TreeSpecialist

As Ekka likely nailed it, the damage on top of the limbs appears to be sun scald. If it were here in Texas, I would also think about animal damage as a possibility (we have tree rats some folks call squirrels.)

Be careful of the "deep root fertilizing" devices. If it's the one I'm thinking of, the tablets that go into the things are a very salty form of man made fertilizer.

The devices are best used for aeration and or deep soaking. A piece of re-bar to make holes in the ground followed by some compost would have the same (or likely) better results.

With the kind of "service" they seem to be getting, cabling would likely be a disaster. Done poorly, cabling is more than worthless.