Now is the time to prune out dead wood and cankers from trees and shrubs. You will probably get scratched up a bit more now than once leaves fall, but it is much easier to see the dead areas to remove.
A canker is a dead area on the stem or trunk of a tree or shrub. The vascular tissue under the canker is dead and usually brown or black. The term "canker" is a general term referring to a symptom on the plant, but it does not indicate cause. A human analogy is the canker sore we get in or around the mouth. Cankers on plants may be caused by injuries (hail, mowers, insect feeding, etc.), environmental stress (cold, heat, scald, etc.), chemicals, or pathogens. They are common on a wide range of trees and shrubs, typically occurring on trunks, older branches, or injured plant areas on smaller twigs.
As the canker girdles the stem, leaves begin to wilt, turning yellow and then brown. Some young twigs may curl downward; their bark 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 dies from that point to the tip. If the stem is not girdled, the stem may show one-sided death or some leaves are 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 bleeding sap or moisture.
Fungi are usually the causal organisms involved in canker development, but occasionally we find a bac-terial 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, infecting only trees under stress. For this reason, canker fungi are known as stress pathogens. Canker pathogens enter through environmental injuries such as sunscald (summer or winter) or through injuries caused by insects, diseases, pruning, animals, and 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, etc.
Remove cankered wood, cutting until you leave only healthy wood on the branch. If cankers occur on the trunk, you may opt to leave them alone or remove as much of the decayed wood as possible so that the tree can more readily callous over the injured area. You can cut off spruce branches that die from Cytospora canker right up to the trunk, but you cannot remove infection from the trunk. 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 where possible. Always try to prune in dry weather to prevent pathogen spread. With oaks, we prune only in the dormant season to avoid attracting beetles that might bring the oak wilt fungus to the tree. Now is the time to prune oaks.
Once pruning is completed, consider how to avoid cankers and dead wood in the future. Because stress is the actual predisposing factor for cankers, the first step toward disease management is identifying the source of stress. Once the source is identified, correct or modify the site, soil, or surrounding plants to make the conditions less conducive to cankers. This approach might involve diverting drainage away from the plant, pruning surrounding plants to allow better air flow, fertilizing the tree, providing water in drought, etc. 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. Report on Plant Disease, no. 636, discussing canker and dieback of woody plants, is available on the University of Illinois Extension VISTA Web site or in local Extension offices.