Butternut canker is an infection caused by a fungus (Ophiognomonia clavigignenti-juglandacearum) that mainly attacks butternut trees. The fungus is considered to be an introduced disease to North America, but scientists are unaware of its origins.
No control for the fungal disease exists and butternut trees are not resistant to it. In Canada, protecting butternut trees has been a part of the Species at Risk Act since 2005.
- Butternut canker has been found throughout the range of butternut trees.
- The fungus creates a wound (canker) that appears as patches of small, long and sunken black blemishes on a tree.
- Damage caused by the fungus was first reported in Wisconsin in 1967, but it was not until 1979 that the fungus was described.
- The fungus was first reported in Quebec in 1990, in Ontario in 1991, and in New Brunswick in 1997.
- Butternut canker has killed up to 80% of the butternut population in Ontario.
Once a tree is infected, it is difficult to halt the spread of the disease. As a result, butternut canker continues to affect Canada’s forests.
- Blemishes are found under the bark of twigs and branches.
- The pathogen penetrates through various wounds and natural openings (e.g., bark fissures), and might be spread by insects.
- Infected trees try to heal the cankers by growing bark and wood over them.
- Rain can spread the fungus from the top of a tree by washing it down branches.
- Dead twigs in the crown of a butternut are the first symptoms of an infection.
- Cracks in the bark of branches and sometimes a blackish fluid are seen in spring and early summer.
- In the summer, a whitish margin may form around the black canker.
CFS scientific research
Scientists have identified strategies to promote the survival of butternut trees.
- Keep at least 10 butternut trees per hectare by creating stand openings that equal twice the height of the surrounding trees to encourage seed germination and seedling growth.
- Keep all butternut trees without cankers that have 50% or more living crown.
- Keep all butternut trees with less than 20% of the trunk’s circumference affected by the fungus and more than 30% of the crown alive.
- Butternut trees growing above the forest canopy and in open fields are preferred.
In a forest stand or managed woodlot, infected trees should be removed as quickly as possible to limit the spread of the disease.
- Remove any tree with at least 30% of its crown leafless and at least 20% of the trunk’s circumference affected by the fungus.
- Remove any tree with more than 50% of its crown damaged even if the trunk is canker-free.
- High-value trees that are severely infected can be conserved by pruning the affected branches and cutting off trunk cankers.
Working collaboratively to develop solutions
Natural Resources Canada, based on the work of its Canadian Forest Service scientists, suggests different measures for controlling the fungus, such as those outlined above.
Want more information on the butternut canker?
Contact Danny Rioux of the Laurentian Forestry Centre.