I am long-term caring for a relative's co-potted lemon and lime tree. I have been caring for it since March and over the past few weeks the leaves have developed yellow spots and eventually fall off (see pictures). The tree has dropped at least 10-20 leaves in the past two weeks. Two weeks ago, the tree sprouted two new branches of new growth. It is currently in the process of growing a lemon. It is located in Central Texas. I water it 1-2x/week, it gets ~6-8h sunlight/day through east-facing window. I do not know the last time it was fertilized. It was repotted in the past 2 years. I have a citrus tree that sits nearby that also has a few leaves yellowing, but without the spotting and I believe to be just old leaves getting old. Does the lemon tree have a pathogen, nutrient deficiency, or am I just caring for it improperly? This tree has an immense amount of sentimental value.