@cavemanrich
https://www.gardeningknowhow.com/edible/fruits/plum/black-knot-tree-disease.htm
Every plum tree I've ever had has gotten this. I ended up removing them and doing without plums, but there are a few products you can use to tone it down. I have used Captan fungicide in the past which mitigated the disease, but decided the risk to our animals and ourselves wasn't worth it. I didn't have much luck with sulfur. The tree will survive, and you can keep cutting the affected spots, but eventually you end up with a gnarly tree that loses its landscaping appeal. Good luck, that's a beautiful tree worth trying to save!
https://www.gardeningknowhow.com/edible/fruits/plum/black-knot-tree-disease.htm
Every plum tree I've ever had has gotten this. I ended up removing them and doing without plums, but there are a few products you can use to tone it down. I have used Captan fungicide in the past which mitigated the disease, but decided the risk to our animals and ourselves wasn't worth it. I didn't have much luck with sulfur. The tree will survive, and you can keep cutting the affected spots, but eventually you end up with a gnarly tree that loses its landscaping appeal. Good luck, that's a beautiful tree worth trying to save!