Cucurbit Downy Mildew Outbreaks Increasing

Cucurbit downy mildew outbreak map. https://cdm.ipmpipe.org.

We have three new reports of downy mildew on cucumbers this week, all from home or community gardens in Coshocton, Muskingum and Geauga counties. Thanks to OSU Extension educators David Marrison and Clifton Martin for the finds in Coshocton and Muskingum counties. The Geauga County outbreak was reported by one of our Ohio Master Gardener Volunteers participating in a multi-state Citizen Science project to monitor and report cucurbit downy mildew (thanks Annie!). Numerous reports in other states show that downy mildew is on the move (see map), mostly still in cucumbers and melons. In Ohio the cucumber/melon outbreaks have been in the northern and central counties, likely caused by the downy mildew “pathotype” that circulates in the Great Lakes region and infects only cucumbers and melons. However, last week downy mildew was reported on squash and pumpkins in Central Kentucky and on squash in Western New York. The downy mildew “pathotype” that infects squash and other cucurbits moves up into the Midwest from the deep South, so Ohio growers, especially in the southern counties, should step up scouting of all cucurbits and apply protectant fungicides such as chlorothalanil products. When downy mildew is confirmed nearby and conditions are conducive (cool, rainy, humid, overcast), more effective fungicides should be applied.

Spray Program Recommendations for Ohio Cucurbit Growers

1. Northern and Central Ohio cucumber and melon growers: add effective fungicides shown in green in the table to the spray program now. Tank mix with a protectant fungicide such as chlorothalanil (Bravo, Equus, etc.), 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.
2. Southern and Central Ohio cucurbit growers (all types): apply a protectant fungicide on a 7-10 day schedule now.

If favorable conditions for downy mildew persist and downy mildew is present in your area, add effective fungicides shown in green in Table 3 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.

1. Forum, Presidio, Quadris, Zampro and Revus are not recommended. Curzate may be moderately effective in some locations.
2. Follow this blog, Twitter @OhioVeggieDoc or the Cucurbit Downy Mildew IPM PIPE for downy mildew reports.

Click to enlarge.