What is That Black Blob on Your Trees?

Black knot on chokecherry tree. Photo by Mike Schomaker, Colorado State Forest Service, Bugwood.org.

by Annie Deutsch, Agriculture Agent, Door County UW-Extension

As the leaves fall off trees, not only do you suddenly remember how close your neighbor’s house is, but it allows for a different view of trees. Looking at trees without leaves lets you clearly see the branch structure, if there is any damage to the branches or trunk, or if diseases are present but were previously obscured.

One very common disease that is apparent at this time is black knot, a disease caused by the fungus Apiosporina morbosa, which forms characteristic black, knobby swellings on tree branches. The fungus only attacks trees and shrubs in the genus Prunus, which includes wild, ornamental, and culinary cherries, plums, apricots and peaches.

Black knot fungal spores overwinter in the black galls and are released when temperatures warm in the spring. The spores will blow onto and infect damaged branches or new tree growth. The initial infection is not apparent on the surface, but the fungus causes chemicals to be released within the plant, which induces it to produce excessive cells, essentially forming a tumor-like gall.

When the gall begins to form, it will have an olive green color. The gall turns black in the second year of infection. After two years, the gall may be attacked by other fungi, giving it a whitish color. Each year the gall continues to release more spores leading to additional infections.

As a black knot gall develops, it encircles the branch eventually cutting it off from water and nutrients. At that point the branch is girdled and will die. Trees with minor infestations may have limited overall damage, but trees overrun with galls can be killed.

Preventing the disease from becoming a problem is ideal. When shopping, select tree species that are more resistant to black knot. Trees/shrubs not in the Prunus genera will not be infected, but some Prunus species are more resistant to the disease than others. A few ornamental varieties that have shown some resistance are Prunus accolade, Prunus sargentii, and Prunus maackii. Apricot trees are also more resistant.

Before purchasing any new tree or shrub, carefully inspect the entire plant for any signs of disease. If you notice anything, do not purchase it.

If black knot is already present in your yard, the best treatment is to prune out the galls. The fungus only infects tissue around the gall, so cutting the branch approximately 6-8 inches below the gall will remove the infection from that branch.

Between each pruning cut, clean all tools with a 10 percent bleach solution or rubbing alcohol to prevent moving spores between cuts. Pruned out galls should be burned or buried. If possible, galls on nearby wild trees should be pruned out as well. There are some fungicides labeled for use against black knot, but they are not effective enough to be worth using.

The ideal time for pruning trees is late winter, so if you notice black knot in your tree now, the best thing to do is wait. In the fall, the fungus is going dormant so it won’t be releasing spores any time soon. Pruning later in the winter, however, helps the tree heal quickly and limits other pathogens from entering the pruning wound. Black knot galls discovered during the growing season can be pruned out at that time, but it is even more important to sterilize pruning equipment between each cut and destroy the galls after they have been removed.

For more information about black knot or other horticulturally related questions, contact the Door County UW-Extension office at 920.746.2260.