Last year, we discovered that our tree out in the front had black knot -- post here -- and we did what we could by cutting out the infected branches. But the tree is well over 25' tall and so we couldn't get some of the branches high up. I didn't call anyone in, and I suppose I am at fault but we let it go to see what would happen.
Well, this season it was the worst I had ever seen. Every young branch had it girdling it to death. Even shoots from the roots had it. I looked up some information and talked to one of my Arboretum teachers and the result is: take it down.
I was even told that cherries and plums shouldn't even be planted here because black knot is so prevalent. This tree was planted by the previous owners so it had at least a good 15+ years. And we've enjoyed it immensely with it's green spring foliage and white fragrant flowers. In the fall it turns this awesome purple and stands out among all the other green/yellow fall trees.
It was let to grow shoots, so we inherited it as a 6-trunk tree (1 trunk was removed in 2006). So we are cutting it down in stages. Three trunks have been taken down so far, leaving the oldest 2 trunks (1 of them has almost no black knot). If we can save it, we will. For now we will just see what happens.