Powdery mildew is one of the most important diseases of hop. If not managed appropriately, powdery mildew can lead to complete loss of marketable crops. Disease management is difficult and often incomplete in susceptible varieties. This is because, in part, no single tactic is adequate. Successful management requires an integrated set of practices — from cultural, biological and chemical tactics to the planting of resistant varieties. These best management practices can help growers minimize damage from the disease.
Powdery mildew susceptibility varies widely among hop varieties, ranging from entirely resistant to extremely susceptible. Planting resistant or less susceptible varieties is the most efficient means of managing powdery mildew. When markets allow, choose varieties that are least susceptible to the disease (Table 1). In general, most hop varieties are more susceptible to powdery mildew when grown in western Oregon than when grown in south-central Washington.
Table 1.Powdery mildew susceptibility in a range of hop varieties
Limited or no fungicides required
Typically <4 fungicide applications/year
Typically 4–8 fungicide applications/year
|Total yards||Yards with flag shoots||Yards without flag shoots||Flag shoot prevalence (%)|
|Poor or moderate||283||23||250||8.1|
|Ensure planting material
|Thorough spring pruning;
|Reduce nitrogen fertilization
as much as feasible
|Remove basal foliage||x||x||x||x||x|
|Manage cover crops/weeds||x||x||x||x||x||x||x||x|
|Avoid excessive irrigation||x||x||x||x||x||x||x||x|
|First fungicide application||x||x|
with contact activity
into downy mildew program
|General period for use
of sulfur fungicides
|Use of drip-applied fungicides||x||x||x|
|Use of foliar-applied fungicides with excellent residual and
|Use of most efficacious fungicides on cones||x||x|
|Apply fungicides up to
preharvest interval for maximum disease control
|Quantity coverage; adjust sprayer for maximum coverage||x||x||x||x||x||x||x||x||x||x|
|Communicate disease levels||x||x||x||x||x||x||x||x||x||x|
|Monitor dry matter to time harvest||x||x|
The powdery mildew fungus can travel long distances — multiple miles — as airborne spores. Knowing where the disease is present in the region helps growers gauge the hazard of infection due to inoculum originating from other yards or farms. Disease spread is mostly localized to a few miles in spring. Later in the summer, it can disperse over longer distances.
Cone dehydration and maturation accelerate in the presence of powdery mildew. In varieties such as Galena and Zeus, monitor dry matter early and harvest before dry matter exceeds approximately 25.5% to maintain cone color and minimize crop losses during picking (Figure 6).
Gent, D.H., G.G. Grove, M.E. Nelson, S.N. Wolfenbarger and J.L. Woods. 2014. Crop damage caused by powdery mildew on hop and its relationship to late season management. Plant Pathology
Gent, D. H., C. Probst, M.E. Nelson, G.G. Grove, S.T. Massie and M.C. Twomey. 2016. Interaction of basal foliage removal and late season fungicide applications in management of hop powdery mildew. Plant Disease
Gent, D.H., W.F. Mahaffee, W.W. Turechek, C.M. Ocamb, M.C. Twomey, J.L. Woods and C. Probst. 2019. Risk factors for bud perennation of Podosphaera macularis on hop. Phytopathology
Nelson, M.E., D.H. Gent and G.G. Grove. 2015. Meta-analysis reveals a critical period for management of powdery mildew on hop cones. Plant Disease
Probst, C., M.E. Nelson, G.G. Grove, M.C. Twomey and D.H. Gent. 2016. Hop powdery mildew control through alteration of spring pruning practices. Plant Disease
Twomey, M.C., S.N. Wolfenbarger, J.L. Woods and D.H. Gent. 2015. Development of partial ontogenic resistance to powdery mildew in hop cones. PloS ONE
Wolfenbarger, S., S.T. Massie, E.B. Eck, G.G. Grove, M.E. Nelson, C. Probst, M.C. Twomey and D.H. Gent. 2016. Distribution and characterization of Podosphaera macularis virulent on hop cultivars possessing R6-based resistance to powdery mildew. Plant Disease 100(6):1212-1221.