Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Watermelon is the least susceptible to powdery mildew of all locally grown cucurbit crops. However in recent years the Southeast region of the U.S. has seen an increase in powdery mildew on watermelon. Yield loss has been observed under some conditions, particularly hot and dry weather. Disease is also favored by low light intensity (such as cloudy weather or low in the canopy). Because powdery mildew doesn’t overwinter here, and is introduced each year, the disease should always be confirmed before applying a fungicide.
To determine if a fungicide spray is warranted, scout at least 50 leaves across a field. For example a minimum of 10 plants and 5 leaves on each plant should be examined. Typical symptoms are chlorotic spots on the upper leaf surface accompanied by sporulation on the lower surface. Alternatively sporulation may occur on both upper and lower leaf surfaces or on petioles or stems. Because sporulation is sparse, powdery mildew on watermelon is more difficult to diagnose than on other cucurbits, and a sample may need to be submitted to University of Maryland or University of Delaware for confirmation.
If powdery mildew is confirmed, an initial fungicide application is warranted. A good program would be to alternate Quintec (6 fl. oz/ A) plus chlorothalonil with one of the following products tank mixed with chlorothalonil: Procure (8 oz/A), or Rally 5 oz/A), or Folicur (4-6 fl. oz/A). Once the first spray is applied, scout the field to determine if a follow up spray is needed. If the disease does not spread or if weather conditions change, additional sprays are not necessary.
Powdery mildew sporulation on the upper surface of a watermelon leaf. (Photo courtesy of Bugwood images and David Langston.)