We tend to see canker diseases in periods of stressful weather when leaves wither and branch dieback is more apparent. The diseases did not just suddenly appear: Cankers develop slowly over months or years when fungi invade the plant following injury or stress. Cankers may be initiated by injuries (hail, mowers, insect feeding, etc.), environmental stress (cold, heat, scald, etc), chemicals, or pathogens.
A canker is a dead area on the stem or trunk of a tree or shrub. Vascular tissue under the canker is dead. Cankers also may appear on herbaceous plant material, usually as sunken (dead) areas on the stems. The term "canker" is a general one referring to a symptom on the plant but does not indicate cause. Cankers are common on a wide range of trees and shrubs, typically occurring on trunks, older branches, or injured plant areas on smaller twigs.
One common canker diseases is Botryosphaeria canker. We see it on a wide range of ornamental plants and fruit crops. Oak, crabapple, sweetgum, dogwood, elm, and redbud lead the list at our clinic.
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. Look for these on drought-stressed trees now. Some young twigs may curl down-ward. Bark on younger twigs may lose color or black-en, depending on the canker or plant involved. When a canker girdles the stem, the twig dies from that point to the tip. If the stem is not girdled, it may show one-sided death, or some leaves are affected and others are green. Botryosphaeria cankers are usually cracked, dry, and discolored. Fruiting bodies of the fungus ap-pear as pinhead-sized black specks embedded in the bark. Often, these fruiting bodies appear as small bumps covering the canker.
Botryosphaeria and other canker fungi infect only stressed trees; therefore, canker fungi are known as stress pathogens. Because stress is the predisposing factor, the first step toward disease management is identifying the source of stress. Try to 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 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 pH for best plant growth. In other words, follow good horticultural practices.
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 the tree can more readily callous over the injury. Prune out stem cankers where possible to avoid entry of wood rot fungi into the trunk.
One last warning: In areas with oak wilt, we prefer to prune oaks in the dormant season to avoid attracting beetles that might bring the oak wilt fungus to the tree. It is probably safe to prune oaks after midJuly. A report on cankers and dieback diseases of trees is available in Report on Plant Disease (RPD), no. 636.