Cucurbit Downy Mildew Fungicide Efficacy in Ohio 2018-2020 and Recommendations for 2021

Now that we know cucumber downy mildew has appeared in the Great Lakes Region (Ontario report – see my post on 6/17/2021), cucumber and melon growers should prepare for fungicide applications. There are a number of products labeled for downy mildew management in cucurbits, but not all are consistently effective due to development of fungicide insensitivity in downy mildew pathogen populations.

The OSU Vegetable Pathology Lab conducts bioassays each summer to assess the efficacy of fungicides commonly used to manage cucurbit downy mildew. We treat potted young cucumber plants with fungicides individually at the labelled rate or with water alone, and the next morning place the plants in a cucumber field with active downy mildew. Depending on the year, tests were conducted in Wayne, Sandusky and Huron counties, all in northern Ohio. After 48 hrs of exposure the plants are returned to the greenhouse and rated for symptom severity 7 days later.  The results for 2018, 2019 and 2020 are shown in the table.

Click to enlarge.

The fungicides Previcur Flex, Ranman, Omega and Orondis were highly effective against cucumber downy mildew for all three years. Zing! and Elumin were only tested in 2018 and 2019/2020, respectively, and both were highly effective. One or more of these fungicides should be part of a downy mildew management program.

Curzate, Forum, Presidio, Quadris and Zampro were inconsistent across years and locations and were ineffective in a least one location in one of three years. Revus was consistently ineffective.

Spray Program Recommendations for Northern Ohio Cucumber and Melon Growers

  1. Apply a protectant fungicide such a chlorothalanil (Bravo, Equus, etc.) on a 7-10 day schedule now.
  2. When downy mildew is reported on cucumbers or melons in northern OH, SE MI or western PA add effective fungicides shown in green to the spray program. Tank mix with a protectant fungicide, alternate fungicides with different modes of action (FRAC codes) and follow the label restrictions and requirements. Shorten the application interval to 7 days under favorable weather for downy mildew: rain, overcast skies, high humidity, and cooler day and/or nighttime temperatures. Follow this blog, Twitter @OhioVeggieDoc or the Cucurbit Downy Mildew IPM PIPE for downy mildew reports.
  3. Forum, Presidio, Quadris, Zampro and Revus are not recommended. Curzate may be moderately effective in some locations.

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