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 thickbarked trees. On young or smoothbarked 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 targetshaped lesion. The size of the canker can range from small inconspicuous lesions on branches to massive dead areas on the trunk.