It does not look like fungal or disease issue. It looks like potential stress from some environmental factor. Are the leaves dying on that single branch in the photo or across the whole tree?

It is likely an environmental stress. Was this tree recently transplanted? If not - has anything changed around the roots, trunk, or soil? It may be water stress - too much or not enough? 

Other things to consider that can add stress:

Is the root flare (the area where the roots begin at the trunk) buried under soil or mulch?

There could also be a girdling on the trunk or larger roots that is impacting the movement of water and nutrients through the tree.

Has the trunk been damaged by a mower or weed whip?