Mature Black Knot
In recent years the spring season in Calgary has been windy and wet, two prime contributors to the outbreak of black knot disease in the city and surrounding areas. Owners and potential purchasers alike are in a fever pitch about the unattractive fungal overgrowth affecting their landscapes. First let me give you the good news. Black knot is not fatal in most cases. Like the flu, if the individual is weak, and symptoms are left unchecked, death can occur, but rarely is it the case. What is most distressing about black knot is how easily, and widespread it can occur, and how truly ugly in appearance it is.
You know what I am talking about, that “poop-on-a-stick” you see all over the park and neighborhood trees. What you are seeing is called a gall, and by the time it is black and highly visible it is 2-3 years old and ready to spread ascospores to susceptible trees within the prunus species. Caused by Apiosporina morbosa, or Dibotryon morbosa (same fungus, different names), it is an airborne fungus that requires wind to spread, and moisture (rain) to stick to the tender new growth on the tips of branches. This new growth is the only part of the tree that can be infected each year. This is good news, as it provides a year to year and a half window of opportunity to prune, and therefore control the spread of black knot. Galls begin to form the first year and become visible in the fall or early spring of the following year. They are small, and green, to light brown in appearance. This is the time to prune as the spores are not mature and viable until the gall is 2-3 years old.
Immature Black Knot
When pruning the immature gall, cut 8″ below the knot, and disinfect your pruners between EACH CUT with a 1:10 solution of bleach and H2O. If you are pruning a mature gall, some arborists recommend 12″ below as the fungus may have extended through the branch itself. Be sure to burn or bury the branches immediately after pruning. Spores can live in a pruned branch for up to 4 months.
Black Knot on trunk
In some cases black knot will appear on the trunk or other critical juncture on the tree. In this instance, hire a certified arborist to remove the damaged tissue.
The best way to control black knot is to prune diseased branch tips as soon as evidence of the disease it noticeable. Do not prune unaffected branches, or you will lose all your fruit bearing capacity for the year! Be sure to check your Nanking Cherry, Sandcherry, and flowering plum shrubs along with Mayday, Schubert Chokecherry, Amur Cherry and Sour Cherry and Plum trees for immature, green galls.
The winter here has been so mild and warm, creating a perfect window to eradicate black knot before the spring season. I encourage you to prune on the next double digit day. The spores that carry black knot fungus are spread throughout the blooming period and up to two weeks after. Best to rid your trees of black knot before it can spread. Good luck!