Black knot on fruit trees is caused by a fungus, Apiosporina morbosa, also called Dibotryon morbosum. Most commonly found in North American fruit trees, especially tart cherries, it can cause the loss of an entire crop if not controlled. Black knot appears as black galls, or “knots,” which can range from 1/2 inch to over a foot. The gall may have a wrinkly “warty” sort of appearance. Black knot is not active in the winter but remains in the infected parts of the tree. According to Cornell University’s Plant Disease Diagnostic Clinic, “In the spring, infective spores (ascospores) are produced in sacs (asci) contained within tiny fruiting bodies on the surface of the knots.” The ascospores are released from the infected area and cause the disease’s spread. Because this fungus spreads so easily it is important to identify and control it as early as possible. There are several natural methods for doing this.
Identifying Infected Branches and Shoots
Check cherry trees for black knot during the late fall/early winter when the branches are bare. Look for large, wrinkly, black growths on branches and shoots that resemble clumps of burned marshmallows. This indicates an advanced infection.
Inspect each shoot and branch for early evidence of black knot. A tree with a mild or early infection of the fungus will show evidence of swollen, smooth greenish lumps forming on shoots or branches.
Check the trunk of each cherry tree, infected or otherwise, for telltale black knots or large greenish lumps that might indicate a black knot infection.
Removing Black Knot from Infected Trees
Locate the lowest point of infection on each branch or shoot.
Measure about 8 inches below each knot and make a small cut with a penknife to mark the spots to cut.
Cut through and remove the branch or shoot using a sharp handsaw (or chain saw for larger branches) . This must be done before the tree begins to bud in the spring.
Carve out any black knots or swollen green lumps on infected trees. Be sure to excise to at least 2 cm of healthy wood around the infected knot.
Collect all knots and affected wood in a plastic lawn and leaf bag as you work to prevent any spread of the infected wood.
Burn all knots and infected wood collected in a fire pit as soon as is practical. This will completely destroy the fungus before it can release any ascospores, which can re-infect the tree even in the winter dormant season. If it is impossible to burn the wood, be sure to use a shovel to bury it in a deep hole well away from any other susceptible trees.
Controlling Black Knot
Inspect all surrounding woods and copses for any wild fruit trees that may be infected and remove them, burning any wood to be sure of eradicating the threat.
Refrain from over-watering cherry trees, especially at sites of previous infections, as too much moisture can create a welcoming environment for black knot to thrive.
Take steps to prevent deer and elk from chewing on new growth on trees as this can open the bark and make a tree more easily susceptible to infection. Trim off and properly dispose of any moist new shoots that show signs of animal grazing.
Plant species that are less susceptible to black knot, such as Mountain Alder, Amur cherry, and Black Tartarian cherry.