Ready, aim...

I came across this interesting looking leaf spot during a short walk recently.



Here is is again, on a different leaf from the same tree (a Chestnut Oak).



This is the kind of thing that we got calls for all the time at the Master Gardener Hotline, where I volunteered this past summer.

But without all the Cooperative Extension's plant books and other resources, I'm at a loss. (And this is one of those occasions where Google brings up too many choices to be useful.)