Cytospora canker is a disfiguring disease of spruce and other conifers. This disease, caused by the fungus Leucostoma kunzei, invades trees that are weakened by environmental stress such as winter injury or drought. In Minnesota, Colorado blue spruce is commonly affected. Infection does not usually begin until trees are 10-15 years old. Infected trees may be severely damaged as they mature.

The fungus initially infects trees through wounds or dead branch stubs. Once established in branches, the fungus produces reproductive structures in the dead tissue. During periods of wetness throughout the growing season, these structures release gelatinous threads of spores. These spores are dispersed to nearby branches by rain, wind, humans, and possibly insects.

Fig. 1 Photo: U of MN Plant Disease Clinic Fig. 2 Photo: U of MN Plant Disease Clinic

Infection usually begins in the lower branches and gradually moves upward (Fig. 1). Cankers, localized dead areas, often form at the base of small infected branches and continue to expand along the branch (Fig. 2). These cankers often appear elliptical or diamond-shaped. Eventually, cankers appear sunken with a ridge of callus tissue surrounding them. Cankers girdle the branch, disrupting the flow of water and nutrients, which causes dieback of the branch from the point of infection outward. Copious amounts of resin may exude from the cankers, run down the bark, and harden into a bluish-white cracked crust.

Dieback is initially observed in the spring and early summer as yellowing needles. Later in the season, these needles turn purple to brown. Needles drop off during the winter leaving the infected branches bare. Trees are seldom killed by the disease, but can become misshapen with bare, often distorted branches (Fig.1).

Chemical control of Cytospora canker is not available. Since trees under stress are more susceptible, the best way to prevent this disease is to maintain good tree growth and vigor. Water trees regularly during dry periods to alleviate drought stress. This is believed to be one of the most important stress factors predisposing spruce to Cytospora canker. Occasional fertilizing will also improve tree vigor on poor soils. Care should be taken in selecting a proper planting site. Avoid overcrowding or planting trees too close to buildings.