MSU Extension

Wet and humid conditions fall 2014 and spring 2015 have provided ideal conditions for downy mildew to become a problem in Michigan hopyards. All hopyards, even new yards not exhibiting signs or symptoms of downy mildew, should be utilizing a season-long – from 6 inch bines through harvest – protectant fungicide program to minimize downy mildew infection. Michigan receives a substantial amount of rainfall and days with high humidity which increases potential for downy mildew infections and makes complete control likely impossible. Even when growers follow best management practices, downy mildew can be a problem.

The severity of downy mildew is increased by high disease pressure from previous outbreaks, conducive environmental conditions, poor fungicide timing, suboptimal spray coverage, fungicide wash-off, cultivar susceptibility or a combination of factors. In addition, fungicide resistance may play a role in some cases. Growers who were able to apply fungicide treatments on a protectant basis ahead of recent rain events are seeing lower infection levels, but still experiencing significant pressure for this time of the season. For more information about downy mildew biology, refer to “Managing hop downy mildew early in the season is critical” from Michigan State University Extension.

Downy mildew management should begin before the hopyard is planted through smart variety selection. Varieties vary widely in their susceptibility to downy mildew and Michigan growers are encouraged to select more tolerant varieties (refer to Table 2 in “Field Guide for Integrated Pest Management in Hops”). Clean planting materials should be used when establishing new hopyards since this disease is readily spread via nursery stock. Growers should consider purchasing a few plants from prospective nurseries and have them tested for downy mildew before committing to a large numbers of plants, particularly if the grower hasn’t worked with the nursery before.

If downy mildew infections are already present in the yard this season, affected shoots and spikes should be removed by hand and healthy shoots retrained in their place. Weeds and lower leaves should also be controlled or removed to promote air movement around the crown and reduce the duration of wetting periods. Remove infected material from the yard and burn or bury it to reduce the spread of downy mildew spores. Growers who do not remove severely infected bines could likely see bines fail to climb the coir and cone infection later in the season. If there is a cover crop, it should be mowed close to the ground. If yards have no cover crop, cultivation can help dry soil and minimize humidity. Keep nitrogen applications moderate, only providing the minimum for plant health.

To control downy mildew, growers must begin applying fungicide treatments on a protectant basis as soon as bines emerge in spring regardless of the presence or absence of visible symptoms of downy mildew. Dormant applications are not recommended. Applications should continue season-long on a seven- to 10-day reapplication interval until harvest. The time between applications may stretch longer when weather is dry and if hopyards don’t have active infections. Several periods in the season are particularly critical for disease control:

• Immediately before and after training.
• When lateral branches begin to develop.
• Bloom.
• Cone development.

Protecting young, developing bracts with a fungicide application before cones close up is critical to preventing downy mildew when conditions for disease are favorable. Getting adequate coverage on undersides of bracts where infection occurs becomes increasingly difficult as cones mature.

Efficacy research performed in the Pacific Northwest and reported by USDA recommends the following fungicides against downy mildew:

• Zampro (ametoctradin+dimethomorph)
• Forum (dimethomorph)
• Ranman (cyazofamid)
• Revus (mandipropamid)
• Ridomil Gold SL (mefenoxam)
• Copper-based products
• Curzate 60DF (cymoxanil)
• Tanos (cymoxanil+famoxadone)
• Phosphonate fungicides (Agri-Fos, Aliette, Fosphite, Fungi-phite)

Strobilurin compounds such as Flint (trifloxystrobin) and Pristine (strobilurin+boscalid) will only provide suppression of downy mildew when used for powdery mildew control.

Growers with hopyards already infected with downy mildew should consider the post-infection characteristics of some of the available fungicides. Research from the Pacific Northwest indicate that cymoxanil (e.g., Curzate) has about two days post-infection activity, but only provides three days of forward protection. This means cymoxanil would be a good choice if growers failed to get a protectant on ahead of a rain event, but would require the grower to tank-mix it with a protectant fungicide with a longer residual for protection moving into the period following application.

The product Tanos is a combination of cymoxanil – the active ingredient in Curzate – and a protectant mode of action called famoxadone and would be a nice choice for growers looking for some curative action as well as five to seven days protection. Curzate and Tanos have a seven-day pre-harvest interval. Dimethomorph (e.g., Forum) and mandipropamid (e.g., Revus) have the same mode of action and offer seven days of protectant activity and one to two days of post-infection activity on actively growing shoots. Forum and Revus have a seven-day pre-harvest interval.

Phosphorous acid fungicides (e.g., Phostrol) have been shown to provide about four to five days protection and post-infection activity of up to five to seven days in field trials in the Pacific Northwest and have a very short pre-harvest interval.

Organic growers have fewer options and will need to focus on keeping tissue protected, selecting downy mildew-tolerant varieties and following cultural practices to limit downy mildew infection. Copper-based products are the mainstay of downy mildew management in organic hopyards and offer five to seven days of protection, but no post-infection activity. The pre-harvest intervals for copper formulations vary and applications are limited; refer to the label.

Actinovate, Eco-mate, Armicarb-O and Sonata are additional products that list downy mildew on the label and are approved for organic use in hops. The pre-harvest interval for these products are one-day or less. At this time, we have no data on the efficacy of these products.

Additional considerations

• Growers should rotate through multiple fungicides in a season to delay resistance development in the downy mildew pathogen.
• Ensure thorough coverage of plant material, particularly for contact fungicides. This means increase spray volume, reduce tractor speed, spray every row and adjust nozzles accordingly.
• Apply fungicides at the highest labeled rate to ensure good post-infection activity.
• Ensure forward protection of healthy plant parts by tank-mixing or following up with materials that have good protective activity.
• Always read the label for the pre-harvest interval, incompatibility with other products and other restrictions.
• Scout to assess if your treatment was effective, keeping in mind that newly developing infections may continue to manifest themselves for a week or more after the spray.