Disease name: canker
Pathogen name: Numerous species of fungi and bacteria can cause canker diseases.
Host: A wide range of trees and shrubs can be affected by canker organisms. Some of the more common cankers are Cytospora canker, found on spruce, pine, poplars, and willows; Phomopsis canker, found on juniper, Douglas-fir, and arborvitae; and Nectria canker, found on honey locust, oak, and maple.
A canker is really a symptom of an infection by various fungal and bacterial organisms. The pathogen often enters the host through an injury or open wound. Once the pathogen enters the wound, it can spread under the bark. Canker diseases frequently kill branches or structurally weaken them, making the infected branch more prone to storm damage.
Symptoms
Cankers are usually oval to elongate, but can vary considerably in size and shape. Typically, they appear as localized, sunken, slightly discolored or dark lesions on the bark of trunks and branches, or as injured areas on smaller twigs.
The bark eventually splits between the diseased and the healthy tissue, and it may ooze sap or moisture. The inner bark turns black as the tissues die. As the fungal pathogen invades bark and sapwood, the water-conducting tissues (vascular system) become blocked or die, causing wilting and dieback to occur. 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. Cankers can take months, or years to enlarge enough to girdle larger branches and trunks.
Disease Cycle
Canker disease organisms overwinter in existing cankers as spores. These disease organisms are not very strong and they generally need a weakened host. Canker diseases are most common on trees and shrubs under stress. Damage results when opportunistic fungal or bacterial pathogens enter a wound during a time of plant stress, such as transplanting, 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, animal damage, pruning wounds, root rot, insect borers, and improper planting. On plants under severe stress, a wound may not be needed for the canker organism to enter. In cases of severe stress, the disease organism may be able to enter through natural openings in the plant.
Most cankers are caused by fungi, which invade bark tissue on current season wood. However, some colonize both bark and inner tissue causing cankers that persist for years. All fungal cankers contain fruiting bodies that appear as pinhead-sized, black or colored raised bumps embedded in the bark. The spores produced by these fruiting bodies serve as inoculum for new infections, mostly in wet or damp weather.
Management
Cultural Management
Cankers are difficult to manage. The best management practices are preventative ones. Keep plants healthy and vigorous through proper planting, mulching, watering, soil management, pruning, and winter protection practices. Grow only trees and shrubs that are adapted to the area and site. If a canker infection occurs on twigs or branches, carefully remove the affected parts several inches below the infection. Pruning cuts should be made at the branch collar and should avoid leaving stubs. Avoid all unnecessary bark wounds, because many pathogen’s main entry is through injuries. Do not prune when the bark is wet, to reduce spread of the fungus. Pruning tools should be sterilized between cuts. 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.
Chemical Management
No chemicals are available for treatment of cankers.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).