Cucurbit Downy Mildew Update: May 28, 2025

Cucurbit downy mildew has been reported in South Carolina on cucumber May 21, 2025.

Optimal conditions for cucurbit downy mildew include at least 6 hours of 100% relative humidity at the leaf surface for the production of sporangia, which are the structure that makes and releases spores. According to the Compendium of Cucurbit Diseases, the optimum temperature for sporangia production is 59–68°F. When higher levels of spores are present, the disease can successfully penetrate if free moisture is present for as little as 2 hours. The spores are dispersed by air currents, which is why the disease is tracked as it moves from south to north.

Symptoms of downy mildew on cucumber are angular yellow lesions on leaves delineated by leaf veins. Spots on cantaloupe are more irregular and may resemble angular leaf spot at first glance. However, since downy mildew is a water mold and produces spores, you may observe gray-purplish sporulation on the underside of leaves infected with downy mildew. Angular leaf spot is bacterial and does not produce spores. To further test suspicious leaves, place a few in a plastic bag with a moist paper towel and let sit on the countertop overnight. The next day, observe leaf undersides for spores. A hand lens with at least 10x power is helpful when looking for spores.