Michelle Grabowski, UMN Extension Educator
Once commonly planted near farmstead houses for the nuts it produces, many Minnesotans would not recognize a butternut tree today. Butternut (Juglans cinerea), also known as white walnut, is a native tree in Minnesota and a close relative of black walnut (Juglans nigra). This nut bearing tree provides food for squirrels and other rodents and is used for wood carving and furniture building. Butternut is hardy to zone 3 and is therefore a valuable tree in northern Minnesota, where black walnut will not grow.
Although butternut is naturally found in small numbers in native forests of the United States, these populations have decreased due to a lethal disease known as butternut canker. Butternut canker is caused by the fungus Sirococcus clavigignenti-juglandacearum and is responsible for the near extinction of butternut in some eastern forests. Populations of butternut still grow in Minnesota, but many of these trees are already infected with butternut canker.
Infected trees can be recognized by dead branches in the trees canopy and elliptical cankers on trunks and branches. The fungus infects through wounds or natural openings like leaf scars, bark cracks or stomata. The first year of infection, dark sunken oval cankers can be seen on infected branches. These often ooze sticky dark liquid in the spring and may have a sooty black center with a white border later in the season. The fungus infects and kills bark and the sapwood beneath it. Black staining of the sapwood can be seen if the bark is peeled back.
Infections on small branches quickly girdle and kill the branch, resulting in dieback within the tree's canopy. On large branches or the main trunk of the tree, cankers do not grow quickly enough to girdle the tree in one season. Rather the bark over the canker cracks open and ridges of wound wood develop around the canker. With time, multiple cankers accumulate on the main trunk of the tree and it succumbs to the disease.
The butternut canker fungus produces spores during wet weather throughout the growing season. These spores are washed down the trunk or splashed onto nearby trees, starting new infections where they find an entry point. It is unknown how the fungus moves long distance. Seed can carry the pathogen, and it is suspected that insects or birds may play a role. People can also move the pathogen by moving infected wood from place to place. The fungus survives in wood infected with butternut canker for two years after the tree has been killed.
Unfortunately there is no strategy to prevent infection with butternut canker. Infected butternut trees often survive many years however. During this time the trees provide shade and nuts. Large branches and trees that have been killed by the disease should be removed. Butternut is not as rot resistant as black walnut. Dead trees and limbs can be a hazard if located in areas near people or valuable property.
The US Department of Agriculture Forest Service has an active butternut breeding program, working to identify canker resistant butternut trees. If you know of a mature butternut tree that is thriving despite having a few cankers or a tree that is healthy despite many neighboring butternut trees succumbing to the disease, contact Dr. Ostry at the USDA North Central Forest Service (firstname.lastname@example.org 651-649-5113). These trees are a valuable natural resource that may provide disease resistance to butternut canker.
For more information on butternut canker visit the USDA Forest Service website.