Black Knot is a common plant disease of the genus Prunus (Plums and Cherries), with all Prunus species susceptible to varying degrees. This includes native species such as Wild Black Cherry (Prunus serotina), Chokecherry (Prunus virginiana), and particularly the Sand Cherry (Prunus pumila). Black Knot also affects cultivated fruit trees (Cherry, Peach, Apricot, Plum) and non-native ornamental cherries.
The name Black Knot refers to the large, irregular black galls that grow on stem tissue, either along branches or a tree’s trunk. These galls are produced by a native North American fungus, Apiosporina morbosa. The fungus produces its spores with these galls, releasing them during wet spring conditions in the hopes of infecting other Prunus and repeating the cycle. Galls can also spread infection through stem tissue the longer they remain attached to a tree.
Remember that since it is a native species, A. morbosa is part of an ancient web of ecological relationships. The fungus is itself preyed upon by other fungi, especially Trichothecium roseum, which grows on galls and interferes with spore formation. In wild trees, mature galls are used for habitat by wood-boring beetles that in turn are food for woodpeckers and other insectivores.
The primary impact of Black Knot in the landscape is aesthetic: the galls look like turds hanging in the branches of an infected tree or ugly bulges on mature trunks. Beyond the cosmetic, galls that grow to completely encircle a stem cut off the flow of water and nutrients (girdling), causing the stem or branch to die back. In a heavily infected tree, this can mean decline and even death if measures are not taken to reduce the impact of Black Knot.
The best intervention on a tree with Black Knot is to prune out infected stems in the late Winter (February – March), ideally on a day with temperatures below freezing. This is also the easiest time to spot galls, before leaves emerge in Spring. Whenever possible, prune with 4-6” or one node (branch intersection) between gall and pruning cut. Remove infected branches from the area or dispose of them by burning. Infected branches left nearby may still spread spores.
Once all galls are removed from an infected tree, continue to monitor going forward and prune galls as they appear (and they will). Diligent manual control will ensure that the galls do not multiply enough to drain the tree of resources or negatively affect its shape.
At Scioto Gardens we do not advocate the use of chemical fungicides which, while common in the orchard industry, are always used in tandem with pruning. The residential gardener or backyard orchardist should have no trouble maintaining tree health with pruning alone.
The next time you see a bare-branched tree laden with lumpy, black galls, remember that while nature may not always be pretty, there is always something to be learned and most problems can be managed with diligence and understanding.
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Black Knot is a common plant disease caused by a native North American fungus, Apiosporina morbosa. It produces lumpy, dark galls on stem tissue of trees in the Prunus genus. This includes native species such as Wild Black Cherry (Prunus serotina), Chokecherry (Prunus virginiana), and particularly the Sand Cherry (Prunus pumila). Black Knot also affects cultivated fruit trees (Cherry, Peach, Apricot, Plum) and non-native ornamental cherries.
March 2024 Newsletter
