Hi Onion - Welcome to the forum! Hope you find it friendly and helpful. I have taken the liberty of a) moving your post to the Trees & Hedges section and b) making the title more specific. By putting your question under Trees & Hedges, hopefully more people will see it who may be able to answer it. Not everyone takes the time to read all the introductions. Showing escallonia in the title should get the attention of people with an interest in them. It must not be a midwest thing, I unfortunately had never even heard of them. But for future reference those are tips on how to get the best answers to your questions.
When someone does come along with some knowledge of your hedge, they will be much more likely to be able to give you some good information if you give us more information about the situation the hedge is in. To start with, are you in Somerset England (there's also one in New Jersey and one in KY, but I think the UK one is the original and most famous)? What's the climate like there? What's the weather been like since you've been noticing the problem? Is your hedge in full sun or what kind of exposure? How have you been treating the hedge since you planted it-- any fertilizers, pesticides, etc that you used on it?
Ideally posting a picture of the plant and a close up of the leaf problems would really help in diagnosing it. Instructions for posting pictures are here:
The better information you can give us, the more likely we can give you good help. Best Wishes!