Cucurbit downy mildew occurrences in the eastern US. https://cdm.ipmpipe.org.
Downy mildew was reported on pickling cucumbers in Seneca County today. The field was heavily damaged, with about 90% of the plants showing symptoms. We also found one confirmed downy mildew lesion in our sentinel plot in Wooster (Wayne County). These are the first reports for 2021 of cucurbit downy mildew in Ohio, following an outbreak in Chatham-Kent, Ontario reported on June 17. Growers in northern Ohio should protect cucumbers and melons with effective downy mildew fungicides such as Orondis Opti, Ranman, Omega, Elumin, Previcur Flex or Zing!. Remember to alternate products in different FRAC groups and tank mix with a chlorothalanil product or other protectant fungicide – except for Zing!, which contains chlorothalanil in the premix. Check the labels carefully for use instructions and restrictions. Fungicides must be applied preventatively – they are far less effective if applied after infection.
Growers in central Ohio should intensify scouting of cucumbers and melons and apply a protectant fungicide. 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.
Smartphone image of a downy mildew lesion with the pathogen sporulating on the underside of a cucumber leaf.
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 Vegetable Disease Diagnostic Lab for confirmation. Instructions for sample submission are here. 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.