The flared callus ridges of infected maples reminded foresters of the hood of a cobra. Commonly called cobra canker, Eutypella canker, caused by the fungus Eutypella parasitica infects red, sugar, silver, Norway, and other maples. Eutypella destroys the cambium; As the tree compartmentalizes the pathogen in the spring and summera ridge of callus develops around the infected area. A dead branch stub in the center of the swollen, cankered area is often visible in young infections, but decays over time.

Young maple trees in urban and ornamental plantings are susceptible to this fungus, with between 2% and 10% of the trees infected. In addition to the loss of aesthetic value, the tree is susceptible to attack by wood decay fungi and then to wind damage, becoming a hazard to people and property in the vicinity.

Symptoms and Signs. The canker is commonly observed between 3 and 10 feet above the ground, although basal cankers (from lawn mower and weed whip injury) do occur in the urban landscape. A large roughened and sunken area of bark with heavy callus around the margin is observed on the branch, around a branch stub or main trunk. Embedded in the bark, black perithecia protrude from the bark near the centers of older cankers. Removal of the bark exposes a cream-colored to tan mat of fungus at the periphery of the canker.

Life Cycle. Initial infection commonly occurs at a branch stub, and the fungus must invade the cambium and sapwood to establish infection. Every year, the tree produces callus during the growing season to compartmentalize the pathogen. The fungus kills this callus and invades more cambium and bark during the tree's dormant season. In the center of older cankers, the fungus produces fruiting bodies that produce ascospores during wet spring weather. The fungus also produces a sickle-shaped conidia, but these spores are thought to be unimportant in the disease spread. Occasionally, the tree “wins,” with the canker compartmentalized, and the fungus dying within the tree. More often, the canker serves as a weak point that fails during high winds.

Management. Remove all cankered branches, cutting at least 6 inches below the canker when the weather is dry to prevent the spread of the pathogen. Remove any trees with cankers on the main stem (Hazard tree). If pruning for aesthetic purposes, remove limbs when those limbs are less than 1 inch in diameter, to minimize the risk of infection. If removing damaged or broken branches, use a clean cut close to the branch collar taking care to not damage the collar.