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A plant disease in which there is a well defined area of necrosis of the cortical tissue, which becomes surrounded by layers of callus tissue. Cankers occur mainly on woody stems but may also appear on herbaceous plants. For example, in apple canker, caused by Nectria galligena, the fungus penetrates the apple stem through a wound or leaf scar. As the fungus grows into the stem the tree produces callus tissue to seal off the infected zone. Further penetration by the fungus results in the production of more callus tissue, or wound wood, forming the characteristic canker.
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