Field Guide for Integrated Pest Management in Hops

14DOWNY MILDEW INCIDENCEFigure 14. Association of spring pruning quality to the incidence ofplants with downy mildew in 97 commercial hop yards in Oregonduring 2005 to 2008. Excellent = No foliage or green stemsremaining after pruning, Moderate = Foliage or green stems onsome hills after pruning, and Poor = No pruning was conducted orfoliage and green stems were present on all hills after pruning.DOWNY MILDEW INCIDENCEDOWNY MILDEW INCIDENCE0.40.30.20.10Figure 15. Association of spring pruning timing to theincidence of plants with downy mildew in 6 commercial yardsof Willamette in Oregon. Hop yards that received the delayedpruning treatment were chemically pruned 10 to 14 dayslater than the growers’ standard pruning timing.10.80.60.40.2010.80.60.40.20NTPruning QualityExcellent Moderate PoorDelayed PruningStandard Pruning2007 2008Moderate DiseasePressureHigh DiseasePressureAliette Flint NT Aliette FlintFigure 16. Efficacy of Aliette WDG and Flint under moderate andhigh disease pressure in Washington. NT = Non-treated.Non-infected rhizomes or softwood cuttings shouldbe selected when establishing new hop yards since plantingmaterial may harbor the pathogen. Thoroughly 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 situationswhere 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.Timely fungicide applications often are needed tomanage 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.The 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) ofthese 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. The 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 showngraphically in Figure 16.