Wild cherry species such as chokecherry sometimes fall prey to black knot. Any walk over old fields or in woods taking over an old field may take you past a tree with this infection. The black galls typical of this infection are caused by a fungus. It is very, very easy to identify.
Sometimes a tree has been infected for a long time and the galls may even have other fungi or lichen colonizing the original gall.
Black knot is visible along trails like the Cole Harbour Heritage Park trails that we walked this past Sunday. This disease also occurs in cultivated trees of the Prunus spp. Manitoba Agriculture has some great information.