“What is that gross, black stuff on my tree?!”
Invariably, that gross, black stuff is a fungal disease called "Black Knot" that is caused by a fungus known as Apiosporina morbosa. The disease appears as conspicuous, 2 to 25 centimetre long, black knotty swellings on branches. The knots can be several times wider than the limbs and look quite grotesque
Black Knot spores are spread by wind and rain and can penetrate injured and healthy tissue of the current season’s growth. The first year of infection, the branches swell somewhat but aren’t black. The second year, the swollen branches burst with masses of black spores.
Trees and shrubs that are infected by Black Knot are limited to members of the "Prunus" family that includes: plum, edible cherry, ornamental cherry (Schubert and chokecherry), and Mayday.
The best way to control Black Knot is by pruning the infected branches and then bagging or burning them. Recognizing and removing first year infections is the best strategy.
- Jim Hole