I agree its probably powdery mildew. I tried growing pumpkins one year and after about a week of beautiful vines and broad leaves, I noticed this whitish stuff on some of the leaves. I bought this organic stuff that was supposed to get rid of the fungus but it was too late by that point, the entire vine ended up shriveling up and never produced fruit. I am starting to see this stuff now on my poor squash plants which are also clearly going to be non-productive.
good luck with this, I know I havent had any