Guess the Pest! Week 14 & 15 Answer: Spider Mites, Powdery Mildew and Downy Mildew

David Owens, Extension Entomologist,

Congratulation to Kathleen Heldreth, Keith McGowan, and Chris Burkhart for correctly guessing the three problems on cucurbits. The first two images were spider mites. Early spider mite infestations on watermelon do not always reveal themselves on the upper side of the leaf because of the thickness of the leaf. On the underside of the leaf, you will see a grayish stipling. By the time advanced stipling and leaf yellowing is present, extremely large populations are present. The third image had both powdery mildew and downy mildew. Both diseases are active and require specific fungicides. Kate Everts with the University of Maryland wrote some excellent posts on both diseases. Last year’s can be found here:, and this year’s here:

Early signs of spider mites on watermelon

Advanced stipling and leaf yellowing indicate that extremely large populations of spider mites are present.

Both powdery mildew and downy mildew infecting a cucurbit leaf.