Witches Broom on Picea engelmannii (Engelmann Spruce)
Eagle Peak Trail August 20, 2007.

Witches Broom appears on a number of trees: maples, willows, pines, and spruces.  The Broom is abnormal tissue growth that results from some disturbance to the tree or from a genetic mutation.  The disturbance can be biological, i.e., an organism such as a parasite might trigger the abnormal growth or it can be environmental, i.e., drought might trigger the growth.

The broom occurs in large masses such as shown above or in much smaller clusters that go unnoticed.