I have a very large generally healthy looking plum tree, but some branches have these black growths. What is this and can it be mitigated?
It's Black Knot, a very common fungus disease on plum trees. I just cut off affected branches a few centimetres below the visible infection as soon as I see the problem. I even successfully removed an infection on the main trunk by just carving it off with a knife. It healed completely after a few years. The sooner you remove the infected growths, the less likely the infections will spread. There are fungicides that will help prevent its spread, but I've never had to use them.
