Hey everyone, looking for a little help. I received 2 Chinese elm trees from a friend a few months ago. Both about the same size and from the same source. One of them is doing really well and has a bunch of new growth. This one on the other hand is not. It began developing what looked like burn marks on the edges of the leaves. It is also not producing any new growth. It has been in either full sun or partial shade here in Southern California (zone 9b) depending on the weather. I have left it in full sun when the days are no hotter than 90 degrees (32 C) and moved it to an area under my large trees where it gets partial sun all day if the heat got any higher. They are both still in nursery pots but I did transplant this one about a month ago to see if that would help and to inspect the roots just in case. Roots looked healthy at the time of transplant. The spots on the leaves have remained the exact same for about the last 2 months. They are not getting any worse or better. They cover the entire tree though. Can anyone tell me what it may be and how to treat it?

Looks like sunburn to me.

Can be also to wet Soil ?! I change my chinese Elm into a mineralic Bonsaisoil, there it grows well (prefer full sun more than half shadow), in the Midsummer if the temperature rise above 90 F and stays longer than a couple days, my Elm loose a lot of Leaves (also in half shadow). I think it´s not the temperature and the sun at the Leaves, it´s the temperatur at the Soil and Roots what my Elm dont like. If the hot period is over, new Leaves comming back.
If your Elms potted normal organic Soil, you can change at the right time the one in in mineralic and test the diference.

Yes sunburn, not wet soil when the temperatures are that high. Not helped possibly by re potting, keep doing what you are doing it will start to grow once the temperatures drop a bit, most trees are not growing when that high.

Thank you all for your replies. When I did transplant it I used a much better draining soil then it came with. I will keep it in the partial shade until temps come down in my region and watch for new growth.
Should I avoid using any fertilizer at this time while it recovers?