We have a Washington cherry tree that we planted about 5 years ago. It produces more cherries each year. Leaves seem to be okay. But... the north side of the truck of the tree has erroded away and is black in colour. It has sap that leaches from it at that point and seems to be spreading higher up the tree. What is this caused from and what can I do to cure it? I really don't want to loose the tree. It's just starting to bud so I would prefer to do something before it flowers. If possible, is there anyting natural I can use? Any help would be appreciated.