Canker diseases are common, widespread, and destructive to a wide range of trees and shrubs. A ‘canker’ is really a symptom of an injury often associated with an open wound that has become infected by a fungal or bacterial pathogen. Canker diseases frequently kill branches or structurally weaken a plant until the infected area breaks free, often in a wind or ice storm. Some of the more common cankers are Cytospora canker found on spruce, pine, poplars and willows, Phomopsis canker found on juniper, Russian olive, Douglas-fir, and arborvitae, and Nectria canker found on honey locust, oak, and maple.
Cankers are usually oval to elongate, but can vary considerably in size and shape. Typically, they appear as localized, sunken, slightly discolored, brown-to-reddish lesions on the bark of trunks and branches, or as injured areas on smaller twigs.
The bark often splits between the diseased and the healthy tissue, and sometimes it may ooze sap or moisture. The inner bark turns black and sometimes gives off a foul odor. The newest leaves on affected branches are usually the first to show decline symptoms. Leaves may appear smaller than normal, pale green to yellow or brown, often curled and sparse. As the fungal pathogen invades bark and sapwood, the water-conducting tissues (vascular system) become blocked or dies, causing wilting and dieback to occur. Cankors are formed by the interaction between the host and pathogen. The pathogen grows within the wood and the host tree tries to contain the growth. Cankers can take months (or years) to enlarge enough to girdle twigs, branches, or trunks.
Canker and stem dieback diseases are most common on trees and shrubs under stress. Damage results when opportunistic, living (biotic), infectious pathogens (fungi or bacteria) enter a wound during a time of plant stress, such as transplant shock, drought, or winter injury. Other stress agents that provide opportunities for canker diseases include prolonged exposure to extremely high or low temperatures, flooding, summer or winter sunscald, hail, high winds, nutritional imbalances, soil compaction, mechanical injuries (lawn mower, vehicles), animal damage, pruning wounds, root rot, insect borers, and improper planting. Most cankers are caused by fungi, which invade bark tissue on current season wood. However, some colonize both bark and inner tissue causing canker rots that persist for years. All fungal cankers contain fruiting bodies that appear as pinhead-sized, black or colored (red-orange on Nectria canker) raised bumps embedded in the bark. When present these are an important diagnostic characteristic. Unfortunately, fruiting structures are not always present and many are not easily distinguished. The spores produced by these fruiting bodies serve as inoculum for new infections, mostly in wet or damp weather.
Cankers are difficult to control. No chemicals are universally registered for treatment of cankers. The best controls are preventative ones to keep plants healthy or to prune out the diseased plant parts when practical.
Grow only trees and shrubs that are adapted to the area and site, and select resistant varieties.
Keep plants healthy and vigorous through proper planting, mulching, watering, soil management, pruning, and winter protection practices.
Avoid all unnecessary bark wounds, because many pathogen’s main entry is through injuries.
If a canker infection occurs on twigs or branches, carefully remove the affected parts several inches behind the infection. Pruning cuts should be made at the branch collar and avoid leaving stubs.
Do not prune when the bark is wet, to reduce spread of the fungus. Pruning tools should be sterilized between cuts using rubbing alcohol or 10% household bleach.
Once a trunk canker develops, the tree may begin to seal off the area by forming a callus around the canker. Avoid cutting into such cankers because it may renew fungal activity and increase damage. Any type of trunk canker removal is best left to a professional certified arborist.