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
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.