/90

Black Root RotFrank Hay and David H. Gent

Disease CycleTe black root rot pathogen survives

in soil as dormant sexual spores (oospores), which can survive 18 months or more. Inthe presence of free water and host roots,oospores or the asexual spores (sporangia)germinate and infect the plant directly ormay release motile spores (zoospores) thatare attracted to compounds released from

host roots (e.g., ethanol and certain aminoacids and sugars). Te motile zoospores set-tle on roots and later produce mycelia thatinfect and grow through the host tissues.

ManagementGrowers should avoid establishing

hop yards in areas with poor water drainage,especially with highly susceptible varietiessuch as Cluster types E-2 and L-8. ClusterL-1 and Galena are considered partially

resistant, while Brewers Gold, Bullion,Cascade, Columbia, Comet, Eroica, Fuggle,Hallertauer, Nugget, Olympic, ettnanger,and Willamette reportedly are highly resistantto black root rot. Reducing cultivation andavoiding injury to crowns and roots canprovide some reduction in disease sinceinfection is favored by wounds. Certainphosphorous acid fungicides are registeredfor control of black root rot, but theirefficacy has not been reported. Phenylamidefungicides (i.e., various formulations of

Ridomil) applied for control of downymildew may provide some control, althoughthese products are not registered specificallyfor control of black root rot.

9

At-A-Glance:

BlackRoot Rot

Plant resistant◆

varieties whenpossible.

Avoid poorly◆drained eldsand excessiveirrigation.

Avoid damaging◆roots duringcultivation.

Phosphorous◆acid fungicidesand variousRidomilformulations mayprovide somecontrol.

Figure 3. Extensive black discoloration caused by black root rot. Notice the distinct

margin between healthy tissue and the black, diseased tissue. (R. A. Beatson)

Te fungus-like organismPhytophthora citricola causes a crown androot rot of hop referred to as black rootrot. Te disease tends to be most damagingto hop plants in poorly drained soils andareas with high water tables. CertainCluster varieties such as Cluster types E-2and L-8 are particularly susceptible. Tepathogen has a relatively broad host rangethat includes cherry, fir trees, raspberry,strawberry, and walnut.

SymptomsInfected roots and crowns have a

characteristic water-soaked and blackenedappearance with a distinct boundary be-tween diseased and healthy tissue (Fig. 3).Infection can spread from the crown for sev-

eral inches up the base of the bine. In severecases, leaves become yellow and bines wiltrapidly during warm weather or when plantsbecome moisture-stressed. Young plantsirrigated heavily to encourage productionin the first year can wilt later in the seasonas a result of black root rot. As the diseaseprogresses, leaves turn black and remain at-tached to the bine. Severely infected plantsare weakened and may die during winteror the following spring. Affected plantsoften are found in areas of hop yards with

poor drainage. Wilting symptoms causedby black root rot can be mistaken for Ver-ticillium wilt, Fusarium canker, or damagecaused by California prionus beetle.

See the Pacifc

Northwest Plant Disease

Management Handbook

at http://plant-disease.

ippc.orst.edu/ for a

current list of registered

herbicides.


15/90

At-A-Glance:

DownyMildew

Select the most◆resistant variety

that is availablefor the intendedmarket.

Establish hop◆yard with disease-free plantingmaterials.

Thoroughly◆remove all basal

foliage duringspring pruning.

Prune yards as◆late as possiblewithout adverselyaffecting yield.

Strip leaves◆from binesafter trainingand removebasal foliagewith chemicaldesiccants.

Apply◆appropriatefungicides duringthe rst year ofproduction andwhen weather is

favorable to thedisease.

Rotate◆and tank-mixfungicides to delaydevelopment ofresistance.

Downy MildewDavid H. Gent and Dennis A. Johnson

Downy mildew is caused by thefungus-like organism Pseudoperonosporahumuli . It is one of the most importantdiseases of hop in the Pacific Northwestand worldwide. Yield and quality lossesfrom downy mildew vary depending on

susceptibility of the variety and timingof infection, and may range from non-detectable to 100% crop loss if signicantcone infection or plant death from crownrot occurs.

10

Figure 5. Profuse sporulation on the

underside of a hop leaf appears dark

purple to black. (D. H. Gent)

Figure 4. Basal spikes: Hop shoots

systemically infected with the downy

mildew pathogen. (D. H. Gent)

Figure 6. Infection of shoots after training.

Notice the yellowing, stunting, and down-

curling of the leaves. (D. H. Gent)

Figure 7. Stunted lateral branches resulting

from downy mildew. Production from these

branches will be lost. (D. H. Gent)

See the Pacic Northwest PlantDisease Management Handbook

at http://plant-disease.ippc.orst.edu/ for a current list of

registered herbicides for downymildew and other diseases.


