I'm not that far from you, and I wanted to show you this:
It looks almost exactly like yours!
I have just this one pocket of 5 plants that looks like your plants. It's hard to image that we've dried out our plants after having the wettest May in our history, but I think that's what we're dealing with. Either that, or we're having some unique issue driven by the fact that we were really, really hot, then we'd get 2 or 3 inches of rain, then it was back to really, really hot, then it was 3 more inches of rain. Could be they are just stressed by these crazy weather swings.
The spot I have the problem is in the fullest sun. These guys never get a break and that streak of 90 degree days maybe fried some of our foliage. If yours also get a lot of sun, maybe we have the same thing.
I think Sue is right, and I also followed Larry's advice. I did take it easy on the water, though, since we just got 3.25 inches in West Chicago last Wednesday. I trimmed off the dried bits, watered a little, and added a light amount of 10-10-10. I was thinking if they are just stressed by the weather, I didn't want to over-do anything. I don't think this is fungus related like leaf streak, but I had some fungicide in the garage, so I gave it a little squirt just in case.
I guess I'll know in a month if that was the right thing to do. Here's how it looks cleaned up.