EM 9256    Published October 2019

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.

Before planting

  • Select the least susceptible variety available

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

Resistant

Limited or no fungicides required

Moderately susceptible

Typically <4 fungicide applications/year

Susceptible

Typically 4–8 fungicide applications/year

Highly susceptible

>8 applications/year

Crystal Centennial Cascade Apollo
Comet Fuggle Cashmere Columbus/Tomahawk/Zeus
Lotus Mt. Hood Nugget
Pahto Newport Simcoe

Susceptibility and fungicide inputs depend on prevailing weather conditions and which race of the fungus is present. For example, Apollo, Mt. Hood and Nugget are resistant to many races of the powdery mildew fungus and require fungicides only when virulent races of the pathogen are present. However, the powdery mildew fungus can develop new races, eventually overcoming resistance in the host. Susceptibility also varies with climate. Varieties in the moderately susceptible and susceptible categories often require more intensive management in cooler climates such as the Willamette Valley of Oregon, as opposed to warmer climates such as the Yakima Valley in Washington.

  • Ensure that planting material is free of powdery mildew

The powdery mildew fungus spreads easily on planting material. Genetic evidence indicates that the powdery mildew fungus spread across the U.S. in association with infected planting material. If plants are produced in a nursery or greenhouse, verify that the planting material is free of powdery mildew. Conduct a close visual inspection or a grow-out to confirm disease-free status (Figure 1).

Cultural measures after planting

Cultural practices after planting are the foundation of disease management. These practices help to delay disease development and create an environment less conducive to the disease, reducing the rate of disease spread.

  • Thoroughly prune yards in spring

In the western U.S., the powdery mildew fungus must survive the winter on living host tissue, which is provided by crown buds. In spring, thorough mechanical pruning is the most effective means of reducing overwintering inoculum of the pathogen (Table 2). Two pruning operations are often necessary to ensure all green tissue is removed thoroughly. For instance, it is more effective to follow a mechanical pruning with a later chemical desiccation of remaining shoots. Consider variety, plant age and weather when pruning to avoid injuring the plants.

Table 2. Association of pruning method and pruning thoroughness on occurrence of powdery mildew flag shoots in Oregon hop yards

Total yards Yards with flag shoots Yards without flag shoots Flag shoot prevalence (%)
Pruning method
Chemical/unpruned 360 22 338 6.1
Mechanical 129 1 128 0.8
Pruning thoroughness
Poor or moderate 283 23 250 8.1
Excellent 206 4 202 1.9
  • Prune as late as possible in spring

In addition to thorough pruning, pruning as late as possible in spring can reduce the severity of powdery mildew. How late pruning can occur depends on variety, plant vigor and other factors (Figure 2).

  • Reduce canopy density

Powdery mildew is favors dense canopies, shade, rapid growth rate of the plant, moderate temperatures (50–85°F) and high humidity. Reduce canopy density where possible by training few shoots per plant, ensuring adequate spacing of plants and strings, and avoiding excessive fertility.

  • Moderate nitrogen rates

Late-season powdery mildew on leaves and cones is associated with the amount of nitrogen applied (Figure 3). Reduce nitrogen rates as low as feasible for yield goals; avoid nitrogen applications after bloom. For well-managed crops, less than 200 pounds per acre of nitrogen generally is adequate for maximizing yield and cone quality.

  • Remove midseason basal foliage

When powdery mildew is present, removing basal foliage at least twice during the late spring and summer months reduces the severity of powdery mildew, especially on cones. Scout basal foliage regrowth late in the season for signs of disease.

  • Manage weeds, cover crops and irrigation to reduce humidity

Powdery mildew favors high humidity and moderate temperatures. Manage other vegetation in the yard to promote air flow and reduce humidity where possible. Excessive irrigation also favors powdery mildew. Matching irrigation to plant demand can help to reduce disease levels.