16/90

SymptomsTe disease first appears in spring on

newly emerged and infected shoots that arecalled “basal spikes.” Basal spikes are stuntedand have brittle, downward-curled leaves(Fig. 4), upon which masses of purple toblack asexual spores (sporangia) are visible(Fig. 5). After training, the main bines

and lateral branches may become infected,arresting the development of these shootsand leading to “aerial spikes” (Figs. 6 and 7).Infection of trained bines causes these binesto cease growth and fall from the string,requiring retraining with healthy shoots andoften leading to yield loss. Lesions commonlyare present on leaves next to spikes. Tese leaflesions are confined between leaf veins andappear angular (Fig. 8). Leaf lesions tend todry quickly in warm, dry weather, becomingbrown areas of dead tissue (Fig. 9).

1

Figure 8. Angular leaf lesions on hop leaves.

The black discoloration is due to sporulation

by the pathogen. (D. H. Gent)

Figure 9. Dry, angular leaf lesions caused by

downy mildew. (D. H. Gent)

Infected burrs turn dark brown, shrivel,dry up, and may fall from the plant. Infectedcones become dark brown, harden, and ceasedevelopment. Bracteoles of affected cones tendto become discolored more readily than bracts,and affected cones may develop a stripedappearance. Under high disease pressure entire

cones may become dark brown (Fig. 10).Sporulation on the underside of bracts andbracteoles is diagnostic for downy mildew oncones, although it is common for sporulationto be absent on infected cones.

In infected roots and crowns, reddish-brown to black flecks and streaks are apparent when roots are cut open (Fig. 11). Te crownmay be completely rotted and destroyed invarieties susceptible to crown rot from downymildew, such as Cluster varieties.

Figure 10. Dark brown discoloration of bracts

and bracteoles on cones severely affected

by downy mildew. (B. Engelhard)

Figure 11. Left, Dark discoloration of rhizomes

infected with Pseudoperonospora humuli .Right, Healthy rhizome. (C. B. Skotland)


17/90

Disease CycleTe downy mildew pathogen

overwinters in infected dormant buds andcrowns (Fig. 12). It spreads into developingbuds during the winter and early spring, andsome (but not all) infected buds give riseto basal spikes when shoots emerge in thespring. Te pathogen sporulates profuselyon the undersides of leaves of spikes when

nighttime temperatures are greater than43 °F and humidity is greater than 90% inthe hop yard. Sporangia are released in mid-morning to early afternoon, and germinateindirectly to produce swimming zoospores when the temperature is favorable and free water is present on leaves, shoot tips, orcones. Zoospores enter hop tissues throughopen stomata, and consequently the mostsevere infections occur when wetness occurson plant surfaces during daylight. Infectionis favored by mild to warm temperatures (60to 70 °F) when free moisture is present forat least 1.5 hours, although leaf infectioncan occur at temperatures as low as 41 °F when wetness persists for 24 hours or longer.

Infection of shoots can becomesystemic, producing secondary spikes andadditional sporangia that perpetuate thedisease cycle. When shoots near the crown(approximately 6 inches in height or less)become infected, mycelia can progressthrough the shoot and invade the crown.

Carbohydrate reserves are reduced insystemically infected rhizomes and theplants become weakened over time, resultingin reduced yield or plant death.

ManagementNo single management tactic provides

satisfactory control of downy mildew.Careful attention to cultural practices, judicious irrigation management, andtimely fungicide applications are needed tomanage the disease successfully. Varieties

vary widely in their susceptibility to downymildew (Table 2), although no varieties arecompletely immune. When possible, selectthe most resistant variety that is availablefor the intended market and plant themost resistant varieties in areas with knowndowny mildew pressure (e.g., next to riversor in low-lying areas with cool air pooling).Cascade, Fuggle, Magnum, Newport, andPerle are among the most resistant to downymildew. Cluster is notably susceptible.

12

Table 2. Disease Susceptibility and ChemicalCharacteristics of the Primary Public

Hop Varieties Grown in the U.S.

Disease Susceptibilitya

Variety UsagePowdery

Mildew

Downy

Mildew

Verticillium

Wilt

Brewers Gold Bittering S MR MR

Bullion Bittering S MR R

Cascade Aroma MR MR MR

Centennial Bittering MR S U

Chinook Bittering MS MR R

Columbia Aroma MS MR S

Comet Bittering R S R

Crystal Aroma R S R

East Kent Golding Aroma S S MR

First Gold Bittering R S MR

Fuggle Aroma MS R SGalena Bittering S S R

Glacier Aroma S S U

Hall. Gold Aroma MS R S

Hall. Magnum Bittering S R MR

Hall. Mittelfrüh Aroma MS S S

Hall. Tradition Aroma MR R MR

Horizon Bittering MS S MR

Late Cluster Aroma S S R

Liberty Aroma MR MR U

Mt. Hood Aroma MS S S

Newport Bittering R R U

Northern Brewer Bittering S S R

Nugget Bittering R S S

Olympic Bittering S MS R

Perle Aroma S R MR

Pioneer Bittering MR MR U

Saazer Aroma S MS S

Saazer 36 Aroma S MS S

Spalter Aroma S R MR

Sterling Aroma MS MR U

Teamaker Aroma MR MR S

Tettnanger Aroma MS MS S

Tolhurst Aroma S S U

U.S. Tettnanger Aroma MS MS S

Vanguard Aroma S S U

Willamette Aroma MS MR S

a Disease susceptibility ratings are based on greenhouse and eld observations in experimental

