Black Knot Fungus

This is a great time of year to spot a PEI fungust that's best known as a disease of Cherry trees: Black Knot (Apiosporina morbosa).

Black Knot fungus forms distinctive, black galls on stems and branches of our native Pin Cherry and Chokecherry, as well as ornamental Cherries and Plums. While it’s unattractive, reduces fruit production, and may kill the infected branch, Black Knot doesn’t usually kill the entire tree unless the infection is very severe.

Within those black galls are fungal spores that will be released in spring, just as Cherry leaves and flowers begin to appear. Spores are spread by wind, water, and animals, and gain entry through flower buds, leaf buds, or wounds on the tree. Once established, the fungus grows unseen within the tree over the summer, releasing chemicals that cause abnormal cell growth. By fall, the first evidence of a new gall becomes visible, and within two or three years it will begin to release its own spores.

Black Knot is native and widespread across PEI; control at the provincial scale would be virtually impossible. Individual, high-value trees can be protected by removing infected branches in winter or early spring, when temperatures are below freezing and spores are inactive. The fungus lives inside the tree – not just within the gall – so pruning must be well below the Black Knot to prevent re-infection. (It’s also important to sterilize tools between cuts and when done to prevent further spread). Pruned galls can continue to release spores for months, so be sure to either seal them tightly in a plastic bag for disposal in your waste bin, or burn infected material if you can do so safely.

Black Knot is sometimes mistaken for Chaga by novice foragers, but it’s not hard to tell the two apart. Black Knot looks like scat on a stick, whereas Chaga looks more like burned wood or charcoal on the side of a tree. Additionally, Chaga is golden-yellow inside and found mostly on Birch (and never on Cherry).

Black Knot may not be popular, but it is an interesting and native part of PEI Untamed!