We have an old growth Cherry tree in our yard. It is loosing leaves which are yellow. The trunk of the tree has knots on it and is excreting sap.
Washington County Oregon fruit trees
From your very brief description it is difficult to determine the cause of the defoliation. My first thought is that it is bacterial canker, but this disease is usually much more prevalent on young trees. It is possible that it is affected by Cytospora canker, which infects older trees and often colonizes bacterial canker infection sites. Here are two links to more information about bacterial canker and Cytospora canker. http://pnwhandbooks.org/plantdisease/cherry-prunus-spp-bacterial-canker