There is something killing your plants check the undersides of your leaves to see if you can see anything eggs etc. It may be mites and if it is they are very hard to see. They drain all the nutrients out of the plant until it is dead. Put a tablespoon of dishsoap in a litre of water and spray both sides of all the leaves. this should do the trick. Good luck Don't forget to spray the runners as well.
Linda, I suspect you have a case of powdery mildew, which is fairly common on zucchini and pumpkins. Depending on how significant the problem is, you might start by spraying the leaves with baking soda (1 teaspoon in 1 quart water), which will raise the pH, creating an inhospitable environment for powdery mildew. If the condition worsens, you may have to resort to a fungicide.
What caused this mold?
The airborne spores of fungi. There are several types that cause powdery mildew.
Thanks for all your help. Problem resolved