What’s this growing on this plum tree?

I walked to this European Plum tree (at least I think that’s what it is) at a nearby public park in Whatcom County, WA to see about potentially picking some. Upon arrival I noticed that the branches of the tree were covered in these strange looking growths and opted not to pick or eat any plums because of it. Is the tree diseased? There was another plum tree nearby that had these same growths on it but far fewer. Thanks in advance.