I'm also in the piedmont of NC. I have struggled with tart cherries, mostly they have died from bacterial canker. It kills quickly. I've been told the wild black cherries (Prunus serotina) are responsible for spreading the canker to the fruiting varieties but I've never found solid evidence of that. It wouldn't surprise me though, most of the black cherries in the woods around my property all have black knot and look diseased.
Do you have a pic you can post of your cherry with PM?

No pics :( My good phone died because of my sweaty gardening, lol. But it's definitely powdery mildew, at least if it looks the same as in roses.

We have 'wild' seedlings from really old ornamental Japanese cherries around here as our only 'wild' cherries, and they grow great. I've never seen black knot, which I hope is a good sign. Cankers on lots of cultivated things are a big problem, though, yeah. My orchard is nice and windy, so it stays drier than many Southern yards, but you know that humidity is still way up there disturbingly often!

I know the cherries are a bit of an experiment. Unfortunately, my very favorite fruits and nuts are hard to grow here. So I grow both easy and hard fruits and hope that the harder ones do well enough to not rip out :) I have room, so that part is easy enough.