We've mentioned cankers in various articles and discussed Cytospora canker of spruce in issue no. 9 of this newsletter. Cankers are dead areas of the vascular tissue and surrounding wood of a tree or shrub, or even field crops. The term "canker" is a symptom, like "wilt" or "leaf spot." Cankers may be caused by injuries (such as from hail or mowers), environmental stress (cold, heat, scald, etc.), chemicals, or pathogens. We see cankers on a wide range of trees and shrubs. Typically they occur on trunks, older branches, and injured areas on smaller twigs.
Fungi are usually the cause of cankers on stressed plants, but occasionally we find a bacterial canker. The fungal cankers contain fruiting bodies of the fungus, a very important diagnostic characteristic. The fruiting bodies contain the spores of the fungus. The size, shape, and color of these fruiting bodies, as well as the spore characteristics, allow us to identify the particular fungus. The fruiting bodies are pinhead-sized black or colored specks embedded in the bark. Often these fruiting bodies will appear as small bumps all over the cankered area. In wet weather, they exude spore masses or tendrils, often brightly colored (bright red-orange on Nectria canker).
Most canker pathogens enter the host through an injury caused by sunscald, insect feeding, pruning, weather extremes, chemical sources, and the like. Weakened tissue from poor growing conditions, transplant shock, water or temperature extremes, nutritional imbalance, or extensive defoliation also provides entry points for the pathogens.
The youngest leaves are usually the first to show decline. Leaves wilt, turn yellow, and finally brown as their water supply is cut off. Some young twigs may curl downward. The bark may be discolored or blackened depending on the canker and host involved. If a canker girdles the stem, the twig will die from that point outward. If the stem is not girdled, it may show a one-sided death, or some leaves will be affected while others are green. Cankers usually take months (or years) to enlarge enough to girdle twigs, branches, and trunks. They may appear swollen, sunken, cracked, or discolored, and they may bleed sap or moisture.
If your plant has cankers, try to determine why they are present. If you can determine the cause of the cankers or stress, then you can try to alleviate those conditions. Next, determine whether or not the cankers need to be removed. If they are on the trunk, you may either leave the area alone or remove as much of the decayed wood as possible so that the tree can more readily form callous tissue over the injured area. Prune out stem cankers if they are unsightly or when 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.
You can help avoid cankers on trees and shrubs by heeding the advice you've been hearing for years. Choose plants adaptable to local growing conditions. Plants growing out of their hardiness zone may do well some years, but they will be more prone to winter injury and more likely to have canker problems. Plant trees and shrubs at the proper depth, at the proper spacing for mature size, and in sites for which they are suited. For more information on cankers, consult Report on Plant Diseases No. 636.