Holly Tree-Black Root Rot??
 

We are in Rio Grande. We have a holly tree that looks like it's dying. Dropped 1/2 its leaves and has many dead branches. I googled and had our lawn company spray with fungicide but am wondering what it is. We just had one of those no name on the truck guys stop by uninvited to tell me the tree has black root rot and there is nothing we can do. Of course he was looking for a job. I am noticing other holly trees in our neighborhood not looking very healthy either.

Does anyone know anything about this issue and if there is anything we can do to save it?