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Eutypella canker

Quick facts

  • Eutypella canker is common on maple trees 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. Trees battle for decades before decay turns the tree into a hazard that requires the tree to be removed.
  • To prevent cankers make proper pruning cuts and avoid wounding trees.

How to identify Eutypella canker

Canker on maple
  • Trunk cankers are typically within 9 feet of the ground. They are usually centered on branch stubs or wounds.
  • Young cankers are round to elliptical in shape and slightly sunken or flattened. These young cankers are hidden behind bark.
  • When bark is removed from the edge of the canker, chalky-white to tan colored mycelia (mats of fungal cells) can be seen.
  • 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 older than 6 years are discolored black at the center, where black, fungal fruiting bodies have formed on the wood.
  • Cankers can grow up to 5 feet long with age.
Old canker on maple

Trees affected by Eutypella Canker

All trees in the genus Acer are susceptible to this disease including all maples that grow in Minnesota, box elder and sycamore trees. No other shade trees are commonly affected by the disease.

  • Black maple, Acer nigrum
  • Box elder, A. negundo
  • Norway maple, A. platanoides
  • Red maple, A. rubrum
  • Silver maple, A. saccharinum
  • Sugar maple, A. saccarum

How does Eutypella Canker survive and spread?

Black, spore-producing structures
  • Eutypella canker is caused by the fungus Eutypella parasitica
  • During rainy weather spores are ejected into the air from infected wood. These spores 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. There, it grows  into the wood and expands outward up to 1 inch per year. 

As the fungus spreads, 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).

The fungus also infects the sapwood of the tree, causing it to decay. This decay can extend up to a foot into the tree and greatly weakens the tree. Many trees infected with Eutypella canker break during strong storms and high winds.

Each year during the growing season, the tree tries 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 fallen off, rings of growth can be seen that reflect this annual battle between fungus and tree.

Mycelia on canker

How to manage Eutypella canker

Make a proper cut when pruning Acer spp.

  • Make the pruning cut just outside the branch collar.
  • Do not leave a branch stub. This is a common entry point for Eutypella parasitica. 

Do not wound trees

  • Prevent accidental wounds to the tree trunk by lawn mowers and weed whips by adding a 3-4 inch deep even layer of wood chip mulch around the tree.

Prune out and destroy infected branches

  • Inspect maples each spring for cankers.
  • Prune out infected branches and remove the infected wood from the site. The fungus can produce spores even on dead wood.
  • Infected wood can be buried or burned.

Monitor trees with trunk cankers

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. These weakened trees pose a risk of breaking and falling on property or people.

Contact a certified arborist to determine the stability of infected trees.

Consider removing infected trees

If healthy maples 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.

Rebecca Koetter and Michelle Grabowski, Extension educator

Reviewed in 2019

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