If you’ve noticed strange black lumps forming along the branches of your cherry or plum tree, you’re likely dealing with Black Knot Fungus. This common tree disease is more than just an eyesore. It’s a sign of active infection that can slowly deform and weaken your tree if not treated promptly. In this guide, we’ll break down what causes Black Knot, how to identify it, and the most effective ways to manage or remove it before serious damage occurs.
What Is Black Knot Fungus?
Black Knot is a fungal disease caused by Apiosporina morbosa. It targets trees in the Prunus genus, including cherry, plum, apricot, and chokecherry. The fungus infects branches and twigs, eventually producing hard, swollen black galls that look like burnt clumps of charcoal. Although it spreads slowly, over time these knots can girdle and kill branches or even compromise the tree’s structural health.
The most obvious symptom of a black knot fungus is the presence of rough, black growths along the branches. These “knots” typically start as olive-green swellings in the first year and darken to hard, brittle black masses by the second.
You may also notice:
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Small swellings or cankers on young branches
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Dieback beyond infected areas
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Reduced flowering or fruit production
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Clusters of knots where pruning cuts were not cleaned properly
What Causes Black Knot Fungus?
Black Knot is most active during wet spring weather when fungal spores are released and carried by wind or rain to nearby branches. The fungus typically enters through wounds, cracks, or pruning cuts, where it quietly establishes itself before symptoms become visible the following year.
Factors that increase risk include:
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High humidity and poor air circulation
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Nearby infected trees (especially unmanaged wild cherry)
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Improper pruning techniques or contaminated tools
Which Trees Are Most at Risk?
Black Knot is particularly aggressive in:
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Schubert and Canada Red Chokecherry
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Sour cherry
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Wild and ornamental plum trees
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Apricot (occasionally affected)
If these are part of your landscape, inspect them yearly for early signs of infection. While Black Knot often develops slowly, untreated infections can eventually girdle major limbs or kill younger trees entirely. Once knots encircle a branch or reach the main trunk, removal may become the only viable option.
When to Call a Certified Arborist
If Black Knot has spread to multiple limbs, is affecting large branches, or keeps coming back despite pruning, it’s time to call in a tree care professional.
A certified arborist can:
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Confirm diagnosis
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Perform strategic pruning safely
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Recommend preventive care or removal if necessary
Conclusion
Black Knot Fungus is a serious but manageable threat to cherry, plum, and other Prunus trees in Ontario. Early identification, proper pruning, and preventive care are essential to controlling its spread and protecting your landscape. Left untreated, this fungal disease can lead to branch dieback and even tree death. If you notice black growths on your trees or need help with pruning and treatment, don’t wait until the damage is irreversible. Croft Tree Experts offers professional tree disease diagnosis, Black Knot removal, and certified arborist services across Ottawa.
Professional Tree Disease Treatment In Ottawa
At Croft Tree Experts, we specialize in diagnosing and treating common fungal diseases like Black Knot throughout Ottawa and surrounding areas. Whether you’re looking for expert pruning, disease control, or professional advice, our ISA-certified arborists are here to help.
