Joseph OBrien, USDA Forest Service, Bugwood.org
This causes a pronounced bulge of callus to develop around the infected area of red, sugar, silver, Norway, and other maples. Dead bark remains attached to the canker which may be on one side of the trunk or completely girdle the tree. Often, there is a dead branch stub in the center of the swollen, cankered area.
Maples, especially young trees, in ornamental plantings and in the forest are susceptible to this fungus. In most cases, between 2% and 10% of the trees may be affected. However, it has been observed in some stands to canker over 20% of the maples. Not only is the aesthetic value of the tree reduced by the presence of the swollen, callused trunk, the tree is very susceptible to attack by wood decay fungi and then to wind breakage. The infected tree becomes a hazard to people and property in the vicinity.
The canker is most often seen between 3 and 10 feet above the ground. This perennial canker enlarges year after year and may become 3 feet long. The tree produces callus during the growing season in response to the presence of the fungus. The fungus kills this callus and invades more cambium and bark during the tree's dormant season. Sometimes the fungus dies in the tree and the tree produces a large roll of callus along the canker's edge.
The fungus produces spores (ascospores) sexually in the centers of old cankers (more than 5 yrs. old). These spores are forcibly discharged during mild, moist weather and are carried by the wind 75 feet or more. Although the fungus also produces a sickle-shaped spore (conidia) asexually, these spores are thought to be unimportant in the disease spread.
Symptoms And Signs
- A large roughened area of bark, sunken in the center with heavy callus around the margin, is observed on the branch or main trunk. Usually there is a branch stub in the center of the canker and the canker is within 10 to 12 feet of the ground.
- When bark is removed from the upper and lower ends of the canker at the junction between the diseased and healthy wood, a light tan to cream colored mat of fungus is observed.
- Black fungal fruiting structures protrude from the bark near the centers of 5 year old or older cankers.
- Remove trees with cankers on the main trunk.
- Remove all cankered branches, cutting 4-6 inches below the canker. Do this pruning when the weather is dry.
- In the case of highly valuable trees, use a sharp chisel to remove all of the cankered wood and fungal mat plus 1" to 1.5" of the surrounding, apparently-healthy bark and wood.
- When the long range plan for a tree is to remove the lower limbs, do so when those limbs are less than 1 inch in diameter.
- When a limbs breaks, remove the remains with a clean cut close to the branch collar without damaging the collar. If the collar has been damaged or the bark has been stripped down the trunk, use a sharp chisel to shape the outline of the damaged area into a clean-edged, vertically-oriented oval if possible. Do not point the ends of the oval. If some callus has already begun to form, do not damage that callus.
Prepared by Gary W. Moorman, Professor of Plant Pathology