plots and commercial yards in the Pacic Northwest as of 2009. Disease reactions may vary

depending on the strain of the pathogen present in some locations, environmental conditions,

and other factors, and should be considered approximate. S = susceptible; MS = moderately

susceptible; MR = moderately resistant; R = resistant; U= unknown


18/90

sporangiophores emerge

with sporangia on

undersideof leaf

zoospores are released

from mature sporangium

zoospores infect leaves,

cones and shoots

mycelia grow systemically

throughout the plant,

infecting the crown and buds

mycelia overwinter in buds and crowns

infected shoots emerge

in spring

oospore

antheridium

oogonium

cycle of sporulation/infectionrepeats throughout the season

1

ABOVE: Figure 12. The life cycle of Pseudoperonospora humuli on hop. (Prepared by V. Brewster) BELOW: Figure 13. Hop plants pruned thoroug

mechanically (A) or chemically by a desiccant (C) in early spring. Notice in A and C that all shoots on the sides of the hills have been removed.

Incomplete mechanical (B) or chemical pruning (D) can result in more severe outbreaks of both downy mildew and powdery mildew. (D. H. Gent


19/90

Figure 14. Association of spring pruning quality to the incidence of

plants with downy mildew in 97 commercial hop yards in Oregon

during 2005 to 2008. Excellent = No foliage or green stems

remaining after pruning, Moderate = Foliage or green stems on

some hills after pruning, and Poor = No pruning was conducted or

foliage and green stems were present on all hills after pruning.

Figure 15. Association of spring pruning timing to the

incidence of plants with downy mildew in 6 commercial yards

of Willamette in Oregon. Hop yards that received the delayed

pruning treatment were chemically pruned 10 to 14 days

later than the growers’ standard pruning timing.

Figure 16. Efcacy of Aliette WDG and Flint under moderate and

high disease pressure in Washington. NT = Non-treated.

1

0.8

0.6

0.4

0.2

0

Moderate Disease

Pressure

D O W N Y M I L D E W

I N C I D E N C E

NT

High Disease

Pressure

Aliette Flint NT Aliette Flint

2007 2008

1

0.8

0.6

0.4

0.2

0

D O W N Y M I L D E W

I N C I D E N C E

Delayed Pruning

Standard Pruning

14

0.4

0.3

0.2

0.1

0

D O W N Y

M I L D E W

I N C I D E N C E

Excellent Moderate Poor

Pruning Quality

Non-infected rhizomes or softwood cuttings shouldbe selected when establishing new hop yards since plantingmaterial may harbor the pathogen. Toroughly removeall basal foliage during spring pruning (Figs. 13 and 14).Pruning yards as late as possible, provided all green tissueis removed, generally reduces the severity of downy mildew(Fig. 15). However, optimum timing for pruning must bedetermined carefully for each variety since pruning too latecan reduce yield.

In high disease pressure situations, strip leaves frombines after training and remove basal foliage with chemicaldesiccants to reduce disease spread higher into the canopy.Decisions on stripping and the intensity of basal foliageremoval also depend on the severity of downy mildew,presence of powdery mildew, and consideration of thenegative impacts on beneficial insects and mites. In situations where downy mildew is threatening late in the season, earlyharvest of yards can minimize cone infection. However, yieldand alpha acid content is reduced when plants are harvestedtoo early and this practice also needs to be considered carefully.

imely fungicide applications often are needed to

manage the disease when weather is favorable to the pathogen.Fungicide applications during the first season a yard is plantedmay be beneficial to help minimize crown infection anddisease levels in ensuing seasons. Under high disease pressurein western Oregon, a fungicide applied just after the first spikeemerges and before spring pruning significantly enhancescontrol of downy mildew later in the season. Later fungicideapplications should be timed to coincide with major infectionevents. See the Pacific Northwest Plant Disease ManagementHandbook at http://plant-disease.ippc.orst.edu/ for a currentlist of registered herbicides.

Te downy mildew pathogen has a high potential fordeveloping resistance to certain fungicides. Strict adherenceto resistance management tactics is essential to delay thedevelopment of resistance. Resistance to phenylamidefungicides (e.g., various Ridomil formulations) and fosetyl- Al (Aliette WDG) is common in the Pacific Northwest.Phenylamide fungicides should not be used where resistantpopulations have been detected, since resistance to this classof fungicides appears to persist for many years (>15 years)in the pathogen population. Where phosphonate fungicidessuch as fosetyl-Al have been used extensively, resistanceto low rates (e.g., 2.5 pounds Aliette WDG per acre) of

these products is likely to occur. High rates of phosphonatefungicides are needed for disease control where this resistanceis present. Strobilurin fungicides (e.g., Flint and Pristine)applied for management of powdery mildew can providesuppression of downy mildew. Te activity of strobilurinfungicides against both downy mildew and powdery mildewcan be exploited on varieties susceptible to both diseases,bearing in mind that strobilurins have a high risk of incitingresistance development in both the downy mildew andpowdery mildew pathogens. Efficacy of Aliette WDG andFlint under both moderate and high disease pressure is shown

graphically in Figure 16.


