Bad news: that's a virus. It could be one of several or even a combination of two but they are all nasty stuff spread by parasites, chiefly whiteflies.
Since there's no cure for any of these virus the only form of control is to use virus-resistant cultivars (plenbty available nowadays, most giving both high yields and high quality) and especially to promptly exterminate any aphid, whitefly or other sap-sucking parasite found on the plants: these little buggers need 18-24 hours of continuous feeding to spread the disease.
All those plants need to be pulled from the ground and destroyed quickly to avoid more damage to be done, and you also need to examine all your other cucurbits for parasites. If you find anything, exterminate with extreme prejudice.
