With recent cloudy and rainy weather, northern Ohio has seen an increase in the occurrence and pressure of the disease known as Downy Mildew (DM) of cucurbits.
Symptoms of Downy Mildew, Pseudoperonospora cubensis, on cucumber leaves
The DM pathogen, Pseudoperonospora cubensis, doesn't overwinter in northern regions of the state and is introduced by wind-borne sporangia from other areas. We have two waves of DM that arrive at different times and from different points of origin. In Ohio and nearby Great Lakes States, the first wave of downy mildew likely originates from regional greenhouse production and typically affects northern Ohio around early July. This wave primarily impacts cucumbers and melons, while pumpkins, squash, and watermelons usually remain unaffected. A second wave of the DM pathogen, which arrives later in the summer from the south, can affect all cucurbits, particularly in southern and central Ohio, with the potential to also spread up through northern Ohio.
Classic checkerboard-like symptoms of Pseudoperonospora cubensis (Downy Mildew) on cucumber leaves
This disease thrives in moderate temperatures (between 60°F and 70°F), overcast skies, and rainy conditions. Growers with cucumbers and cantaloupes should act quickly, as Cucurbit Downy Mildew can cause rapid leaf loss and overall plant decline under these favorable conditions.
Close up symptoms of Downy Mildew, Pseudoperonospora cubensis, on cucumber leaves
Scouting for DM symptoms, such as yellow or tan angular lesions on leaf surfaces and fuzzy growth underneath, is essential. If DM is suspected, samples should be submitted to the C. Wayne Ellett Plant and Pest Diagnostic Clinic, in Wooster, Ohio for confirmation. This service is free for Ohio vegetable growers, thanks to support from the Ohio Produce Growers and Marketers Association.
Downy (black) sporulation on the underside of a cucumber leaf infected with Downy Mildew
The Plant Pest Diagnostic Clinic always wants to identify areas of disease appearance to update the Cucurbit Downy Mildew IPM Pipeline website. Growers can sign up for disease alerts by visiting this website: https://cdm.ipmpipe.org/
General preventative fungicide applications, like the use of chlorothalonil, help to slow the initial infections of this disease. However, once the disease is confirmed in an area, the consistent application/usage of specific recommended fungicides are crucial and are really the best and only approach to managing this disease. These fungicides include: Orondis® Opti, Ranman®, Omega®, Previcur® Flex, and Elumin®, mixed with chlorothalonil or mancozeb. It’s important to always rotate fungicides to prevent resistance.
The list and effectiveness of fungicides on DM, based on field trials and greenhouse assay, can be found at: https://u.osu.edu/miller.769/2022/07/11/first-report-in-ohio-of-cucumber-downy-mildew-for-2022/