Cucurbit Downy Mildew Update: June 18, 2025

On June 10, 2025, a new report of downy mildew on acorn squash, summer squash, and cucumber was reported in South Carolina. 

Downy mildew is a disease caused by the water mold pathogen Pseudoperonospora cubensis. There are 2 types of P. cubensis that affect different species of cucurbit crops - they are referred to as Clade 1 and Clade 2. Clade 2 primarily infects cucumber and cantaloupe, and Clade 1 is known to infect watermelon, pumpkin, and squash. 

In South Carolina, downy mildew has been reported on cucurbit crops that are affected by both clades of the pathogen, indicating that many different types of cucurbit crops could be at risk for disease development under optimal conditions. 

Optimal conditions for cucurbit downy mildew include at least 6 hours of 100% relative humidity at the leaf surface to enable the production of sporangia, which are the structures that make and release spores. According to the Compendium of Cucurbit Diseases, the optimum temperature for sporangia production is 59–68°F. Under high disease pressure (more spores are present) and favorable conditions, the pathogen can successfully infect in as little as 2 hours. The spores are easily dispersed long distances (hundreds of miles) 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 and squash 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 opposite the yellow lesions. Angular leaf spot is a bacterial disease and does not produce spores. To further test suspicious leaves, place a few in a plastic bag with a moist paper towel and let them 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.

Extension educators have begun visiting farms and scouting for disease. Please email Leah Fronk at lxf339@psu.edu to contribute to this report.