I have a 7 year old Chinese Elm bonsai. So far it has been growing new shoots without any problems. As of last week I noticed most of the leaves of the old shoots have started going yellow at the center and brown at the tip. After a while they become crispy and fall of the tree. It only seems to affect old shoots, as far as I can tell all the new shoots look normal to me. I’ve attached a picture for reference.
I normally wait until the soil looks dry (it changes color) before giving it a thorough watering.
Is this natural? Or could this be caused by overwatering? Under watering? Over fertilizing? What should I do?
I’ve also noticed a thin layer of white crust forming on top of the soil.