Dibotryon morbosum or Apiosporina morbosa is an infectious organism which causes black knot.
It affects the cherry and plum trees of Canada.
The disease produces rough, black areas which surround and kill the infested parts of the tree or shrub and provide habitat for insects.
In the late nineteenth century, it was a very destructive disease for plums and cherries.
These days, it is relatively well controlled though seen in poorly managed orchards. We see it frequently on choke cherry shrubs during our walks.
Olive-green swellings can be seen in late spring. As it spreads and matures, the rough black knots circle and kill the affected parts.
Older knots can kill trees by promoting insect infestations.
These abnormal outgrowths will produce and release a vast amount of spores during the bloom period, resulting in a rapid increase in infections. The fungus continues to grow internally and externally, with the branch eventually becoming girdled and dying.
Pruning infected parts during winter and spraying buds with a fungicide are forms of treatment.