Cucurbit Disease Update for Mid-May 2025 (Downy Mildew Found in SC)

From Clemson Plant Pathologist Tony Keinath

Despite relatively dry conditions this spring, two foliar diseases have shown up on cucurbits in different parts of South Carolina. Gummy stem blight was found on watermelon in the Central Savannah River Area, and downy mildew was found on cucumbers at Coastal REC.

Five of 23 watermelon samples (that’s 22%) were positive for gummy stem blight. This level of disease was unexpected, given how good the field looked from the pickup traveling down the drive rows. Gummy stem blight starts inside the canopy, and on a healthy crop, the first symptomatic leaves are often hidden by the healthy foliage above them. This camouflage is why proper scouting must be done on foot by walking diagonally across the field.

The watermelon field had already been sprayed once with Miravis Prime, so the presence of expanding lesions is a bit surprising. Two points explain this discrepancy:

1. No fungicide works 100%. Delaying disease is the “name of the game.”
2. Even though the days have been hotter than normal and dry, nights have dew and temperatures that are favorable for the gummy stem blight fungus.

Growers should follow the 2025 Watermelon Fungicide Guide to stay on top of gummy stem blight and other diseases.

Downy mildew was found on May 21, 2025, in a research plot at Coastal REC. It appears that spores arrived approximately May 11. The popular picking cucumber ‘Gershwin’ was the first cultivar infected, as it had 10 times as much disease as the other susceptible check cultivars, which had only 1% leaf area diseased.

All conventional and organic cucumbers in the lower half of South Carolina should be sprayed with a recommended fungicide. The resistant cultivar DMR-NY-401 does not need to be sprayed under organic production, and can be sprayed with only chlorothalonil or mancozeb in conventional production. Applications should be repeated weekly until 1 week before the last harvest.