It is hard to tell from the pictures, but I suspect either pumpkin downy mildew or bacterial leaf spot. I would lean toward bacterial leaf spot. If you notice purplish growth in small spots on the underside of the leaf in periods of damp weather, that would help confirm that it is actually downy mildew.
Have you had similar problems in the past when pumpkins were planted at the same location? What has the rainfall/irrigation situation been?