It occurs mostly on older trees that have begun to lose their natural resistance to environmental stress. The fungus produces spores that are spread to open tree wounds by rain and wind.
Infection produces a dark, sunken, dead area on the bark, known as a canker.
The lower portion of a Spruce is usually affected first. The disease then slowly progresses up the tree. Cankers on White and Colorado Spruce usually start at the base of a lower branch, eventually killing the entire branch. Less commonly, Norway Spruce is subject to trunk cankers which can girdle the entire tree. Shallow roots are also susceptible to the canker.