Early symptoms of cucumber downy mildew. Photo by Raven Schaffter.
With the recent mild, cloudy weather, we could see an early arrival of downy mildew in northern Ohio. Downy mildew is favored by moderate temperatures, overcast skies and rain. The pathogen, Pseudoperonospora cubensis, does not overwinter in northern regions and is introduced via sporangia (a type of spore) on wind currents from other areas. In Ohio and other Great Lakes states and Canada, the first “wave” of downy mildew likely originates in greenhouse production systems in the region. This wave rarely extends to central or southern Ohio. Over the years the first report of downy mildew on cucumbers in northern Ohio has been about 7-10 days before or after July 4. Melons are also susceptible to this pathogen “type” that occurs in the first wave. Pumpkin, squash, watermelon and other cucurbits are usually not affected. The second wave of this pathogen occurs later in the summer, with a variant of the pathogen arriving on wind currents from the south, affecting all cucurbits. This variant affects mainly southern and central Ohio, although it may spread north.
Underside of cucumber downy mildew lesion showing sporulation of the pathogen. Photo by Marty Bauer.
Cucumber and melon growers in northern Ohio should intensify scouting these crops and apply a protectant fungicide such as chlorothalanil. Look for yellow or tan angular lesions delimited by veins on the top surface of leaves, and fuzzy grey/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. Once downy mildew is reported in the area, growers should ramp up their spray programs to include fungicides highly effective against downy mildew such as Orondis Opti, Ranman, Omega, Previcur Flex, and Elumin. More information on these fungicides can be found here.
If you suspect downy mildew in cucumber or melon please text or email pictures to Sally Miller (330-466-5249; miller.769@osu.edu) of both sides of lesions, with the underside in the highest possible magnification. I can often confirm downy mildew from photos, but if not will ask you to send a sample to the OSU C. Wayne Ellett Plant and Pest Diagnostic Clinic (CWEPPDC), now in Wooster. Instructions for sample submission are here. Digital images may also be sent to the CWEPPDÇ.
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.