Up here here it's not that humid in general, but we do tend to have rainy spells and those can wreak havoc. A couple of seasons back, my melons and zucchini looked fine until some time late July they all got white, practically overnight. I tried everything, but it's hard save them once PM takes holds. I replaced the squash with new plants, but couldn't save the melons. Last year, I sprayed regularly with sulfur and copper a couple of times, and that helped quite a lot. I saw some PM and just removed the affected leaves.
Another thing I heard is good is Kaolin Clay. I used to spray my cukes and grapes with it to fight cucumber beatles and Japanese beatles. But I heard once you spray it on leaves prone to PM, it creates a barrier. It's messy and unsightly, and you have to reapply after each moderate to heavy rain, but apparently it works. At least most cucumber beetles left my cukes alone that year.
