Cucurbit downy mildew has been confirmed in Ohio, specifically in Sandusky, Ashland, Holmes, Medina and Wayne counties.

The observations have been made by area growers, the OSU Vegetable Pathology Lab and the Wayne County IPM Program and were confirmed by the C. Wayne Ellett Plant and Pest Diagnostic Clinic at the OSU CFAES Wooster campus.

Ontario, Canada and Southern Michigan had several confirmed observations that were reported the last week of June and first week of July. Ohio incidences started being found during the second and third weeks of July.

Growers who have cucumbers and cantaloupe in their fields should take quick action to protect their crops. Cucurbit downy mildew is best managed with preventive applications before infection takes place. Once visual symptoms appear, control becomes increasingly difficult, and yield reductions may occur.

In conditions favorable for disease development and without effective, timely management, cucurbit downy mildew can cause rapid decline of the plant, severe defoliation and ultimately plant death in just a matter of days. Disease development is favored by rainy, humid conditions − relative humidity greater than 85% − and cooler temperatures of 60-70 degrees.

According to research trials completed in Ohio, Michigan and other states and provinces around the Great Lakes region, the best fungicide options are Orondis Opti (FRAC 49+M05), Ranman (FRAC 21), Omega (FRAC 29), Previcur Flex (FRAC 28) and Elumin (FRAC 22). These should be tank mixed with chlorothalanil (Bravo, Equus, et cetera) or mancozeb (Dithane, Manzate, et cetera). Orondis Opti is a premix already containing chlorothalanil, but at a reduced rate.

Fungicides have restrictions on how much product can be applied and how often, so follow the label. The label is the law. Fungicide products with higher efficacy ratings should be rotated to avoid resistance development in the pathogen.

Please note, the inclusion of pesticide name is for educational purposes and does not imply an endorsement of that product. We do not attempt to give specific chemical recommendations, but we do summarize research, efficacy trials and basic concepts and principles for growers to use when considering the use of pesticides.

Growers in Ohio also should intensify scouting of cucumbers and melons. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves and fuzzy gray/brown growth on the undersides of the lesions. With a good hand lens or a smartphone camera with high magnification, you may be able to see small dark brown/purple spots within the fuzzy growth. These are the spores of the downy mildew pathogen. You also can use the Cucurbit Downy Mildew IPM Pipeline website, where you can sign up for alerts notifying you of nearby confirmed cucurbit downy mildew observations.

If you suspect downy mildew in cucumber or melon, submit samples to the C. Wayne Ellett Plant and Pest Diagnostic Clinic for confirmation. Instructions for sample submission are on its website. Call or email Dr. Francesca Rotondo at 330-263-3721 or rotondo.11@osu.edu with questions. You also can work with your county extension educator to get samples submitted to the lab. A major 'thank you' to Dr. Rotondo and her staff in the Plant and Pest Diagnostic Clinic and Dr. Andres Sanabria-Velazquez and the Vegetable Pathology Lab for their critically important work. Also, thanks to financial support from the Ohio Produce Growers and Marketers Association’s Ohio Vegetable and Small Fruit Research and Development Program. There is no fee for this service for Ohio vegetable growers.

Frank Becker is the Agriculture and Natural Resources Extension Educator with Ohio State University Extension – Wayne County, and a Certified Crop Adviser, and may be reached at 330-264-8722 or becker.587@osu.edu
This article was previously published (incorrectly as an "Opinion" piece) in The Daily Record.