It's August. That means in about 2-3 weeks powdery mildew will start showing up on the pumpkin leaves. A few years ago I started spraying Actinovate on the leaves on my plants in early August and in late August I could see the difference. On one leaf, a couple of years ago, you could see where another leaf was preventing it from being sprayed on and in the exact outline of the leaf above the leaf below had powdery mildew. Actinovate probably won't prevent your leaves from getting powdery mildew but I know from experience that it can greatly reduce the severity of the powdery mildew and delay the onset.
I used to only find Actinovate online but now it is in my local garden centers. Actinovate is a bacterium that is unique because it can be effective when added to the leaves and soil. It is an organic fungicide that I think is well worth the price. Consider it a preventative measure. No biological I've found can eliminate a fungus problem so once you see the problem you are too late. However, it can help keep things in your favor so you won't get problems.