From Clemson Plant Pathologist Tony Keinath.
Powdery mildew was found on three cultivars of seedless watermelon at the Clemson Coastal REC on May 23, 2023. All watermelon crops in South Carolina should be 1) scouted for powdewry mildew and 2) if found, sprayed to prevent powdery mildew from reducing yields by up to 40%.
When scouting, look for bright yellow spots at the tips of leaves. On the leaf underside, the spots are pale green to pale yellow.
White, powdery fungal growth may be present, but it does not develop on watermelon as much as it does on squash.
The slightly cooler temperatures in the past week is the most likely reason that powdery mildew showed up on watermelon. This year’s weather has not been as dry as in previous years when powdery mildew was found.
Spraying any conventional fungicide or one of several organic fungicides, like Organocide or sulfur, will suppress symptoms on the tops of leaves. Systemic or translaminar conventional fungicides are best at reducing powdery mildew on the undersides of leaves. Be careful spraying sulfur and oil products, like Organocide or Neem, when temperatures are above 90°F on sunny days. These products can burn leaves.
Crops that already have been sprayed with a protectant fungicide, such as chlorothalonil, as part of the watermelon fungicide program are somewhat protected against powdery mildew but should be scouted. Chlorothalonil is only partially effective. If cool weather continues, a powdery-mildew specific fungicide is needed.
In a previous study at Coastal REC, chlorothalonil rotated with sulfur had the highest weight of marketable fruit. This program can be used by growers who want to minimize the cost of powdery mildew fungicides. Sulfur volatilizes and helps control powdery mildew on the bottom of leaves.