Thanks for writing. The white spots you're seeing are probably powdery mildew, a very common disease of pumpkins, squash, and their relatives.
Help keep powdery mildew at bay by making sure the plants are in a spot where there's good air circulation. Train them outward so the leaves aren't all clumped together in one spot.
Also: Avoid getting the foliage wet when you water, if possible. Wet foliage encourages disease.
You may also need to use a fungicide to keep the problem at bay. There are conventional and organic fungicides that work against powdery mildew; look for them at your local garden center or from a reputable online source.
---Justin, Senior Garden Editor, BHG.com "