HI Anna, I'm sorry for the delay in my reply. I'll go take a look and report back. Thank you for the direction. I'll be back!
I checked the underside and see no "webbing" or signs of insects. Just the general leaf pattern of veins or whatever it's called. They're just turning yellow, then brown, and curling up. I scraped some of the stem with my fingernail and it's moist and green underneath, so that's a good sign, right? I have several other varieties of lilac on my property, some closer to the busy street than this one, and none of the others are doing it. I assume this particular variety might be more susceptible? I don't remember what kind it is. So, what do you suggest I do, if anything, at this point? I don't usually cut it back in the fall, and I just let all the leaves fall off and it's a stick bush for the winter (I'm in zone 7 and it gets very cold and snows). I did fertilize it back in June, and I side dressed it with composted manure. That's something different I did this year that I just thought of. If there is anything I should do now, let me know. Otherwise, I'll go ahead and rate you; you've been helpful.
Thank you, Anna. I appreciate the help. Have a nice day.