Q: My magnolia tree has tufts of brown leaves at the ends of several branches.
A: I believe you have an infestation of black twig borer.
UGA horticulturist Matthew Chappell says: “The borer will enter the stem 6″ to a few feet from the tip – when twig size is about the size of a pencil to pinky size (I have never seen it on stems greater than 1″ diameter) yet is hardened off/is hardening off (not softwood or early semi-hardwood stage). It will create a chamber that creates a weak spot in the twig and the twig will fail at that point – yet typically be held onto the plant loosely by the remaining bark. Rarely do you see dieback past the point of entry. So what you end up with in severe infestations is something that looks like the plant was tip pruned by hand snapping a bunch of stems. Very unsightly and unique damage. “
All you can do is prune out affected branches.
These references give more information.