The leaves on my pumpkin plants have what looks like a white mold growing on them.
What is this?
If it is harmful, how do I treat it?
This is almost certainly Powdery Mildew (see photo), a fungal disease that prefers high humidity and occurs most often during a spell of cool days and humid nights. A severe attack will destroy plants' leaves and reduce their yield. You can control it chemically with a copper or sulfur-based fungicide, or organically, using the following recipe, which I haven't tried myself but I gather is very effective. It is based on research done by Cornell University:
1 tablespoon each of baking soda (sodium bicarbonate) and either a light horticultural oil or regular vegetable oil, to 1 gallon of water. Shake well before and during application.