Friday, March 18, 2011

What Are the Black Growths on Tree Branches?

Black Knot Gall

Every time I've taken a walk in the woods this winter, I've noticed this distinctive black growth on a number of trees and shrubs.  Today I took a photo to remind me to find out what it is.

It's a gall, caused by a fungus that has two names:   Dibotryon morbosum,  Apiosporina morbosa.  Commonly known as Black Knot disease, it's found only on Prunus species - cherries, plums and apricots.  The presence of this gall is an easy clue to winter identification of these trees.  

In the spring, fungus spores discharged from sacs on the black knots infect new shoots after the buds pop.  The "knots" are visible in the fall and into the second spring.  They turn black during the second winter.

After a year or so, the black knots may also host a whitish pink mold and insects.


1 comment:

  1. I have these all over one of our trees in our yard. I attempted to prune them early spring and now more are emerging. How do I treat this tree?