Issue 17, September 28, 2009

Clean up Tree Cankers Now

Now is the time to prune out dead wood and cankers from trees and shrubs. If you can get the task done before leaves fall, it is much easier to see the dead areas that need to be removed.

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 as described in the previous article on Cytospora canker. The term "canker" is a general term referring to a symptom on the plant, but does not indicate cause. Cankers on plants may be caused by injuries (hail, mowers, insect feeding, etc.), environmental stress (cold, heat, scald, etc), chemicals, or pathogens that invade stressed plants. They are common on a wide range of trees and shrubs, typically occurring on trunks, older branches, or injured plant areas on smaller twigs. 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. The images show two common cankers. 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 will appear as small bumps covering the cankered area. Bacterial cankers will not contain fruiting bodies.

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. 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.

Keep in mind that cankered wood is infected with a pathogen. Remove affected wood from the site. Disinfect pruning shears between cuts where possible, or at least prune diseased wood last. Always try to prune in dry weather to prevent pathogen spread. With oaks, we only prune in the dormant season to avoid attracting beetles that might bring the oak wilt fungus to the tree.

Once pruning is completed, consider how to avoid cankers and dead wood in the future. Since 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 conditions 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, 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 discusses canker and dieback of woody plants.--Nancy Pataky

Nancy Pataky

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