If as you say this started over the summer then it currently being fall isn't the issue.

Did you see this last summer?
Have you checked the underside of the leaves to look for aphids, spider mites and other sucking insects?
Was your weather unusually hot and dry and it didn't get enough water to accomodate its needs?
Do you have a vole problem, a small mammal that tunnels in mulch or just under the soil surface and eats plant roots? (pushing down with your hands around the plant will answer this - if there are vole tunnels, the soil will sink more in some places than others when you push.)
Do you have other animals that might have bothered the plant?

I did have a little bit of this problem last summer too. The leaves were not as brown but they did curl. The leaves have not fallen off.

I think the plant got enough water this summer. I am outside of Boston , MA. We did not have as much drought as other parts of the country. I watered when it was the hottest in July and Aug.

I do not have voles. I have seen a garter snake in the area. I have cats, but have never seen them near the lilac.

We had a leaf hopper infestation in the lawn which we sprayed for. But the brown curling had happened long before the spray.

I have not seen any bugs on the plant.

Ken's point about fall is that the plant is about to shed its leaves - foliage problems are really of no immediate concern at this time of year :-)

Your leaf problem looks like typical lilac blight - and not a very severe case of it, either. This is a bacterial disease lilacs (and a bunch of other plants) are prone to. Tends to be somewhat climate-related and you can treat for it if you wish but I find it more of a cosmetic issue than a serious health threat, at least with an established lilac. Can be pretty gruesome with certain other plants, however.

And pinhole insect damage to leaves is a no-brainer NBD and even more so at this time of year. Just make sure you rake up and properly dispose of all leaves this fall and then follow whatever treatment plan you prefer. A dormant spray with copper sulfate is often used in my area.

Here is a link that might be useful: lilac blight fact sheet

the bugs.. if any are gone...

the weather.. if any are gone ...

the leaf hoppers are gone.. but drifting spray could have been the culprit ...

when it was planted.. was it extremely rootbound in the pot.. did you fix that .. if not.. then it is probably in drought ... during the heat.. but that is all speculation ... did you amend the planting hole ..

its great the puzzle this all out.. but i dont really think there is anything to do .. this time of year ... clean up all the leaves for winter ...

ken

I suggest it hasn't yet rooted in well because the browning is the pattern of water shortage rather than lilac blight.

Looks like it's got good bud set - That's what matters right now. You really can't do much about it at this point; look again at it in the spring when they leaf out.