Black knot on plum and cherry

Black knot is one of the most visually distinctive plant diseases in the eastern US -- the hard, black, corky, elongated galls on plum and cherry branches are unmistakable once you've seen them, and in neglected plantings they can cover branch after branch until the tree is structurally.

Black knot is one of the most visually distinctive plant diseases in the eastern US — the hard, black, corky, elongated galls on plum and cherry branches are unmistakable once you've seen them, and in neglected plantings they can cover branch after branch until the tree is structurally compromised. The disease is caused by a native North American fungus and spreads readily from wild plums and cherries (including ornamental species) that grow throughout the region.

I don't grow plums or cherries at my Long Island property, so this guide draws on Cornell Cooperative Extension, Penn State Extension, and Clemson HGIC research.

The pathogen

Black knot is caused by Apiosporina morbosa (formerly Dibotryon morbosum). Per Cornell Cooperative Extension, the fungus is native to North America and infects primarily Prunus species — plums, cherries, and closely related trees.

The disease is endemic to the eastern US, where wild black cherry (Prunus serotina) and chokecherry (P. virginiana) serve as reservoirs. Per Penn State Extension, spores are released from existing knots in spring when temperatures exceed 55°F (13°C) and there is rain. They infect succulent green shoots of the current season.