Cytospora canker of spruce

Pathogen and disease type. The disease is caused by the fungus Cytospora kunzei, which is classified as a necrotrophic pathogen. It is a common cause of branch and trunk cankers on conifers. The fungus invades the bark and cambium layers, disrupting the tree’s vascular system and causing significant tissue necrosis.

Host species. This fungus primarily attacks spruce trees (genus Picea), with Blue Spruce (Picea pungens) being particularly susceptible. Other conifers, including firs and larches, can also be infected, especially when they are under environmental stress, such as drought, nutrient deficiency, or physical trauma from landscape maintenance.

Symptoms and signs. The first visual sign is the needle dieback, where foliage turns brown and eventually drops off. On the branches, sunken, discolored cankers appear, often accompanied by heavy resin flow. During humid weather, small black or orange fungal fruiting bodies (pycnidia) can be seen on the affected bark, releasing spore masses that spread the infection further.

Development and impact. The fungus thrives in cool, moist conditions. Spores are primarily disseminated by splashing water, rain, and pruning tools. The impact on the tree is severe; as the infection progresses inward, it girdles the branches, cutting off nutrients. This results in progressive canopy loss, often starting from the lower branches and moving upward, eventually causing the tree to decline and die.

Prevention and management. Controlling Cytospora kunzei requires a proactive strategy. Key practices include:

• Pruning and destroying infected branches while the foliage is dry.
• Sterilizing all pruning tools with alcohol or a bleach solution between cuts.
• Providing adequate water and mulch to prevent tree stress.
• Applying preventive copper-based fungicides in early spring to protect susceptible trees.