I doubt it's phytophthora. If so, it would be dead by now.
As long as the tree is doing well, please don't complicate its survival with unneeded treatments.

Actually, this does look like advanced phytophthora to me, and you can certainly try treating with Agrifos - it is a biorational fungicide. It has minimal impact on the environment and quite safe. It is phosphorus acid, and very effective against phytophthora, but your poor tree has sustained so much bark damage, I'm not sure if it can heal itself or not. I would contact a citrus expert (try contacting the UC Riverside folks), to see what the best course of action would be. It may certainly be approaching the end of its lifespan, although we do have heirloom orange orchards here in California that are at least this old and still going strong, but then, they aren't dealing with the degree of bark and cambium damage your tree has sustained.

Patty S.

Thanks Patty, good advice, I'll try doing that.