I'd be very tempted to consider angular leaf spot, a bacterial disease. None of the other foliar diseases of cucurbits fit the image you've presented. Perhaps your local extension staff will be able to help.

If the conditions are favorable for a disease outbreak, even resistant varieties can become infected if the pressure is high enough.

Your state universities Cooperative Extension Service can be of real help. Click on the link below and then click on your state, or go to the text list, and the information about your states university should be linkable.

Here is a link that might be useful: USDA Cooperative Extension