Hi Debbi, thanks for the question.

This looks like Angular Leaf Spot, a bacterial disease.  It can by introduced through infected seed and survive on plant debris. in can infect other cucurbits.   It can easily be spread with hands and tools, so it is important to avoid working on wet plants and to disinfect tools.  Although you can't control the weather, avoid getting the leaves wet. Keep plant debris cleaned up. Use of a bactericide (e.g. copper) can slow it down.  Infected leaf removal can help limit spread.   Removal of the plant can also be an option. Be aware that Cucurbit Downy Mildew (not yet seen in your area) looks similar but is usually found later in the season.  Check out the following link:

Angular leaf spot | UMN Extension

I hope this helps.