Black knot is a serious fungal disease caused by the fungus Apiosporina morbosa. It primarily targets trees in the Prunus genus, including plum, cherry, apricot, and chokecherry. If left unmanaged, the black, knotty growths restrict the flow of water and nutrients, eventually weakening and potentially killing the infected branches or the entire tree. Effective treatment requires immediate action to remove the source of the fungal spores.
Identifying Black Knot
The disease is identified by the formation of hard, rough, black galls, or knots, on the woody parts of the tree, typically on branches and twigs, but sometimes on the main trunk. The disease cycle begins subtly, often going unnoticed during the first year of infection.
Initial infections appear as small, light-brown or greenish swellings on the current season’s growth. By the following spring, these swellings enlarge and develop a velvety, olive-green coating, indicating the fungus is actively producing spores. As the season progresses, the knots harden, turn dark black, and develop a cork-like, elongated shape. The knots continue to grow, sometimes girdling the branch, which causes leaves to wilt and die back beyond the knot.
Pruning and Removal Techniques
Pruning is the most direct and necessary step for eliminating the black knot fungus. It is best performed during the dormant season (late fall to early spring) when the tree is leafless and the fungus is inactive. Pruning during this period minimizes the risk of spreading infectious spores, which are released during wet, warm spring weather.
When pruning, the cut must be made far enough below the knot to ensure all infected tissue is removed. Make the cut at least four to six inches below the knot and into healthy wood. This margin is necessary because the fungal infection extends internally beyond the black gall itself. For knots on the main trunk or large structural branches, a certified arborist may need to chisel out the infected wood and at least one inch of healthy bark surrounding the knot.
Tool sterilization is required to prevent the accidental spread of the fungus to healthy parts of the tree or to other trees. Pruning shears must be cleaned with a sterilizing solution, such as a 10% bleach solution or 70% rubbing alcohol, after every cut. After all infected material is removed, it must be disposed of immediately by burning or burying to destroy the spores. Infected branches should never be left on the ground or placed in a compost pile, as the spores can remain viable.
Fungicidal Treatment and Long-Term Prevention
Fungicides are not a standalone treatment for black knot but are used preventatively to protect new, healthy growth from infection. Chemical control is most effective when used in conjunction with a pruning and sanitation program. The timing of the fungicide application is important, as it must coincide with the period when the fungus releases its spores in the spring.
The initial application should occur in late winter or early spring just before the flower buds open. The fungicide protects the tender, newly emerging green tissue, which is highly susceptible to infection. Multiple applications are usually required, spaced every one to two weeks, especially during periods of high moisture and warm temperatures (55°F to 75°F), until the new shoots have fully matured. Effective fungicides often contain active ingredients such as copper, chlorothalonil, or thiophanate-methyl, but products must be registered for use against black knot in your region.
Long-term management relies on cultural practices designed to reduce disease pressure. This includes:
- Regularly monitoring the tree for new swellings to allow for early intervention.
- Removing nearby wild host trees, particularly wild plum and chokecherry, which harbor the fungus.
- Selecting varieties of plum and cherry that have genetic resistance to black knot when planning new plantings.