From Clemson Plant Pathologist Tony Keinath and Extension Agent Zack Snipes.
Downy mildew was found on non-sprayed “sentinel” cucumbers at Coastal REC, Charleston, SC, on June 1. Two susceptible slicing cultivars (Silver Slicer and Poinsette) had 5% leaf area diseased, while the resistant cultivars Citadel pickling and DMR-NY401 slicing and the moderately resistant Bristol slicing didn’t have symptoms yet. This delay illustrates one benefit to planting resistant cultivars.
Downy mildew also was reported on spaghetti squash (Cucurbita pepo) from a large home garden in Charleston County on June 2.
June 1 is a week earlier than normal to find the first outbreak of downy mildew in Charleston County. Between 2009 and 2022, June 8 was the average first detection date, with a range from May 25, 2021, to June 23, 2014. Cucumber was the first host in 10 years, including the last four (2019-2022), and squash was the first 4 times. That’s why growers are encouraged to a) scout cucurbits for downy mildew, b) get text alerts from https://cdm.ipmpipe.org/, and c) start conventional or organic fungicide applications when you receive a text alert of high predicted risk or in mid-May.
This year there have been several calls about powdery mildew on squash that looks like downy mildew. By “looks like” I mean small, yellow flecks on the upper surface and a tan spot underneath. It’s possible that the heat and sunny days have suppressed powdery mildew growth on the upper leaf surface.
Using a hand lens or a 30X microscope, an observer should be able to see the difference between the white, flat, “spidery” growth of powdery mildew pressed flat against the leaf underside with short spikes of spores sticking up and the white, “tree-like” growth of downy mildew with larger purplish lemon-shaped spores. Downy mildew spores are more likely to still be attached to the leaf before noon, whereas powdery mildew spores can be seen any time of day (or night).
Spraying any type of conventional fungicide or one of several organic fungicides, like Organocide or sulfur, also will suppress symptoms on the top of leaves, but only systemic or translaminar conventional fungicides reduce powdery mildew on the leaf underside. Be careful spraying oil products, like Organocide or Neem, and sulfur when temperatures are above 90°F. on sunny days. These products can burn leaves.