Black Knot Fungus

Posted by shane - January 25, 2011 - Diseases, Insects, Diseases & Other Problems - No Comments

Black knot in mayday at commercial lot in Red Deer. Shane LePage photo.

 Black knot is one of the most commonly encountered diseases among ornamental plantings.  I see it most often on mayday and ‘Schubert’ chokecherry trees.  Besides poor planting techniques and poor plant quality, black knot is the most common cause of plant death among those species.

Easily recognizable, black knot starts as swellings on the twigs in the fall, after being infected earlier in the spring.  These swelling often go unnoticed until they begin growing larger the following year.  When conditions are right, trees will often become infected year after year and require significant care and maintenance in order to remain healthy.

Pruning and sanitation are the preferred treatments.  I prune out all “knots” back to a lateral branch, making sure that I cut back at least 6″ from the bottom-most part of the “knot”.  You can see the brownish discolouration in the wood that results from the infection.  Keep cutting back until you no longer see the discolouration.  In trees that have a “systemic” infection, that won’t be possible unless you remove the entire tree, which is exactly what I do in severe cases.  Make sure all infected branches are removed from the property.  Ideally, cut branches should be buried or burned, but I have yet to see or hear of anyone who does this.  Tree services will chip the infected branches, which later become part of mulch or compost piles.  Many branches end up at a local dump and are either chipped or buried.

Through regular observation and pruning, this disease is easily controlled and, provided you planted the tree properly in a suitable site, you can have a healthy mayday or ‘Schubert’ for many years.  All too often, I am called in to prune an infected tree after the disease has spread beyond the point of complete control.  To make matters worse, many people do not follow through with regular maintenance, and the disease quickly recurs, and spreads to neighbouring trees.

I have seen mayday trees in commercial parking lots, like the Galaxy Cinemas on St. Albert Trail in Edmonton, that are so heavily infested with black knot, that they take on an ornamental appearance, perhaps similar to Christmas lights.  The trees in the surrounding neighbourhoods haven’t much of a chance with a breeding ground like this nearby.  Annual maintenance is critical.

Acreage owners and city dwellers that border natural areas should be warned that native chokecherries often harbour the black knot fungus.  The disease will easily spread from the forest to landscape trees.  Check your trees for black knot each season.

(c) 2011 Shane LePage, Wild Rose Garden & Tree Service, Red Deer, AB