Black Knot Fungus, caused by the pathogen Apiosporina morbosa, is a common and destructive disease primarily targeting stone fruit trees belonging to the Prunus genus, such as plums and cherries. This fungal infection can severely damage and eventually kill susceptible trees by restricting the flow of water and nutrients within the woody tissue. Controlling this disease requires a combination of timely physical removal of the infected wood and ongoing preventative measures.
Recognizing the Signs of Infection
The earliest sign of Black Knot infection often appears as a small, slightly swollen area on a twig or branch, sometimes with a velvety, olive-green color during the first year of infection. These initial swellings frequently occur on the youngest growth or at the site of a prior wound. As the fungus develops, it stimulates the host tree to produce excess woody tissue, which leads to the formation of the characteristic galls.
Over two to three years, these galls mature into hard, rough, brittle, cylindrical knots that are distinctly black. These knots can range from half an inch to several inches in length and may partially or completely encircle a branch. Girdled branches will eventually wilt and die beyond the point of the infection, making annual inspection for these unique growths an important diagnostic tool.
Essential Pruning and Sanitation Steps
Physical removal of the galls is the most effective method for eradicating the disease and should be carried out during the dormant season, typically from late fall to early spring before bud break. Pruning during this time is preferable because the tree is leafless, making the black galls easy to spot, and the fungal spores are less likely to be released and spread in cold, dry conditions.
When removing a gall, it is necessary to cut the infected branch well below the visible swelling to ensure all fungal tissue is removed, as the pathogen can extend beyond the visible knot. Make the cut at least four to six inches below the gall, cutting back to a healthy branch collar or a lateral side branch. This practice removes the part of the stem that has been infiltrated by the fungus but does not yet show symptoms.
To prevent spreading spores to healthy parts of the tree, tool sterilization is necessary between cuts. Pruning shears and saws should be dipped in a solution of 70% rubbing alcohol or a 10% bleach solution for several minutes. Cleaning the tools prevents the transfer of fungal spores from the infected wood onto the fresh pruning wound, which would create a new site for infection.
Disposal of the infected wood must be done, as galls can continue to release spores for up to four months after being removed from the tree. Never leave the infected wood on the ground or place it in a compost pile, as this will perpetuate the disease cycle. The best disposal methods are burning the wood where permitted, or securely bagging the prunings for municipal waste disposal.
Preventing Recurrence and Further Spread
Long-term management focuses on cultural practices and targeted chemical control. Choosing resistant varieties of Prunus species when planting new trees is important, as some Japanese plum cultivars show higher tolerance than European or wild varieties. Ensuring proper tree spacing for adequate air circulation also helps maintain a dry canopy environment, which is less favorable for fungal spore germination.
Chemical control, using fungicides, serves as a secondary protective measure to supplement pruning, especially in areas with high disease pressure. Fungicides are designed to protect new, susceptible growth from infection and are most effective when applied just before or during the spring bud break period. This timing targets the period when mature galls release spores in warm, wet conditions.
Common active ingredients for protection against Black Knot include Chlorothalonil or certain Copper-based fungicides. Fungicide use alone is not enough to control the disease, and protective sprays must be reapplied at regular intervals throughout the wet spring period until shoot growth matures.
Management involves regularly inspecting and removing nearby wild or abandoned Prunus species, such as chokecherries, which can harbor the fungus. These infected trees act as a continuous source of spores carried by wind and rain. Annual inspection during the dormant season allows for the early detection and removal of new galls before they become mature spore-producing knots.