Sap bubble at base of weeping cherry tree
The sap indicates an injury of some kind. Can't guarantee it's a borer. However, it is extremely common for weeping cherry to be infested and killed by peach tree borer.
One of the ways to prevent this is to keep mulch away from the base of the tree (this attracts egg laying there.)
Actually, mulch should never touch the base of any tree or shrub. It should be kept back a few inches and not be more than 2-3" deep.
Here is a page on managing borers: https://extension.umd.edu/hgic/topics/borers-trees