Kwanzan cherry tree 16yrs old has one dying limb
There are a couple of canker diseases that attack cherry trees. From the pictures it appears most likely to be bacterial canker caused by the bacterium Pseudomonas syringae. Here is more information about the disease and its control http://pnwhandbooks.org/plantdisease/cherry-flowering-prunus-spp-bacterial-canker. The best control is to prune out the infected branch during the dry part of summer. Chemical control measures are not all that effective. It is possible that more of the tree will become infected, but but bacteria need open wounds and rain in order to infect new branches. Leaf fall is one time when wounds from leaf scars and rain are often present at the same time, and it may help to apply a copper bactericide at this time, although efficacy my be questionable. Be very careful never to prune in the rain or prior to rain.