• looks like maybe Alternaria fungus.. you can Google Alternaria citrus. Take off the affected leaves, spray it with a good quality fungicide and check the soil to be sure you are not over watering it.

I will try removing those affected leaves and try fungicide. I only use organic solutions. I have been hand-watering for the past year, so there's no over-watering. The rest of the leaves are very yellow compare to the other yuzu plant I have. It's too dark outside now, so I will try taking photo tomorrow, but the other one has very dark green leaves and looks very healthy. They are both in the same potting soil mixture, and they both get same fertilizer with same watering schedule (by me). I just don't understand why one will start looking like this all of a sudden.

BTW, I live in San Diego, California.

Thank you for your comment.