This is a picture of black knot tree fungus dibotryon Morbosum. It is very common growing on plum or other stone fruit trees like dates, or peaches. It grows on the ends of branches and eats away and rots the branch. if you do not catch it in the beginning you will have to cut down the tree because it is hard to stop it. We had two trees that had to be cut down because of this fungus.
-J.P.
-J.P.