A canker is a dead area on the stem or trunk of a tree or shrub. The vascular tissue under the canker is dead as well. Cankers also appear on herbaceous plant material, usually as sunken dead areas on the stems. The term canker is a general term referring to a symptom on the plant but does not indicate cause. Cankers may be caused by injuries (hail, mowers, insect feeding, etc.), environmental stress (cold, heat, scald, etc.), chemicals, or pathogens. They are common on a variety of trees and shrubs, typically occurring on trunks, older branches, or injured plant areas on smaller twigs.
If the canker itself goes unnoticed, the newest leaves are usually the first clue to a problem. As the canker girdles the stem, leaves begin to wilt, turn yellow, and then brown. Some young twigs may curl downward. Bark on the younger twigs may lose color or blacken, depending on the canker or plant involved. The cankers produced by fire blight are often black on pear and brown on apple. If a canker girdles the stem, the twig will die from that point to the tip. If the stem is not girdled, the stem may show one-sided death or some leaves will be affected while others are green. Cankers usually take months, sometimes years, to enlarge enough to girdle twigs, branches, and trunks. Canker appearance may be swollen, sunken, cracked, discolored, or may bleed sap or moisture.
Fungi are usually the causal organisms involved in canker development, but occasionally we find a bacterial canker. The fungal cankers often contain fruiting bodies of the fungus. These appear as pinhead-sized black specks embedded in the bark. Often these fruiting bodies appear as small bumps covering the cankered area. In wet weather, they may exude colorful spore tendrils. Bacterial cankers do not contain fruiting bodies.
Although we find a pathogen in association with many cankers, the pathogens are usually opportunistic fungi. They do not cause problems on healthy trees. They only infect trees under stress. For this reason, canker fungi are known as stress pathogens. Canker pathogens enter through environmental injuries such as sun scald (summer or winter) or through injuries caused by insects, diseases, pruning, animals, mechanical and chemical sources, or through weakened tissue caused by poor growing conditions, transplant shock, excess or deficient soil moisture, rapid temperature changes, nutritional imbalance, extensive defoliation, and so on. Because stress is the actual predisposing factor for cankers, the first step toward disease management is identification of the source of stress. Once the stress is identified, correct or modify the site, soil, or surrounding plants to make the con-ditions less conducive to cankers. This might involve diverting drainage away from the plant, pruning surrounding plants to allow better air flow, fertilizing the tree, providing water in drought, and so on. Reduce risk of cankers by using plants adapted to your area. Buy vigorous, healthy-looking plants. Plant at the proper depth. Space plants based on mature size. Grow plants in well-drained, fertile soils with the needed soil pH for best plant growth. In other words, avoiding cankers is one of the major reasons for following all of those good horticultural practices we have all learned.
Once a canker problem is noticed, you have the option of leaving the canker alone or trying to remove the affected area. If it is on the trunk, you may opt to leave it alone or remove as much of the decayed wood as possible so that the tree can more readily callous over the injured area. Prune out stem cankers where aesthetically unappealing or where it is obvious that they will soon girdle the stem. Some cankers, such as anthracnose on sycamore, cannot be removed without removing most branches. Leave these on the tree and take measures to promote tree health.
When pruning out cankers, keep in mind that this wood is infected with a pathogen. Remove affected wood from the site. Disinfect pruning shears between cuts when possible. Always try to prune in dry weath-er to prevent pathogen spread. With oaks, we only prune in the dormant season to avoid attracting beet-les that might bring the oak wilt fungus to the tree. A report on cankers and dieback diseases of trees is available in RPD No. 636.