Posted on by Professor Good Ales

The Technical Edge: Hop Solutions

Hop (Humulus lupulus): Downy Mildew

Note the shoots with shortened internodes, characteristic of hop downy mildew


From OSU University’s Website: content edited by: C. M. Ocamb and D.H. Gent

Aerial spike present on hop bine
Cause: The fungus-like microorganism Pseudoperonospora humuli persists from year to year in infected hop crowns or plant debris in soil. It is an obligate parasite specific to hop. Disease is promoted by wet or foggy weather.

Symptoms: In early spring, spike-like infected bines rise among normal shoots from the crown. Spikes are silvery or pale green, rigid, stunted, and brittle. The undersides of leaves may be covered by profuse sporulation by the pathogen and appear dark purple to black. Tips of normal branches may become infected and transformed into spikes. Leaves of all ages are attacked, resulting in brown angular spots. Flower clusters become infected, shrivel, turn brown, dry up, and may fall. Cones also are affected, becoming brown. Severe infection in some susceptible cultivars may produce a rot of the perennial crowns.

Hop cone with downy mildew infection

Cultivar Resistance: ‘Fuggle’ and ‘Tettnang’ are resistant. ‘Willamette’, ‘Mt. Hood’, ‘Chinook’, ‘Liberty’, ‘Cascade’, ‘Bullion’, and ‘Brewers Gold’ are tolerant. ‘Clusters’, ‘Galena’, and ‘Nugget’ are susceptible.
Cultural control:

  1. Destroy escaped hop vines near or in hop yards.
  2. Prune crown before growth starts in the spring or burn back green tissue before training. Complete removal of green tissue or pruning of entire hill is necessary for most effective disease management.
  3. Remove diseased hills and mark for replanting.
  4. Train bines early to prevent them from coming in contact with soil.
  5. Begin suckering as soon as vines are strung. Continue at regular intervals until warm, dry weather prevails (June to July).
  6. Strip leaves from bines at a height of 4’ soon after training to reduce the spread of downy mildew up the canopy.
  7. Avoid overhead irrigation, especially during and after burr development.
Chemical control note: using metalaxyl products is strongly discouraged in the Willamette Valley of Oregon due to resistant pathogen strains. Metalaxyl-resistance is also found in the Yakima Valley of Washington. Insensitivity to Aliette (fosetyl-Al) is widespread in strains of P. humuli found in Oregon and northern Idaho. The highest labeled rates of Aliette should be used where insensitive strains are known to occur. Drench crowns after pruning but before shoots are 6 inches long and/or before training.

  1. Copper products:
    1. Champ Formula 2 at 1.33 pt/A. Do not apply within 14 days of harvest. 24-hr reentry.
    2. Kocide DF at 2 lb/A. Do not apply within 14 days of harvest. 24-hr reentry.
    3. Nordox 75 WG at 1.25 lb/A on 10-day intervals as needed. Apply up to 14 days before harvest. 12-hr reentry.
  2. Metalaxyl products such as Ridomil Gold SL at 0.5 pint/A or MetaStar 2E AG at 1 quart/A in at least 20 gal water as soil drench over crowns when plants are 6 inches or less, tank mix with a copper fungicide reigstered for hop. Do not apply within 45 days of harvest. 48-hr reentry.

Apply during the season.

  1. Aliette WDG at 2.5 lb/A or in Oregon only at 2.5 to 5.0 lb/A (SLN OR-040018). Do not tank-mix with copper products. Do not apply more than 10 lb/A/season (20 lb/A/season in Oregon only (SLN OR-040018)) or within 24 days of harvest. 12-hr reentry.
  2. Copper products:
    1. Champ Formula 2 at 1.33 pt/A. Do not apply within 14 days of harvest. 24-hr reentry.
    2. C-O-C-S WDG at 4 to 6 lb/A. Label states only one spray is allowed. Not widely used due to low efficacy. 24-hr reentry.
    3. Cuprofix Ultra 40D at 1 to 1.5 lb/A on 7- to 10-day intervals. Discontinue use 2 weeks before harvest. 12-hr reentry.
    4. Kocide 2000 at 1.5 lb/A or Kocide 3000 at 0.75 to 1.5 lb/A on 10-day intervals. 24-hr reentry.
    5. Nordox 75 WG at 1.25 lb/A on 10-day intervals as needed. Apply up to 14 days before harvest. 12-hr reentry.
    6. Nu Cop 50 WP at 2 lb/A on 10-day intervals. Do not apply within 2 weeks of harvest. 24-hr reentry. O
  3. Curzate 60DF at 3.2 oz/A on 10- to 14-day intervals. Use only in combination with another protective fungicide. Apply no more than 4 applications per year. Do not apply within 7 days of harvest. 12-hr reentry.
  4. Dimethomorph products (Group 15 fungicides) such as Acrobat 50WP at 6.4 oz/A or Forum at 6 fl oz/A on 10-day intervals in combination with another fungicide that has a different mode of action. Do not apply more than once before alternating with another fungicide with a different mode of action. Do not apply more than three (3) times per season. Do not apply within 7 days of harvest. 12-hr reentry.
  5. Flint at 1 to 4 oz/A in 15 to 200 gal water/A on 10- to 14-day intervals when used for powdery mildew control will provide suppression of downy mildew. See label for specific rates and concentrations. Do not apply more than one (1) foliar application of Flint or other Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Do not apply more than four (4) times per season or within 14 days of harvest. Note ‘Concord’ grapes are sensitive to Flint and may be injured if accidentally sprayed. 12-hr reentry.
  6. Folpan 80 WDG at 3 lb/A on 28-day intervals. Do not apply more than four (4) times per season or within 14 days of harvest. Folpan 80 WDG when used for downy mildew control will provide some suppression of powdery mildew. 24-hr reentry.
  7. Legion 80 WDG at 2.5 lb/A. See label for timing. Do not apply within 24 days of harvest. 12-hr reentry.
  8. Phosphorous acid based products.
    1. Agri-Fos at 2.5 pints/A in 100 gal water/A. When conditions favor disease, applications should occur when shoots are 0.5 to 1 foot long, post training when bines are 5 to 6 feet high, 21 days after the second application, and during bloom. 4-hr reentry.
    2. Fosphite at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. Apply no more than six (6) applications per year. 4-hr reentry.
    3. Topaz at 1 to 3 quarts/100 gal water/A on 14- to 21-day intervals. Apply no more than six (6) applications per year. 4-hr reentry.
  9. Pristine when used for powdery mildew control will provide suppression of downy mildew. See supplemental label for specific rates and concentrations. 12-hr reentry.
  10. Tanos at 8 oz/A. Do not make more than one (1) application of Tanos before alternating with a fungicide that has a different mode of action. 12-hr reentry.
Biological control: Efficacy unkown in the Pacific Northwest

  1. Sonata at 2 to 4 quarts/100 gal water or per acre, depending on growth stage, on 7- to 14-day intervals. Can be applied up to and on the day of harvest. 4-hr reentry.

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