Have the same symptoms on some of my 8 varieties and plenty of black branches even on a beautiful 10' Reed. Sent sticks off to the U. of Fl. got back a great response, said it was cultural (I use rainwater almost exclusively) which makes things really tough to figure out. My Gwen and Sharwil completely lost ALL their leaves but came back. I also drenched with MagnaBon. Here's a Oro Negro branch that looks like yours. Tree is now very healthy and loaded. Good luck!
Waiting on fruit to ripen so all is not lost.
We really need to know your cultural activities to guess what's going on. What plant food and the NPK, etc. applied how often for instance?
Good luck!