how big is the plant.. can we see a pic of the whole..

i see no damage on the other leaf in the pic.. so i wonder if anything need be done at all ...

understand that even with the brown.. that leaf is still converting light into plant food.. doing its job ....

and hail storms this spring??? .. or frost or freeze weather since it leafed out???

ken

When browning or discoloration is contained within cell walls (or limited by the veining), that is typically an indication of a disease pathogen. Cold or weather damage just doesn't manifest in that fashion. However, Stewartia is known for having almost no disease issues and very few insect problems. In all my years of growing these trees (my favorites....even more so than Japanese maples), I have never encountered ANY issues with them other some munching on a leaf now and again and a need for consistent soil moisture (not the slightest bit drought tolerant).

I would just monitor it and if it gets worse, then take a sample in to your local extenson service office for a diagnosis. Right now, that amount of disfigurement doesn't cause me any concern.

Did you spray any repellant on it? Sometimes spraying during daytime hours causes leaves to scorch and turn brown. Also, I have lost a Stewartia to Japanese Beetles, so being part of the rose family, they are susceptible to insects., although a bit early for JB's and the leaves would be looking more like a skeleton.

Stewartia belong to the Theaceae (or tea family), not the rose family. Although it could certainly be bothered by Japanese beetles, as they are by no means restricted to roses or their relatives.

As noted, the patterning of the discolortion is very distinct and that limits its sources. Neither insects, pesticides nor weather would make that sort of defined pattern, cleanly bordered by cell walls or veins.