What kind of a tree was that on?
It does look like a gall, but there is another disease which infects stone fruit trees, called black knot.
The galls are similar when green, but turn black when mature.http://i55.photobucket.com/albums/g122/ThePlantDoc/blackknot2.jpg?t=1177763937http://i55.photobucket.com/albums/g122/ThePlantDoc/blackknotdamage.jpg?t=1177764152
Sorry about the clarity of the pictures, they were taken with my cell phone.
If it is black knot, it is not curable, but you may prolong the life of the tree by pruning out the damaged areas, and making sure that the plant gets enough water, and fertilized yearly.