Have had a dwarf flowering cherry tree for ten years this spring.
Hard sap premature foliage
Answer
So sorry about your cherry tree. It sounds like it has a borer, especially possible with the sap coming out of it. I hope it isn't too late to save it. Take a skewer, insert it where the sap is, just under the bark. Run the skewer up and down in what is probably the borer pathway, trying to skewer any insect in there! Then, wearing gloves, carefully spray an insecticide into the hole. Be careful you don't get any on yourself or in your eyes. Wipe off the insecticide on the bark and plug the hole with a caulk of some sort to keep the insecticide in. Next spring monitor the tree for evidence of borer activity and repeat this process. Insecticides externally don't usually work. The branches that have the small leaves or dying branches usually will be the ones that show the borer sap but the whole tree should be monitored. Thank you for contacting PHS. ep