02/21/2026
Time to prune for Valsa canker.
I bring up Stigmina needlecast of spruce on a regular basis, but it is not the only important problem that spruce can have. There is a branch canker disease that is called Valsa canker (old name is Cytospora canker). If it is present in mature trees or in young trees that are not very vigorous, it can cause very serious damage. it can also cause damage in vigorous trees that are planted near diseased trees. Even though I consider Stigmina needlecast to be the most damaging spruce disease overall in the upper midwestern states, Valsa canker is the one that takes priority for management if it is present. In Fargo, I think Valsa canker is the overall most important disease of spruce.
Valsa canker is characterized by its killing of individual branches, with all of the needles on that branch discoloring, dying, or falling off .all about the same time. The first branches affected tend to be near the base of a tree. When more branches are affected, they can be right next to a branch that already died, or can be on the other side of the tree or higher in the tree.
The most important single action to effectively manage Valsa canker is proper pruning. The dormant season is the best time to prune because there is less danger of causing wounds on other branches and less danger of those wounds becoming infected. Pruning can be done any time of the year, but if done during the growing season, prune at a time when it will not rain for at least 24 hours and be very careful to cause little wounding to healthy branches.
Because of the way infections occur and branches die, careful pruning is required for two consecutive years to get ahead of it. After that, relatively few branches will die each year, and they should be removed each year.
Branches that died from other causes can also be infected by the Valsa canker pathogen, so they should also be pruned out. All spruce branches should be destroyed (e.g., burning or mulching) or placed where the fungus cannot spread to healthy branches (buried or moved at least 1/4 mile away).
If regular pruning is done to manage Valsa canker, then actions that improve the vigor of trees so that they are less susceptible in the future. If there are other problems, especially Stigmina needlecast, actions to reduce damage by the other problems will help maintain tree vigor. If shoot growth or needle length is relatively short, fertilization can help improve vigor. Mulching around the base of the tree, out to the drip line, is usually very beneficial to the tree. Thinning out trees or pruning up alternate trees so there is less competition for soil nutrients and water by the roots and less competition for light by the needles will improve tree vigor. White spruce and Black Hills spruce are less susceptible to Valsa canker, so they should be favored when thinning or pruning. Avoiding exposure to herbicides, especially dicamba (Banvel) and glyphosphate (Roundup) is beneficial.
Pruning alone can bring Valsa canker under control, and if pruning is not done, then other actions will not be very effective.
If you have spruce trees that have dead branches, the trees might have Valsa canker. This disease tends to kill more and more branches each year that it is not managed. If left unmanaged, Valsa canker often results in loss of the benefits expected from a tree.
So, if you have spruce trees that you want to continue to keep healthy, check to see if they have Valsa canker. If so, now is a great time to prune out the dead branches.
