Holes in the leaves of my cherry blossom tree

Mustang Sally

Hi.  I have a number of small holes (2mm approx) in the leaves of my weeping cherry blossom.  I can see no pests other than a few greenfly, but am wondering whether it could be ants.  Also the tree is generally not looking healthy - many branches are bare of leaves as clumps of leaves died where the blossom was.  Also have what looks like sap coming out of many of the nodes on the branches.  Please help!