  • Promote root health

There is a general correlation between root health, late-season water stress and the impact of powdery mildew on cones. Areas of hop yards with rocky soil or other factors that limit root development tend to be most severely impacted when powdery mildew occurs on cones. That is because diseased cones mature more rapidly than healthy cones. Cultural practices that promote a larger root system can help to delay this accelerated maturation of cones. Irrigating for longer periods does not compensate for poor root health.

Chemical controls

In susceptible varieties, it is usually necessary to apply fungicides in regions where disease pressure is high. Find an updated list of registered fungicides at pnwhandbooks.org/plantdisease. An online risk index can help you determine disease hazard levels (see uspest.org/risk/models). Note that fungicides alone are not enough to manage the disease in the absence of cultural practices or other control measures.

  • Ensure timely fungicide application

The efficacy of fungicide programs depends on disease pressure. The timing of the first fungicide application is critical to keeping disease levels low. In high disease-risk situations, this corresponds to when disease is present on less than 1% of plants.

  • Incorporate products with contact activity in fungicide programs

Most synthetic fungicides have limited or no postinfection (kick-back) activity. Using fungicides with postinfection activity can enhance disease control. Examples of fungicides with postinfection activity include horticultural oils, bicarbonates and, to a lesser extent, sulfur.

  • Apply the right fungicide at the right time

Using the most effective fungicides during bloom and the early stages of cone elongation can significantly enhance control of powdery mildew on hop cones at harvest. Fungicides containing quinoxyfen (Quintec), fluopyram (Luna products), and flutriafol (TopGuard Specialty Crops) have been shown to be most effective during this period (Figure 4). You’ll need short intervals of the most effective fungicides for maximal disease control. Fungicides applied after mid-August can reduce disease levels on cones and improve cone color, but these improvements tend to be marginal. The timing of the last fungicide application depends on disease pressure, variety susceptibility and harvest date.

  • Consider other fungicide factors

Postharvest fungicide applications for powdery mildew generally are not recommended in established yards in the western U.S. Information is not available on the benefit of fungicides applied after harvest in first-year plantings or where the sexual stage of the fungus (forming chasmothecia) occurs.

  • Ensure thorough application coverage

It is exceedingly difficult to obtain adequate spray coverage on hop (Figure 5). Measure actual spray coverage using dyes and water-sensitive paper at various heights in the canopy and in the interior of plants. Where permitted, include a surfactant to improve coverage.Always apply fungicides at rates specified on the product label.

Other considerations

  • Scout, sample and monitor for disease

Disease outbreaks are more easily managed when discovered early. Monitor powdery mildew levels throughout the growing season to ensure timely and appropriate application of various disease management strategies. Disease scouting is critical for identifying problems with fungicide applications, such as spray coverage issues or disease control failures due to fungicide resistance. Sampling should ideally be conducted weekly. Carefully examine at least 100 to 200 plants in a hop yard. Spread out sampling over the entire yard, but specifically target known problem areas.

Table 3.Seasonal calendar of management considerations for powdery mildew

The timeframe shown here is approximate. The necessity of any given practice depends on variety, disease pressure and other factors.

Off-season March April May June July Aug Sept
Before planting
Variety selection x
Plant population/spacing x
Ensure planting material
is disease-free
x x x x
Cultural practices
Scouting/monitoring x x x x x x x x x x
Thorough spring pruning;
adjust timing
x x x x x
Training density x x x
Reduce nitrogen fertilization
as much as feasible
x x x x x x
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
Chemical controls
First fungicide application x x
Incorporate fungicides
with contact activity
x x x x x x x x
Incorporate copper
into downy mildew program
x x x x
General period for use
of sulfur fungicides
x x x x
Use of drip-applied fungicides x x x
Use of foliar-applied fungicides with excellent residual and
post-infection activity
x x
Use of most efficacious fungicides on cones x x
Apply fungicides up to
preharvest interval for maximum disease control
x x
Quantity coverage; adjust sprayer for maximum coverage x x x x x x x x x x
Other considerations
Communicate disease levels x x x x x x x x x x
Monitor dry matter to time harvest x x

  • Communicate disease risks to neighbors

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.

  • Harvest in a timely manner

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).

Resources

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.

© 2019 Oregon State University.