Strange leaf damage (Chinese elm)

[MCHarrison]

I have noticed that the leaves of my Chinese elm have begun to die, but in very localized areas typically around the petiole and then spreading up across one whole half of the leaf, but apparently stopping there. At first I thought it could be spider mites or some type of pest, but the damage is so specific and constrained that that seems unlikely. More evidence against a pest is that the damaged region of an affected leaf is not missing. The localized nature of the damage also seems to preclude a fungus, which - while perhaps not being entirely systemic - would at least affect entire leaves at a time. (...right?)

As far as which leaves are affected, it seems to be hitting all leaves (old, new) pretty much the same.

I have had this tree for two winters and it has spent both of them outside. It sprung back from the last one quite well, with the exception of some branch tips which died, I believe due to the cold. The weather here recently has been unseasonably cold and rainy for the last two weeks, which might hint toward disease, but I'm not convinced based on the reasons I laid out above (of course I'm happy to be corrected, I'm still pretty new to this). The only other thing worth mentioning is that I went on vacation and my plant was watered by a friend for the week I was gone. When I returned, the most exterior half of a larger branch had withered, and I cut it off. The leaves looked to me as if the branch had been broken but I couldn't find any damage. I cut that branch off. The leave on that branch looked unlike this one and nothing similar appears to be happening since. This seems unrelated to me, but you never know!

Thanks for any thoughts or opinions you may have!

 

[Bonsai Nut]

Welcome to the site! What does your soil look like, and when is the last time you repotted? Anything else about care - like when you fertilize and with what? Do you ever treat prophylactically for insects or fungus like with a dormant oil or other product? Do you own any other trees that show similar symptoms - or other elms that don't show symptoms at all?