Battling downy mildew as hop harvest approaches

With cones developing in Michigan hopyards, many growers continue to battle downy mildew and should carefully consider their management programs as harvest approaches.

Downy mildew on centennial leaves, bracts and cones. Photo by Erin Lizotte, MSU Extension
Downy mildew on centennial leaves, bracts and cones. Photo by Erin Lizotte, MSU Extension

Unusually wet and humid weather during the 2014 season has provided plenty of opportunity for hop downy mildew to become a problem on bines, leaves and cones. The general approach to downy mildew management is to select less susceptible cultivars, apply preventative fungicide sprays, integrate good sanitation practices and properly time harvest. In addition, field scouting for disease plays a key role in an integrated disease management approach.

Downy mildew thrives under moist, rainy conditions, so growers should be applying fungicide treatments on a protectant basis from basal spike emergence in the spring through harvest. For most producers, this means reapplication of fungicides at least every 10-14 days during the season. The time between applications can stretch longer when the weather is dry and if hopyards don’t have active infections.

Protectant sprays should be reapplied before forecasted rain events as the label allows regardless of the presence or absence of active downy infection. Covering young, developing bracts before cones close up is critical to protecting against downy mildew when conditions for disease are favorable. Getting adequate coverage on undersides of bracts where infection occurs becomes increasingly difficult as cones mature. These proactive treatments are by far the most efficacious method of controlling downy mildew; post-infection treatments should not be relied on for control.

Unfortunately, even when we follow best management practices, downy mildew can take us by surprise due to high disease pressure, poor fungicide timing, suboptimal spray coverage, fungicide wash-off due to rain, cultivar susceptibility or a combination of factors. In addition, fungicide resistance may play a role in some cases. If needed, growers should be prepared to apply post-infection treatments.

Research from the Pacific Northwest indicates that cymoxanil, such as Curzate, has about two days post-infection activity, but only provides three days of forward protection. This means that cymoxanil would make a great treatment if you 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. Both Curzate and Tanos have a seven-day pre-harvest interval.

Dimethomorph, such as Forum, and mandipropamid, such as 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 both have a seven-day pre-harvest interval. Phosphorous acid fungicides, like 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. The strobilurin fungicides including trifloxystrobin, such as Flint, and pyraclostrobin/boscalid, such as Pristine, have less post-infection activity and are not recommended for downy mildew at this time.

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. Growers should select clean planting material, remove heavily diseased plants early in the season and eliminate primary basal spikes as late as possible. Early harvest can also minimize cone infection when infection pressure is high.

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, so 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. The pre-harvest interval for these products is one day or less. At this time we have no data on the efficacy of these products.

Additional considerations

  • Growers should rotate through multiple fungicidal modes of action in a season to delay resistance development in the downy mildew pathogen.
  • If disease symptoms are showing up on leaves and shoots, you can assume that there is plenty of disease pressure to infect the cones as well.
  • Avoid spraying systemic fungicides on heavily sporulating lesions since this is not very effective and can encourage fungicide resistance development. Rather, apply a contact fungicide to kill the spores first and then follow up with systemic fungicide applications.
  • Ensure thorough coverage of plant material, particularly for contact fungicides, which 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.
  • Some growers may have additional limitations based on their purchaser, so be sure to consult with your customer to ensure you aren’t applying materials that are prohibited by the brewer.

For more information on downy mildew of hops, refer to the Michigan State University Extension article, “Downy mildew pressure is high in northern Michigan hopyards.”

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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