Yoshino cherry decline
I have a Yoshino cherry tree that appears to be in rapid decline. It did flower this spring, but now many of the leaves that intially formed have dried up on the branches. There are a few places on the trunk and some branches where a dark substance appears to have oozed out of the tree. I would say the tree is at least 50% defoliated now. Any idea of the cause and whether it is treatable?
The sap you're seeing is oozing out of holes made by the peach tree borer. The number of sites with sap indicates that your tree has been attacked by many borers. The borer's adult phase is a clear-wing moth, but the larval stage is a borer that will remain in the tree for a year. The boring tunnels cut through the cambium layer beneath the bark where water and nutrients travel up and down the tree--in effect cutting this off. Once the borer/borers are in the tree, there is nothing you can do to combat them.
Because of the extent of your infestation,the upper tree is dying and you'll probably have to remove the tree.
Mulch piled on the trunk base encourages this insect. Be sure to keep mulch pulled back a few inches from the trunk of all tree and shrubs. Also, a preventative cover insecticide spray can be put on the trunk each year to keep the borers from getting into the tree but timing the spray application can be difficult.