20/90

At-A-Glance:

FusariumCanker

Avoid◆

propagation fromdiseased hills.

Mound soil◆around thebase of bines topromote growthof healthy rootsand reducewilting.

Reduce free◆moisture near thecrown.

Add lime to◆increase pHnear the crownand avoid useof ammoniumnitrogen

fertilizers.Minimize injury◆

to bines duringeld operationsand from pests.

Arching strings◆may help toreduce bineinjury.

1

Figure 17. Swollen basal portion of a bine

affected with Fusarium canker. (D. H. Gent)

Figure 18. Wilted bine due to Fusarium

canker. Notice that wilted leaves remain

attached to the bine. (D. H. Gent)

Fusarium CankerDavid H. Gent

Fusarium canker is caused by thefungus Fusarium sambucinum. Te diseaseis often present at a low incidence in hopyards, although in some circumstances ahigh incidence of plants may be affected.Symptoms of the disease are conspicuousand diseased plants are easily identified.

Yield losses from Fusarium canker have notbeen quantified rigorously.

SymptomsTe base of an affected bine is swollen

and tapers near the point of attachment atthe crown (Fig. 17). Affected bines can bedetached from the crown with a gentle tug.Older leaves on the lower part of the binemay become yellow. Disease symptomsoften are not recognized until affected

bines wilt suddenly (Fig. 18) at floweringor in response to high temperatures andmoisture stress. Leaves on wilted binesremain attached. Bine wilting is often mostevident after mechanical injury to binesfrom cultivation, pesticide applications withan air blast sprayer, or high winds, sincebines break off from crowns at these times.Severely affected plants may be killed duringthe winter, particularly when the diseaseoccurs on young plants.

Disease CycleTe disease cycle of Fusarium canker

has not been investigated thoroughly. Tefungus that causes the disease is widespreadin soil and also can be found in association

with plant debris, diseased crowns, andapparently healthy planting materials. Itis thought that the pathogen infects hopplants primarily through wounds createdby mechanical damage (e.g., wind, tractors)at or below the soil line. Insect feeding alsomay create wounds that allow the pathogento gain entry into the hosts.

Management Growers should remove diseased

tissue from affected hills, if practical, andavoid propagation from diseased hills.Hilling up soil around the base of binespromotes growth of healthy roots andcan reduce the incidence of bine wilting.Reducing free moisture near the crown

due to irrigation can help. Application oflime to increase pH near the crown andavoiding use of acidifying ammoniumnitrogen fertilizers can help to reducedisease incidence. Minimizing injury tobines during field operations, arching (i.e.,tying bines and strings together to facilitateequipment passage), and preventing damageto bines from arthropod pests can all helpto reduce wounds that allow the fungus togain entry into the plant. No fungicides are

registered for control of Fusarium canker.


21/90

At-A-Glance:

FusariumCone TipBlight

Time irrigations◆to reduce periodsof wetness oncones.

Fungicide◆applications donot appear to beeffective.

This sporadic◆

disease does notwarrant speciccontrol measuresin most yards.

16 Fusarium Cone Tip BlightDavid H. Gent

Cone tip blight generally is adisease of minor importance in the PacificNorthwestern United States, although insome instances up to 30% of cones can beaffected. Te disease has been attributedto several Fusarium species, includingFusarium crookwellense , F. sambucinum, and

F. avenaceum.

Symptoms Affected bracts and bracteoles at the

tip of the cone become a medium to darkbrown as the cone matures (Fig. 19). ebrowning may be limited to a small portionof the tip of the cone or in severe casesencompass as much as 60% of the cone. A characteristic symptom of the disease isthat all bracts and bracteoles in the whorl of

the cone tip tend to be affected. Browningand death of the tip of the strig (central axisthat bears the nodes) generally are apparent when the affected bracts and bracteoles areremoved (Fig. 20).

Disease CycleLittle is known about the disease

cycle. Te pathogens may survive in soil,plant debris, and/or in association withhop crowns. Te cone tip blight pathogens,as well as other Fusarium species, may be

recovered from apparently healthy burrs,bracts, strigs, and stigma. Anecdotal reportssuggest that the disease is favored by highhumidity during cone development.

Figure 20. Discoloration of strigs due to cone tip blight. (Courtesy J. C. Bienap. Reproduced

with permission from Compendium of Hop Diseases and Pests, 2009, W. Mahaffee,

S. Pethybridge, and D. H. Gent, eds., American Phytopathological Society, St. Paul, MN.)

Figure 19. Medium brown

discoloration of bracts and

bracteoles on a cone with cone

tip blight. (S. J. Pethybridge)

Management Control measures have not been

developed for cone tip blight, but the diseaseoccurs sporadically enough that specificcontrol measures are not needed in mostyards. Limited evaluations of fungicidesindicate Fusarium spp. are recovered at a

lower rate from burrs and cones treated withstrobilurin fungicides, but these treatmentshave not been successful for management ofcone tip blight.


