I planted the avocado tree this spring and it was healthy with fruit then. Since then it has dropped most of its leaves and all of its fruit. It is planted on a small hillside. It has been mulched with red mulch since planted and occasionally gets light fertilizer. We live in Jacksonville, FL and I believe it's a Lila. There were both Lila and Lula tags on it when purchased.

After some reading, I thought it may be root rot. How can I know for sure and what can I do about it?