Why are the leaves of my citrus (orange tree) turning yellow?
Thanks for writing. I'm afraid that without being able to see the tree and where it's growing, I can't say why the leaves are turning yellow. This free tool really isn't meant for diagnosing plant problems like this because we don't really have the ability to investigate the problem.
I'd suggest bringing a sample branch to the folks at your local garden center, nursery, or county cooperative extension office. After seeing it in person, they should be able to help more.
---Justin, Better Homes and Gardens