22/90

1Gray MoldDavid H. Gent

Gray mold generally is a disease ofminor importance in hops of the PacificNorthwestern United States. Te disease isfavored by prolonged wet, humid conditions,and can result in cone discoloration andpoor cone quality. Te disease is caused bythe fungus Botrytis cinerea , a widespread and

common pathogen found on numerous cropsincluding bean, raspberry, strawberry, andtree fruit.

Symptoms Affected cones have light to dark

brown spots on the tips of bracts andbracteoles, which can enlarge with timeand cause discoloration of entire cones.Bracteoles are more susceptible to damagethan bracts, and diseased cones can develop

a striped appearance. Gray mold symptomsare similar to Alternaria cone disorder butcan be distinguished by the presence ofgray, fuzzy fungal growth that begins at thetip of the cone (Figs. 21 and 22). Signs ofthe pathogen may not be present in dry weather.

Figures 21 and 22. Medium brown discoloration and fungal growth

on the tip of a cone due to gray mold. (S. Radisek)

At-A-Glance

Gray Mold

Minimal◆damage to PNW

hops.

Control◆measuresgenerally notneeded.

Manage◆irrigation andpromote airmovement to

reduce wetnesson cones.

Manage◆arthropod pestsat economicthresholds toprevent injury tocones.

Fungicide◆

applications canreduce gray molddamage to hopcones during wetweather.

Disease CycleTe gray mold fungus may survive as

a decay organism on organic materials, inand on leaves, and in the soil as dormantresting structures known as sclerotia.Te pathogen is active over a range oftemperatures when free moisture isavailable, with an approximate temperatureof 68 °F being optimal. Te fungus can

remain dormant in or on plant tissuesduring unfavorable conditions and becomeactive when weather or host factors arefavorable. Infection on cones is favoredby wet weather and injury caused by fieldoperations, insect feeding, or other diseases.

ManagementFungicide applications can reduce

gray mold damage to hops. (See the PacificNorthwest Plant Disease ManagementHandbook at http://plant-disease.ippc.orst.edu/ for a current list of registeredherbicides.) However, in most years thedisease causes minimal damage to hops inthe Pacific Northwest and special controlmeasures have not been necessary. Culturalpractices such as increasing row and plantspacing and management of overheadirrigation to reduce the duration of wetnesson cones help to reduce the incidenceof gray mold. Damage to cones frominsect feeding can exacerbate gray mold,

and efforts should be made to managearthropods at economic thresholds.


23/90

At-A-Glance

PowderyMildew

Select early-◆maturing orresistant varieties

when possible.

Apply◆adequate butnot excessiveirrigation andfertilizer.

Remove all◆green tissuesduring spring

pruning.

Apply◆appropriatefungicides assoon as possibleto protectregrowth afterpruning andthroughoutseason.

Eliminate◆basal growthwith chemicaldesiccants toremove diseasedtissue.

Apply highly◆effectivefungicides to

protect burrs andyoung cones.

◆ Harvest timelyto minimize croplosses in the eldwhen powderymildew occurs oncones.

18 Powdery MildewDavid H. Gent and Mark E. Nelson

Figure 23. Powdery, white colonies on a leaf severely affected by powdery mildew. (D. H. Gent)

Powdery mildew is caused by thefungus Podosphaera macularis , and is one ofthe most important diseases of hop in thePacific Northwest. Te disease can causesevere crop damage, in some cases resultingin complete loss of marketable yield due tolost production and reduced cone quality.

SymptomsDisease symptoms appear as powdery

white colonies on leaves, buds, stems, andcones (Fig. 23-25). During periods of rapidplant growth, raised blisters often are visiblebefore sporulation can be observed. Infectionof burrs and young cones causes abortion orsevere distortion of the cone as it develops. Affected cones may develop a characteristic white powdery fungal growth, although

in some cases fungal growth is visible onlyunder bracts and bracteoles and only withmagnification. Affected cones become reddish-brown as tissues are killed (Fig. 25), or mayturn a medium brown after kiln drying.

Disease CycleIn the Pacific Northwest, the

pathogen is known to overwinter onlyin infected buds. Where sexual matingoccurs there is potential for overwintering

structures (called chasmothecia orcleistothecia) to form and survive in and oncrop debris and soil. However, the sexualstage of the fungus has not been confirmedin the Pacific Northwestern United States.Shoots that emerge from an infected budoften are rapidly covered with fungalgrowth, and are termed “flag shoots” (Fig.26). Flag shoots occur on a small percentageof hills, on average approximately 0.7% in Washington and 0.02% of hills in Oregon,and provide the initial spores to beginoutbreaks each spring. Flag shoots oftenare not detected until they become heavilycovered with powdery mildew, althoughinfected shoots can be found at a low levelas soon as shoots emerge in spring.

