Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityDecember, 1995
Tree Trunks, Bark, Cankers, and Frost Cracks
Keywords: tree health, disease, prevention, weather, environmental problems/injury, insect damage, landscape, nursery
T runk injuries, cracks, and cankers may seriously compromise the health and lower the value of nursery stock. In the past year, I have seen many instances of trunk cracks and cankers in shade and ornamental tree species. In a few cases, large numbers of trees in nursery plantings were affected, and the cause unknown. Often when no pathogen has been isolated, the severe winter of 1994 has been blamed for the injury. In this article, I'll review some of the established causes of non-mechanical trunk injury.
First, a brief anatomy lesson. We first notice a canker or trunk injury by noting a bark irregularity, or by noting that the integrity of the bark is disrupted. The bark of a tree consists of inner and outer bark. Phloem cells make up the inner bark. The phloem tissue transports the carbohydrates produced in the leaves and other substances throughout the tree. The outer bark performs the critical function of protecting
all inner tissues from injury and infection. The vascular cambium lies just inside the bark, and consists of actively dividing cells that produce new xylem (wood) to the inside and new phloem to the outside.
Cankers caused by pathogens. Cankers are caused by the necrosis (death) of stem or trunk tissue. Pathogenic organisms cause many types of cankers in trees. Canker-causing organisms kill bark and cambium; some types also kill the xylem. Cankers may appear as sunken areas or lesions in the bark. Other types appear as round, elliptical, or long, narrow 'target' cankers with surrounding ridges of callus tissue. Some types of cankers may be accompanied by gum formation or bleeding.
The pathogens that form annual and perennial cankers are relatively weak. Trees wall off annual cankers in a year's time. The host and pathogen reach a standoff in the perennial cankers, whereby each year the tree grows and walls off the invader during the growing season, but during the dormant season the pathogen invades new tissue. In contrast, some types of canker-causing organisms are very strong pathogens that infect, girdle, and kill trees in a relatively short period of time. When a tree is girdled, the cambium is killed in a complete circle around the trunk. Girdled trees eventually die because the transport of photosynthates from the leaves to lower regions is interrupted.
Frost cracks and sun scald. Where canker-causing organisms have been ruled out as the cause of the injury, growers often suspect frost cracks or sun scald. Frost cracks appear as shallow to deep longitudinal cracks in the trunks of trees. The current view is that frost alone does not cause the deep cracks. To use the analogy of Alex Shigo, a trunk injury of some kind first creates a weakness that 'loads and cocks the pistol,' and the mechanical stress imposed by the frost event 'fires the shot.' Frost cracks occur on the south or southwest sides of trees because this area usually experiences the greatest variation in temperature. Commonly affected species include linden, London plane, elm, horsechestnut, oak, willow, crabapple, beech, walnut, and Norway maple. Drought has also been implicated in injury similar to frost cracks.
Sun scald develops on the south or southwest side of trees as well. In the winter, the temperature of the sun-warmed side of the trunk may exceed air temperatures by 20 F; this is thought to trigger deacclimation of trunk tissues. The tissues that are no longer acclimated to the cold subsequently die when temperatures plunge at night. Usually the bark and cambium are killed in a localized area, and the bark eventually cracks and falls away.
Sometimes only the cambium is killed and a sunken area appears on the trunk. Thin-barked trees are most often affected, including American beech, yellow birch, flowering cherry, maples, callery pear, white pine, weeping willow, and fruit tree species.
Tree wrap has been recommended in the past for use on young trees to protect them from sun scald. Unfortunately, tree wrap, when left on in the spring and summer, can provide a habitat for disease organisms. If tree wrap is to be used on a newly planted tree for winter protection, be sure to remove the following spring.
Cankers caused by herbicide injury. Glyphosate (Roundup), paraquat (Gramoxone-Super), and glufosinate-ammonium (Finale) are postemergence herbicides that should be used with care around woody ornamentals because they have been implicated in canker formation. These herbicides may be absorbed through the bark of young, thin-barked species that still have green, yellow, or red pigment in the bark. Tree trunks will not exhibit herbicide injury for a year or two after application.
Injury from paraquat or glufosinate-ammonium appears as a sunken canker; healthy tissue grows up and around tissue that was injured. Glyphosate injury appears as a slit in the bark. Depending on the amount of herbicide intercepted by the bark and species sensitivity, the split may extend a few inches or few yards. Glyphosate injury may be confused with frost cracks, however, cracks associated with herbicide injury may occur on any side of the tree.
Other causes of bark injury and cankers. The injury from some insect pests may mimic canker diseases. The bronze birch borer feeds on cambium tissue, which causes a sunken canker on the bark. Allegheny mound ants inject formic acid into the trunks of trees that shade their mounds; blister-like swellings appear at the base of the trunk, and eventually, trees are girdled and killed. Cicadas and related insects also cause injury by slitting the bark and inserting their eggs directly into the sapwood of twigs and branches. Finally, injury from hailstorms may severely
damage trunks and branches of young, thin-barked trees. Damage from hail can be very severe, appearing as multiple cankers along the length of the branch.
