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Fungal Twig and Limb Cankers
In addition to fire blight (caused by a bacterium), twig and limb cankers are caused by several different fungi. It will not be necessary to determine the actual cause of cankers other than fire blight; however, it is important to note the occurrence of cankered limbs when they are observed. Many fungi that cause limb cankers also cause fruit rot diseases.
SYMPTOMS: Generally, cankers will appear as small to large sunken areas on twigs, limbs and/or tree trunks. Depending upon the cause of the cankers, the sunken areas may exude a liquid or develop sporulating structures which in turn exude a gelatinous material laden with fungal spores. This is especially true during humid, rainy weather. Cankers may become cracked or appear to blister in a way which is not characteristic of "normal" apple bark. Many times cankers will be associated with lenticels or tree wounds, especially those resulting from improper pruning techniques. Additionally, healthy wood bordering cankered tissue will often be swollen with callous tissue as the tree attempts to "heal" cankered wood.
DISEASE CYCLE: Canker-causing fungi normally overwinter in diseased or dead wood on the tree, and produce and disseminate spores during rainy periods in the growing season. New infection sites then become sources of additional spore production. Weakened or poorly-growing trees are especially susceptible to fungal canker diseases.
SCOUTING: Be on the lookout for twig and limb cankers as you scout for leaf and fruit diseases, and record their occurrence in the comments section of your scout form.
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