As the plant develops, the pathogen

spreads and infects young leaves, moving upthe bine in sync with plant growth. Leavesbecome increasingly resistant to infection asthey age, especially when they are producedduring hot weather (> 85 °F). Diseasedevelopment is favored by rapid plantgrowth, mild temperatures (47 to 82 °F),high humidity, and cloudy weather. Underideal conditions at 65 °F, the fungus cancomplete its life cycle in as little at 5 days.Burrs and young cones are very susceptible


24/90

11

conidiophore

conidium

conidia reinfect leaves,

cones, buds and shootscrown

bud

ag shoot

ascospore

discharge in

spring, after

a wetting

event, infects leaves

close to the ground chasmotheciamycelioid appendages

overwintering chasmothecia chasmothecia on

cone and leaf litter

cross section of bud

with internal chasmothecia

overwintering mycelia

in and on crown buds

PHOTOS THIS PAGE,

CLOCKWISE FROM

FAR LEFT

Figure 24.

Leaves and stems extensive

colonized by the powdery

mildew fungus surrounding a

originating from a ag shoot.(D. H. Gent)

Figure 25. Cone with severe

browning caused by late-

season infection by the

powdery mildew fungus. Note

white fungal growth (myceliu

on bracts. (D. H. Gent)

Figure 26.

A young shoot with severe

powdery mildew (“ag shoot”

resulting from bud infection a

overwintering. (D. H. Gent)

Figure 27.

Life cycle of Podosphaera

macularis on hop. The sexua

stage of P. macularis (shown

by arrows on the bottom and

left side of the gure) is not

known to occur in the

Pacic Northwestern U.S.

(Prepared by V. Brewster)

to infection, and their development isarrested by infection, resulting in reducedcrop yield and quality. Infections occurringlater in the season are thought to leadto browning and an apparent prematureripening. Extremely cold weather during theoverwintering period is thought to reduce,but not eliminate, survival of the fungus ininfected buds (Fig. 27).


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ManagementControl of powdery mildew requires

integration of crop sanitation practices,adequate but not excessive fertilization andirrigation, and timely fungicide applicationsto keep disease pressure as low as possibleduring the season and up to harvest. Al-though growers often have little ability toselect resistant varieties because of market

factors, many resistant varieties are available(Table 2, page 12). Newport, Nugget, andseveral proprietary varieties are resistant topowdery mildew, while Cascade and Libertyhave useful levels of tolerance. Selection ofearly-maturing varieties (e.g., Fuggle) canhelp to escape late-season powdery mildew.

Management of powdery mildewshould begin in early spring by thoroughlyremoving all green tissues during springpruning, including shoots on the sides ofhills (Fig. 13, page 13, and Fig. 28). etiming of the first fungicide applicationafter spring pruning is critical in affectingthe severity of disease later in the season.Tis application should be made as soon aspossible after shoot growth resumes.

Regular fungicide applications areessential for economic production of mostsusceptible varieties. However, fungicideapplications alone are not sufficient tomanage the disease economically. Underhigh disease pressure, removal of basal

growth with chemical desiccants is essentialto remove diseased tissue and delay diseasedevelopment. Desiccants should be applied

20

Figure 28.

Association of spring pruning

quality to the incidence of

cones with powdery mildew

in 50 commercial hop yards

in Oregon and Washington

during 2000, 2005, and2006. Excellent = No foliage

or green stems remaining

after pruning, Moderate =

Foliage or green stems on

some hills after pruning,

and Poor = No pruning

conducted or foliage and

green stems present on all

hills after pruning.

once bines have grown far enough up thestring so that the growing tip will not bedamaged. Achieving adequate cover of densebasal growth during fungicide applicationsis difficult, and removal of basal foliage iscritical for reducing later infection of leavesand cones. Results of a field trial usingdesiccants alone are shown in Figure 29.

Several factors influence the

development and severity of powderymildew on cones, including disease severityon leaves, temperature and rain duringcone development, late-season fungicideapplications, and harvest date. Highlyeffective fungicides, such as Quintec,applied to young, developing cones cansignificantly reduce incidence of powderymildew on cones at harvest (Fig. 30). eefficacy of any fungicide, however, can varygreatly depending upon the severity ofthe disease pressure present (Fig. 31). Te

incidence of cone infection is also correlated with timing of the last fungicide application,and applications should continue untilthe pre-harvest interval as specified by thelabel. Te powdery mildew pathogen hasan extremely high ability to reproduce,therefore careful attention to fungicideresistance management guidelines is criticalto delay the development of resistance.

When powdery mildew is presenton cones near harvest, timely picking will

minimize crop losses in the field. Earlyharvest also can help to reduce damage tocones, although yield can be reduced.

Excellent Moderate Poor

Pruning Quality

P E R C E N T D I S E A S

E D C O N E S

20

15

10

5

0

See the Pacic

NorthwestPlant DiseaseManagementHandbook athttp://plant-

disease.ippc.orst.edu/ for a currentlist of registered

herbicides.


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12

Moderate Disease Pressure

Figure 29.

Incidence of hop leaves with

powdery mildew in relation

to herbicide treatments to

remove basal leaf growth

in Washington. Applications

of Aim EW were applied 6July, 3 Aug, and 20 August.

Applications of Gramoxone

Max + Desicate II were

applied 6 July and 3 August.

No fungicides were applied

in this trial.

Figure 30.

Effect of Quintec timing

on incidence of cones

with powdery mildew in

Washington in 2008 under

extremely high disease

pressure. NT = Non-treated.

