This fungal disease is one that can have a devastating consequence to many members of the plum or cherry family.  This can include Sandcherry, Flowering Plums, Chokecherry and Flowering Almond in addition to cultivated fruit trees like plums or cherries.

The disease is characterized by large black tar-like swellings on the branches.

At first, a small, olive-green swelling will develop at a soft growing point on the branch as a result of spores landing and infection happening. This swelling will grow until it is mature after a couple of years. The mature 'tumours' (or galls as they are known) are hard, black, and about 4 to 6 inches in size and may be somewhat ruptured. It is the mature galls that will produce and release a vast amount of spores during the flowering period of the tree, resulting in a rapid increase in infections within that tree, or on those susceptible species nearby. The fungus continues to grow both inside and outside this infected area, and when the tumour encompasses the branch, it the branch will die. If it is in the trunk, then of course that's pretty much it.

So what can you do to control Black Knot?

• Removal of sources of spores to prevents the fungal population from building up
• Prune out all knot-bearing branches during the late fall, winter or very early spring when the plant is dormant and the knots are easy to see. Checking for these knots should be a regular 'thing' in the early spring and in the late fall.
  
• Remove these infected branches to at least 15-20 cm (6-8 inches) below where the knot is.  It is preferable to prune the infected branch further back to where it would be suitably pruned (such as to healthy collar or node), rather than to leave a raw stub. It's not a bad idea to seal this wound with pruning sealant.
• As a precaution, pruning blades should be cleaned and disinfected after pruning, if possible, especially if cuts have been made through obviously infected materia. This can be done by dipping the pruners in a mix of 1 part bleach to 4 parts water.
• For knots that are on major branches or on the trunks and that can’t be easily removed, cut away diseased tissue down to good wood and at least 1 cm (1/2 inch) beyond the edge of the knot. A carpet knife might be used for this.
• If you don't remove the branches beyond the internal growth (which is hard to identify), it will result in re-growth of the fungus on the branch.
• DISEASED WOOD MUST BE DESTROYED IMMEDIATELY (burned, buried or removed from site), as diseased knots can produce and release spores for up to 4 months after their removal. While some would say that compost temperatures will kill the spores, it is not recommended to compost these trimmings.
  
• Feed and water the tree to keep its health and vigour. While this won't guarantee non-infection, a healthy plant is surely better able to fight the problem.
• Ensure adequate canopy ventilation through proper pruning of the crown. When in doubt, contact Grobe's to arrange for a Certified Arborist to be used.
  

At present, for homeowners in Ontario there are no chemical means by which to control this pathogen. Orchards have few such tools as well, and there is hope that breeders will, in time, manage to develop more disease resistant strains.  While some fruiting plum varieties that are resistant do exist, those growing plum trees in the home have been resistant to change to new, unfamiliar fruit varieties. This lack of demand has meant that fewer of these varieties tend to be available for the consumer market.