The most common canker disease observed in the Iowa State University Plant and Insect Diagnostic Clinic is Cytospora canker on blue spruce. Cytospora canker is observed most often on older trees, especially those that are planted in poor sites. Trees weakened by environmental stresses, such as drought, freeze injury, or high temperatures, also are more susceptible to canker diseases. The Cytospora canker fungus may attack many different species of hardwood trees, conifers, and shrubs.
Spruce trees infected with the Cytospora canker fungus typically show scattered branch dieback, often starting on the lower branches. A close look at the dead branches usually reveals the presence of sticky white sap. Infected trees produce this resinous sap in response to the infection by the canker fungus.
The Cytospora fungus gains entrance into branches or twigs of trees through wounds or branch stubs. Over time, the fungus encircles or girdles branches, causing death. Brown needles can be observed on killed branches, but they eventually fall off, leaving bare branches.
As with many diseases, the best control for Cytospora canker is prevention. Plant trees in a good site, one that is well-drained and allows unrestricted growth as the tree matures. Adding mulch around trees increases overall health in many ways, including reducing competition from turfgrass. If dry conditions occur, water deeply if feasible. Any cultural practice that promotes good tree vigor helps prevent canker diseases.
Pruning out diseased branches is the primary means of treating trees showing symptoms of Cytospora canker. Scout declining trees closely for cankers. Prune at least 4-6 inches below any visible cankers. Some branches may need to be pruned back to the trunk. To minimize spread of the disease, prune only during dry weather. The fungal spores of Cytospora can be easily spread when conditions are wet. Fungicide sprays are generally not effective at controlling canker diseases.
For more information on canker diseases, see this bulletin.