Clemson Extension Plant Pathologist Tony Keinath prepared the following statement on powdery mildew on watermelon:
Powdery mildew was found today (5/16) on seedless watermelon at the Coastal Research and Education Center in Charleston, SC. Symptoms of yellow and brown spots looked similar to those in the photo below. White mildew growth and short chains of clear powdery mildew spores were visible on the leaf underside.
Because powdery mildew can reduce yields of watermelon, and it is still early in the growing season with plenty of time for powdery mildew to develop until harvest, all watermelon growers in South Carolina should apply a fungicide with specific activity against powdery mildew. The research field had been sprayed with Switch on 04/04, Bravo on 05/04, and Switch on 05/11. I recommend using fungicides such as Luna Experience, Quintec, or Torino that were rated as better than Switch in recent fungicide trials on powdery mildew on watermelon (http://newsstand.clemson.edu/mediarelations/clemson-research-improves-watermelon-yields).
More information on fungicide options are in the following resources: