I am afraid that looks like rot setting in where the tree has been damaged by a trimmer/lawn mower/other object hitting it.  You can test it by inserting a screwdriver.  If there is resistance, it may be something else.  You may wish to have a certified arborist come out and make an evaluation.  Go to this site and click on Find An Arborist.  Fill in your zip code and a list of certified arborists in your area will appear.