Other = Another fungicide

applied during 18 July to 30

August.

Figure 31.

Efcacy of powdery

mildew fungicides under

moderate and high disease

pressure in Washington.

Notice that most fungicides

provide acceptable control

when disease pressure is

moderate.

P E R C E N T D I S E A S E D C O N E S

Quintec 18 July & 1 Aug Quintec 15 Aug & 30 Aug Other NT

100

80

60

40

20

0

P E R C E N T D I S

E A S E D L E A V E S

100

80

60

40

20

0

Non-treated

Gramoxone

Aim

19 Jun 25 Jun 3 Jul 10 Jul 17 Jul 24 Jul 30 Jul 13 Aug 22 Aug 28 Aug

P E R C E N T D I S E A S E D C O N E S

100

80

60

40

20

0

High Disease Pressure

Q u i n t e c

F l i n

t / A c c

r u e

F l i n

t / F o l i c

u r

A c c r u e

/ F o l i c

u r

R a l l y

N o n - t r e

a t e d

Q u i n t e c

F l i n

t / A c c

r u e

F l i n

t / F o l i c

u r

A c c r u e

/ F o l i c

u r

R a l l y

N o n - t r e

a t e d


27/90


28/90

2

At-A-Glance

SclerotiniaWilt or WhiteMold

Control◆measures usually

are not neededin the PacicNorthwest.

Utilize less◆susceptiblevarieties wherepossible.

Limit excessive◆basal growth and

time irrigations toreduce wetness onplants and soil.

Commercial◆formulations of abiological controlagent are available.

ABOVE: Figure 35. White fungal

mycelia and sclerotia (small black

survival structures) on hop bines

affected by Sclerotinia wilt. (T. J. Sm

AT LEFT: Figure 36. Sclerotium of

Sclerotinia sclerotiorum that hasgerminated to produce an apotheciu

Numerous apothecia can be produc

from a single sclerotium. (D. H. Gen

Sclerotinia Wilt (White Mold)David H. Gent

Sclerotinia wilt, also referred to as white mold, aects nearly 400 weed andcrop plant species, including importantcrops in the Pacific Northwest such asnumerous legumes (e.g., green bean andpea), canola, carrot, lettuce, potato, andsquash. Te disease is caused by a fungus,Sclerotinia sclerotiorum, and is an occasionalproblem on hop in wet, cool climates suchas those found in the hop productionregions in New Zealand or western Oregon.However, the disease occurs rarely on hop inthe Pacific Northwest. Sclerotinia wilt cancause damage when soil and plants remaincontinuously wet and temperatures are mild.

SymptomsDisease symptoms generally appear in

late spring or early summer as soft, water-soaked lesions on bines just below or nearthe soil surface at the crown. Te infectedtissue collapses, creating a light brown tograyish lesion approximately 1 to 4 incheslong. During wet weather, fluffy whitegrowth of the fungus may form on theinfected tissue (Fig. 35). Small, hardenedblack overwintering structures (sclerotia)form on and in diseased bines. As thedisease progresses, the lesions expand and

may girdle the bine, causing a wilt. Leavesgenerally remain green until the bine isgirdled completely. Disease symptoms mayappear similar to those caused by Fusariumcanker or Verticillium wilt. However, thepresence of fluffy white mycelia and sclerotiaare diagnostic for Sclerotinia wilt.

Disease CycleTe pathogen overwinters as long-

lived resting structures (sclerotia) in infestedcrop debris and in the soil. Sclerotia

can germinate directly and infect roots,or, if conditioned by exposure to moistconditions and cool temperatures, cangerminate to produce one or numeroussmall mushroom-like structures calledapothecia (Fig. 36). Te soil surface mustremain wet for several days or longer forapothecia to form, and with hops thisgenerally occurs when plants produceabundant, lush foliage that shades the soilnear the crown. A sclerotium may produce

one or numerous apothecia, and each

apothecium may produce produce severalmillion airborne spores called ascospores. Ascospores require a nutrient source upon which to grow before invading a host,and often this nutrient source is senescentleaves or other plant tissues near the crown.Severe epidemics of Sclerotinia wilt on hopreportedly are associated with hilling soilinfested with sclerotia onto crowns and withfrost injury of developing basal buds. Newsclerotia are formed in and on infected binesand are returned to the soil, where they maysurvive five years or longer and perpetuatethe disease cycle. Te pathogen also maysurvive on numerous broadleaf weeds in andaround hop yards.

Management

Control measures for Sclerotinia wiltof hop usually are not needed in the PacificNorthwest. Avoiding varieties reported to beespecially susceptible (e.g., Fuggle, Bramling)might be useful in wet, mild areas. Culturalpractices that reduce the duration of wetnesson plants and the soil surface can reducedisease incidence. Tese cultural practicesmay include limiting nitrogen fertilization,removing excess basal shootsand leaves, stripping leavesfrom lower bines, and timingirrigations to allow the toptwo inches of the soil to drycompletely between irrigations.Formulations of the parasiticfungus Coniothyrium minitans (marketed under the trade nameContans WG) are available forbiological control of Sclerotiniasclerotiorum. Te efficacy of thisproduct for Sclerotinia wilt inhop has not been investigated.


