What is this stuff???!!!!

So Springtime in Alberta is beautiful.  Fresh clean air, blue skies, birds chirping, tulips starting to bloom and your Mayday Tree should be blooming just around the….wait a second.  WHAT IS THAT STUFF???  

Ah yes.  Black Knot.  A client recently commented that it looks as though someone tossed dog poo into a tree (yick!)…but it is rather disgusting stuff. Black Knot is a fungal infection (Apiosporina morbosa), that infects trees in the Prunus genus.  What does that mean for us here in Edmonton?  Well.  In the city, mostly Maydays and Schubert Chokecherries are getting this nasty affliction.  However, there are other trees that are at risk and a close eye should be kept on them as well.  They are (and this is not in any order);  Amur Cherry, Apricot, Mongolian Cherry, Black Cherry, Nanking Cherry, Chokecherry, Cultivated Plum, Flowering Almond, Wild Plum, Flowering Plum, Prunus Hybrids, Japanese Plum, Sand Cherry, Korean Cherry, and Sour Cherry.

Have I seen it on this other list of trees that are at risk?  No.  Not yet.  I’m crossing my fingers.

Now for the good news.  You can keep Black Knot at bay.  All it takes is a sharp eye, patience and pruning.

However, a little info about Black Knot.


Black Knot is a plant pathogen.  What’s that? A disease.  It is caused by the fungus Apiosporina morbosa. It starts out as a slight swelling or gall (lump) on the tree branches.  In 2 – 3 years it matures, turning first green, then hard, black and rather disgusting looking.  Mature Black Knot galls will produce and release spores during the blooming period of the tree (May), which will increase the number of infected trees in the surrounding area as well as the tree of origin.  If the fungus is not controlled it will continue to grow until it is an internal, systemic problem, which at that time, the tree has a limited life span and the fungus will rapidly fill up the tree.  At this point it is beyond saving and it is suggested to get it removed by a certified Arborist.

So.  How can we control this?

  •  If you have it, PRUNE IT. If you struggle with knowing HOW to prune, call a gardener or an arborist.  Your local garden centers (I recommend Greenland Garden Centre in Sherwood Park for good info) will have plenty of information and tools that you will need if you decide to go on this adventure solo.
  • When should I prune? Prune out all black knot bearing branches during late fall, winter and in early spring.  When the trees leaf out it is far more difficult to see the diseased parts.
  • DESTROY THE DISEASED WOOD IMMEDIATELY.  Burn it. Bury it.  Or remove from the site.  Don’t toss it into your neighbors yard or the ravine!!  Also taking it to the Eco Station is good too, they will dispose of it properly.  Don’t attempt to chip your diseased tree and use it as mulch either.  It will spread the fungus!
  • Talk to your neighbors. Get your community involved! Is there a tree in your neighborhood that is infected?  That one tree can be providing spores that will infect all of the trees in quite a large radius as well as trees in our beautiful river valley.  It might be time to have THAT conversation.  A bit nervous about that?  Contact the City of Edmonton (311) and ask for advice on what to do.
  • Are there chemicals that can help?  No.  And no, tears don’t work either. The best advice is to make sure your tree is healthy ie. has a healthy immune system, is well watered and fertilized.
  • Happy Black Knot Hunting!


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