There are a couple of (I think they're pumpkin but could be zuchini or some other squash) plants in my garden that look really bad. The leaves are very yellow and have white stuff on them. I hope it's not too late.
Pull the older yellow leaves off and see if the new ones develop the same issue.
It's called Powdery Mildew - had lots of rain lately? Overhead watering? Too much shade? A fungicide will help control but not eliminate it. Organic methods of control including spraying with potassium bicarbonate with a small amount of vegetable oil added. If you can't find K bicarb you can sub baking soda but it isn't as effective. 1 T baking soda in 1 quart of water and add 1 tsp light vege oil, shake well and lightly mist the affected leaves. Avoid spraying in the heat of the day or in bright sunshine. Again, only a control, not a preventative. The only real preventative I know of is using resistant varieties.
Most important - increasing the sun exposure as much as possible. Good luck. ;)
Here is a link that might be useful: Powdery Mildew
My first thought was powdery mildew, but that yellowing doesn't look right from what I remember. Back when I used to get it, the leaves would get completely coated in white powder, and then the leaf would brown (not yellow) all at once.
Either way, if it is powdery mildew, there's one simple cure: don't do overhead watering on your squash. Switching from sprinklers to drippers completely eliminated it from my garden.
Oh I agree karenrei that the yellow isn't typical of powdery mildew. But it is typical of excessive moisture and given that the OP is from Whidbey Island, WA I'd bet on excessive overcast and rain being the cause of both. ;)
But if you haven't been having typical WA island weather, then be sure, as karenrei says, to eliminate any overhead watering.
Agree with what Dave says. It may not be powdery mildew, but it's some kind of mildew. I am sorry to say that when a cucurbit plant start out life like this, it is pretty much toast. It will never have time to recover, especially in your relatively cool growing environment.
So try to keep the leaves as dry as possible when the plants are young so they can get off to a good start. Watch for insects like cucumber beetles and squash bugs that can bring diseases like mildews to your plants, and if you see any zap them with an insecticide before the plants begin to show female blossoms.
Don Yellman, Great Falls, VA
I don't think this is fungus or mildew at all. White patterns like this are quite normal. The yellowing is a separate problem. See the thread just before this one called "pics of a few leaf/vine questions".