Black Knot Fungus

Black Knot is a fungus that attacks only Cherry trees. This one is growing on a Choke Cherry branch in Fish Creek Park.

"The characteristic feature of black knot is the presence of thick, black, irregular swellings on twigs and branches. The galls are often noticed in the winter when leaves are not present. The disease is difficult to notice during the early stages of infection. Initially, the disease appears as small light brown swellings on current or previous seasons growth. The following year, the swellings appear olive green with a velvety texture. By the end of the season, knots darken and harden. Numerous knots may be present on one tree. Often the branch beyond the knot will either fail to leaf out or wilts suddenly.

Black knot is caused by the fungus Apiosporina morbosa, which infects trees in the spring, about the time of bud emergence." From www.umanitoba.ca.