The mystery spots...on Pumpkin plant leaves.

Asked July 7, 2020, 3:15 PM EDT

Can you help me identify the spots on these pumpkin plants. The first thought was powdery mildew. BUT it is a pretty uniform pattern not the same as the brushed look of a mildew. Im sending some images from last weekend.

Washington County Oregon

7 Responses

Thank you for your question. I believe your plants have Angular Leaf Spot, described here: https://pnwhandbooks.org/plantdisease/host-disease/cucumber-cucumis-sativus-angular-leaf-spot

The only one of the chemicals available to homeowners is Liqui-Cop (copper), which "has reduced disease spread but reportedly has caused some stunting and leaf chlorosis in Oregon." So, your leaves may have fewer bacteria, but may yellow, a problem typically associated with a lack of nitrogen (or inability of the plant to utilize the nitrogen there). Hopefully, you can use resistant varieties next year--and we won't have so much rain! Hope this is helpful. Good luck!

Hello K. A.
The spots on my plants are not circular and there isn't any yellowing present. Were you able to see the images I posted?

We had ruled out powdery mildew and angular leaf spot. IF you can't see the triangular spots in the images I posted by blowing hem up I can take higher resolution images and send them. Thank you for your response.

That is what "angular leaf spot" is. Compare your plants' spots with those in photos #1 and #3 in the Disease Management Handbook. Were the spots round, the diagnosis would be septoria leaf spot: https://pnwhandbooks.org/plantdisease/host-disease/squash-cucurbita-spp-septoria-leaf-spot

Thank you for being patient with me. I didn't look far enough in the handbook. Learning something new every day!

Another expert believes this pattern is a normal leaf variation. You can have a lab examine a leaf if you would like to submit it to the OSU Plant Clinic: https://bpp.oregonstate.edu/plant-clinic