Pruning black knot disease on a mature schubert cherry tree

Asked August 3, 2016, 11:52 AM EDT

I have a mature Schubert cherry tree with a serious infection of black knot disease. Instructions say to prune infected branches 2 to 4 inches below the infection. However, almost every branch is infected. Experts disagree over the use of fungicides to cure it. What do I do? - Prune as much as possible but leave x (50%) of the tree so it might survive? - If so, prune now or wait until Winter/March? - Try a fungicide? If so, which one? - Start over?

Hennepin County Minnesota

1 Response

I can't add much to your research. Here is the University of Minnesota's discussion:
http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/black-knot/

Pruning out the galls, if every branch is affected, will kill the tree. If the tree is tolerating the galls, perhaps you can live with the infected tree. Fungicides are best used on young trees. Furthermore, the old galls must be pruned out and destroyed - they are a source of new infection - before the fungicide is applied. And don't expect the fungicides to "cure" the black knot.

You might want to consult a certified arborist to help you decide a course of action:
http://www.isa-arbor.com/findanarborist/arboristsearch.aspx

If you decide to replant, remember that chokecherries are highly susceptible to blacknot and the black knot fungus can be blown in from wild prunus spp. far away. You would be wise to select a tree from the University's list of resistant varieties and species. I might even advise not replanting prunus at all.