Cytospora canker, caused by the fungus Cytospora kunzei (also known as Valsa kunzei var. piceae), is the most prevalent and destructive fungal disease of Norway and Colorado blue spruce. Occasionally, Cytospora canker is found on Douglas-fir, hemlock, and larch. Susceptibility varies widely among species, but generally trees under stress or growing outside their natural range are more prone to the disease. Cytospora canker rarely affects trees less than 15 to 20 years old. Infected trees are weakened substantially, but are rarely killed.
The disease normally starts on the lowest branches of the tree and, over a period of several years, progresses upward. At first, needles have a purplish hue, eventually turning brown and dropping, leaving dry, brittle twigs and branches.
On severely infected trees, the fungus will enter the trunk through wounds (usually where the branch meets the trunk of the tree), killing the cambium layer and leaving dead bark. This dead tissue is called a “canker.” A conspicuous white resin or “pitch” covers the cankered portion of the branch or trunk, sometimes flowing several feet down the trunk of the tree. This is an important means of diagnosing Cytospora canker; however, resin flow can also be associated with other tree injuries and is not exclusively symptomatic of Cytospora canker. Within the cankered area, black, pinhead-size fruiting structures (pycnidia) of the fungus can be seen with a microscope or hand lens and are a positive sign of the disease.
The fungus survives long-term as mycelium and spores in diseased stems. The canker grows slowly, eventually circling and killing a twig or branch. The fungal spores (conidia) are the principal means by which the disease spreads to other branches, entering through bark wounds and injuries. Infections occur in cool, wet weather. Spores are dispersed by splashing rain, wind, sprinklers, pruning tools, and possibly by movement of insects and birds. Canker development is most severe in trees under stress from drought, insect damage, crowding, nutrient imbalance, and mechanical damage to branches, trunks, or roots. Symptom development becomes more common one or two years following a severe summer drought.
Because Cytospora canker is a stress-induced disease, trees should be planted in sites that are favorable to their growth (e.g., avoiding places where they become too crowded). Minimize stress of established trees by taking care not to injure the root system or compacting the surrounding soil. Use a three-to-four-inch layer of organic mulch to retain moisture and reduce rapid soil temperature fluctuations. Water well in dry periods and provide adequate moisture in late fall before the ground freezes. Improving soil quality will reduce stress. Infected branches should be removed to improve the appearance and reduce the chances of further spread. Avoid pruning or working around trees when foliage, twigs, and branches are wet because water disperses the fungal spores. Clean tools thoroughly and disinfect with rubbing alcohol, a 10% bleach solution or comparable disinfectant after each cut when pruning out diseased wood.
Chemical control is not useful in controlling this disease.