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Sooty MoldDavid H. Gent

At-A-Glance

Sooty Mold

Sooty mold◆is controlled bycontrolling hopaphids.

Natural◆enemies ofhop aphidcan providesignicant levelsof control when

not disrupted bybroad-spectruminsecticides.

Figure 38. Black sooty mold on a cone. Noticethe white aphid castings present under the

bracts and bracteoles. (D. H. Gent)

Figure 39: Hop aphids are a major contributing

factor in sooty mold. This is the winged form

of the hop aphid. For aphid photos and control

information, see the arthropod pest control

section of this handbook. (L. C. Wright)

Figure 37. Black sooty mold on hop leaves.

(D. H. Gent)

24

black and sticky, but sooty mold tends to bemost prevalent on the undersides of bractsand bracteoles and on leaves shaded fromthe sun (Fig. 38).

ManagementSooty mold is managed by controlling

hop aphids (Fig. 39) when populations ex -

ceed economic thresholds. Natural enemiesof hop aphid can provide significant levels ofcontrol when not disrupted by insecticides,therefore when possible broad-spectrum in-secticides should be avoided.

Sooty mold is not a disease, but rathera complex of common fungi that growsuperficially on insect excretions depositedon leaves and cones. Te appearance ofsooty mold is due to the presence anddevelopment of phloem-feeding insects,most importantly the hop aphid. Hopaphids probe the phloem strands of hopplants, ingesting more plant fluids thancan be processed by their digestive systems. Aphids expel the excess plant fluids as adilute solution known as “honeydew,”comprised of sugars, amino acids, and othersubstances, which provides a food sourcethat supports the growth of dark-pigmentedfungi that grow on the surface of leaves andcones, reducing the quality of cones.

SymptomsOnce aphids colonize and commence

feeding, plant tissues become covered with sticky honeydew and develop a shinyappearance before sooty mold becomesevident. Signs and symptoms of sootymold soon develop on this honeydew as aflattened, black mass of fungal growth thatresembles a fine layer of soot (Fig. 37). Burrsand developing cones later may becomecovered with honeydew, quickly becomingblack and sooty in appearance. Entire bracts,bracteoles, and lupulin glands may become


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Verticillium WiltDavid H. Gent and Mark E. Nelson

symptoms often appear initially on lowerleaves as yellowing and death of tissuebetween major veins and upward curlingof leaves (Fig. 40). Aected bines becomenoticeably swollen (Fig. 41) and when thesestems are cut open the vascular tissue isdiscolored a medium to dark brown (Fig.42). ese symptoms generally are rstrecognized near flowering or when plants

become 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 variety Willamette, 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 plants

in Washington than Oregon.

At-A-Glance

VerticilliumWilt

Plant resistant◆

varieties whenpossible.

Clean equipment◆between yards tominimize spreadingthe pathogen.

Plant only◆disease-freerhizomes or

cuttings.Do not return◆

trash or compostfrom yards with

Verticillium wilt tohop yards.

Control weeds◆with herbicidesand reducecultivation wherepossible.

Reduce nitrogen◆fertilization asmuch as possible.

Figure 41. Swollen bine with wilted leaves

resulting from infection by a non-lethal

strain of Verticillium albo-atrum, one of theVerticillium wilt pathogens. (D. H. Gent)

Figure 40. Upward curling and wilting

of leaves associated with Verticillium

wilt caused by a non-lethal strain of

Verticillium albo-atrum. (D. H. Gent)

2

Verticillium wilt is a potentiallydamaging disease of hop and numerousother crops including alfalfa, cherry, maple,mint, potato, as well several herbaceousplants, woody ornamentals, and common weeds. On hop, Verticillium wilt may becaused by two related fungi, Verticilliumalbo-atrum and V . dahliae . Te host rangeand severity of disease caused by thesepathogens varies. Several strains of V . albo-atrum have been described. Some may causerelatively minor wilting symptoms (non-lethal or fluctuating strains) while others cancause severe symptoms (lethal or progressivestrains) that rapidly can kill susceptiblevarieties. Non-lethal strains of V . albo-atrum are common in the Pacific Northwest andhave been reported on hop. Lethal strains

of Verticillium albo-atrum have not beenreported from the United States. Verticilliumdahliae causes a relatively minor wilt diseaseon hop. Tis pathogen has a broader hostrange than V. albo-atrum, and occurscommonly on hop in the United States.

SymptomsDisease symptoms vary depending

on the aggressiveness of the Verticillium pathogen that is attacking the plant. Withnon-lethal strains of V. albo-atrum, disease


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Disease CycleTe Verticillium wilt pathogens

survive in soil, invade hop roots, and latergrow into water-conducting tissues. Fungalgrowth and plant toxins produced by thepathogen disrupt the movement of waterand nutrients, leading to the wilt symptoms.Te fungus also spreads systemically in theplant and invades leaves.

Te pathogens are spread in hopyards during soil cultivation, in hop trash,and in planting materials from infestedyards. Several common weeds of

Hop Handbook 2009

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