Field Guide for Integrated Pest Management in Hops

20See the PacificNorthwestPlant DiseaseManagementHandbook athttp://plantdisease.ippc.orst.edu/ for a currentlist of registeredherbicides.ManagementControl of powdery mildew requiresintegration 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. Althoughgrowers often have little ability toselect resistant varieties because of marketfactors, 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). Thetiming of the first fungicide applicationafter spring pruning is critical in affectingthe severity of disease later in the season.This 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 basalgrowth with chemical desiccants is essentialto remove diseased tissue and delay diseasedevelopment. Desiccants should be appliedonce 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 thedevelopment 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). Theefficacy of any fungicide, however, can varygreatly depending upon the severity ofthe disease pressure present (Fig. 31). Theincidence of cone infection is also correlatedwith timing of the last fungicide application,and applications should continue untilthe pre-harvest interval as specified by thelabel. The 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 willminimize crop losses in the field. Earlyharvest also can help to reduce damage tocones, although yield can be reduced.20Pruning QualityFigure 28.Association of spring pruningquality to the incidence ofcones with powdery mildewin 50 commercial hop yardsin Oregon and Washingtonduring 2000, 2005, and2006. Excellent = No foliageor green stems remainingafter pruning, Moderate =Foliage or green stems onsome hills after pruning,and Poor = No pruningconducted or foliage andgreen stems present on allhills after pruning.PERCENT DISEASED CONES151050Excellent Moderate Poor