Eutypella canker is common on maple trees (Acer spp.) in landscape plantings and in natural areas. Cankers often form on the main trunk or major branches of the tree. Small trees that are less than 4 inches in diameter are commonly killed when the canker girdles the main trunk. On older trees a perennial canker forms. This is a very slow growing disease that trees will battle for decades before decay turns the tree into a hazard that requires the tree to be removed.
Pathogen and susceptible plants
Eutypella canker is a fungal disease caused by Eutypella parasitica. All trees in the genus Acer are susceptible to this disease (Table 1) including all maples that grow in Minnesota, box elder and sycamore trees. No species, or cultivars of the genus Acer (maples) have resistance to Eutypella canker, but no other shade trees are commonly affected by the disease.
Table 1. Maples affected by Eutypella canker
|Common name||Scientific name|
|Black maple||Acer nigrum|
|Box elder||Acer negundo|
|Norway maple||Acer platanoides|
|Red maple||Acer rubrum|
|Silver maple||Acer saccharinum|
|Sugar maple||Acer saccarum|
- Cankers are typically within 9 feet of the ground, centered on branch stubs or wounds.
- Young cankers are round to elliptical, slightly sunken or flattened and hidden behind bark.
- Bark near the center of cankers that are 6 to 8 years old are darkened by black fungal fruiting bodies.
- When bark is removed from the edge of the canker chalky white to tan colored mycelia (mats of fungal cells) can be seen.
- On some tree species, canker edges are raised or appear swollen with a flattened or sunken center.
- Bark falls off the face of old cankers, revealing a target shaped pattern of annual rings of cork wood.
- Cankers can grow up to 5 feet long with age.
During rainy weather spores are ejected into the air from infected wood and can travel more than 75 feet on the wind. The fungus infects recently wounded or newly pruned small branches. Once in the tree, the fungus makes itself at home underneath the bark where it will penetrate into the wood and expand outward up to 1 inch per year. It kills the phloem (vascular cells that transport sugars from the leaves throughout the tree), the cambium (undeveloped cells that grow into new vascular cells) and can even invade and decay the sapwood of the tree. This decay can extend up to a foot into the tree and many trees infected with Eutypella canker break during strong storms and in high winds.
Each year during the growing season the tree will try to defend itself by creating a layer of wound wood around the edge of the canker. When the tree goes dormant for the season, the fungus breaks into this barrier and continues its progress. This back and forth growth can continue for decades. In very old cankers where the bark has finally sloughed off, rings of growth can be seen that reflect this annual battle between fungus and tree.
- Avoid wounding the trunk or branches of susceptible hosts.
- When pruning maples or other Acer spp. it is necessary to make the cut correctly. Avoid wounding the main trunk. Never leave a branch stub. For details on how to make correct pruning cuts visit here.
- If a branch is infected, it should be pruned out and destroyed. Infected wood can be buried or burned (the fungus can produce spores even on dead wood).
- Cankers on trunks cannot be pruned out but should be monitored. Because Eutypella parasitica is capable of causing wood decay, severely affected trees may be weakened and pose a risk of breaking and falling on property or people.
- Contact a certified arborist to determine the stability of infected trees.
- If healthy maples or other Acer spp. are located near a tree infected with Eutypella canker, it may be worthwhile to remove the infected tree to reduce the chances of pathogen spread. Spores of Eutypella parasitica are released from existing cankers and carried short distances by wind to infect new trees.