CANKERS ON TREES
Fig. 1. Canker on maple tree
Photo: Chad Behrendt
Cankers are localized dead areas, which may appear on the branches, twigs, or trunk of a tree. They can be caused by mechanical damage (especially weed whips and lawn mowers), environmental conditions (frost cracks, sunscald etc.), chemical injury, insects, or microorganisms (fungi and bacteria). Cankers may appear sunken on young and thin- barked trees or hidden on older thick-barked trees. On young or smooth-barked trees, the surface of the canker may appear discolored. Callus tissue formed around the canker may cause excessive enlargement of the stem, while some perennial cankers form a target-shaped lesion (Fig. 1). The size of the canker can range from small inconspicuous lesions on branches to massive dead areas on the trunk.
Fig. 2. Reproductive structures of canker
Photo: U of MN Plant Disease Clinic
Small, cushion-like reproductive structures can often be seen on the surface or along the margins of the fungal canker (Fig. 2). Spores released from these structures during wet weather infect trees through natural openings or wounds. Trees under stress are especially prone to infection and canker development.
Cankers caused by environmental conditions or physical injury are often invaded by saprophytic fungi, which grow on dead tissue. These fungi may form reproductive structures on dead wood, complicating the diagnosis.
Cankers on young trees and fruit trees can cause serious damage and may kill the tree. Cankers seldom kill established shade trees but can deform and weaken them, making them more susceptible to wind-throw and invasion by wood decaying fungi. However, few fungi are capable of invading healthy trees and killing them. One example is chestnut blight. The fungus responsible, Endothia parasitica, decimated American chestnut trees earlier this century.
However, few fungi are capable of invading healthy trees and killing them. Most healthy trees respond to injuries quickly and form a defense barrier that halts further expansion of the canker. Stressed trees may not be able to form this defense, so cankers spread rapidly. Some fungi, halted by defense mechanisms of the tree, are able to break through the host's defenses the next year and reinfect healthy tissue. The production of callus tissue followed by the reinfection of the healthy tissue results in the formation of perennial cankers (Fig. 1).
Most canker-causing fungi attack stressed or injured trees. Therefore, the best treatment for cankers is PREVENTION. Keep trees healthy and prevent wounding. In winter, wrap thin- barked trees, such as maples and apples, to help prevent sunscald and frost cracks. In periods of low rainfall, water trees thoroughly. Do not plant trees too close together, since overcrowding causes stress in trees competing for water and nutrients. Proper fertilization and removal of dead wood is also beneficial.
In the case of infectious cankers, remove branches six to 12 inches below the canker margin. Dead or dying branches should also be removed. Prune during dry weather to minimize the spread of the disease.
Chad Behrendt, Crystal Floyd