An easy, cheap, and non-toxic solution for preventing powdery mildew is a baking soda solution 1/2tsp. baking soda to a quart of water, sprayed on the plant's leaves a couple of times per week. I just keep a spray bottle of this out in my garden all the time and spray it on whenever I'm walking through. Nothing will "cure" the spots that are already there, but you can prevent it from spreading any more and when the plants get bigger, just cut off the affected leaves.
But unless you can move some of those plants out into the sun, away from the trees, I think the powdery mildew is the least of your problems.
The squash need to be spaced out twice as far apart as they are now if you plant them in ground somewhere else. Each plant can get to be 3ft across so bear that in mind. I would take out every second plant and put them in some big pots out in the sun. The smaller plants will transplant easier at this stage of the game.
