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Often referred to as 'poop on a stick', black knot is a canker caused by a fungus. The fungus forms a growth on the outside of twigs, encircling the young twig and choking it. It tends to start in the outer canopy and work its way in towards the main stem. The growth, or canker, is hard, dark grey to black, and crusty. If caught early, the fungus can be pruned out and the tree can be saved. If left too long, the fungus will spread to the point where the tree can't survive all the pruning that would be required to eliminate the damaging fungus.
Black knot affects over 400 species in the Prunus genus, such as cherries, plums and choke cherries.