What species of trees does black knot affect in our state? I realize it is a very serious problem. How can one diagnose it ....
Stearns County Minnesota black knot
It is a problem for Prunus or the plums. It is pretty easy to see. It looks like a black thickened area on branches. Here is a link that has very good info about it. http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/black-knot/