You have powdery mildew a fungal disease. It usually comes with cool wet conditions wet but mostly by high humidity. You can get it in hot conditions too if there is enough humidity.It also attacks if there is poor air circulation--like in densly packed plants. The fruit will rot too so if you have any that are ripe I suggest you pick them, and wash them off

As soon as you see it pick off the infected leaves and spray with a funficide for powdery mildew and try to give the plants better air circulation. If you get it early and pick the leaves off, a spray made with garlic will help. Spray the whole plant, and you will have to do it more than once.

Garlic has anticeptic properties and will help with mild outbreaks of fungal diseases, which is why I said get it early and pick the infected leaves off. Your yard will smell like a pizzeria for a day or 2, but it's better than chemicals.

To make a garlic spray--Whirl 4 or 5 cloves of garlic in the blender with a little water. You want basicly mush. Put it in a container and add 2 to 2 1/2 cups of water, rinsing the blender out with the water so you get all the garlic and juice. Let sit for 24 hours and then strain into a spray bottle.

Soak the blender bowl in water and baking soda to get rid of the smell.

I had no idea that squashes turned into large smothering plants. The next question is how long can a squash plant (vine?) get?

Sulfur combats powdery mildew. Try some h2o3 3% peroxide.