What you need to do is prune out the dead parts of thr tree.you do this from the inside of the tree.that means climb inside and take all of the dead out. Cut as close to the main branch.don't forget those little offshuts(they suck the life out of the tree.)when oyu are finished with this you need to loosen the ground around the tree,this would be a good time to fertilise the tree. After you do all of that get some bark mulch (not colored) water the tree every two days(water well) now you will be doing the same thing next spring. You should see an improvement by this time next year. If that doesn't work you'll have to get someone in to see if the tree is infested. Good luck,but what I told you should work.I have done this to hundreds of trees with great success
