Butternut canker is an infection caused by the fungus Ophiognomonia clavigignenti-juglandacearum (V.M.G. Nair, Kostichka & J.E. Kuntz) Broders & Boland (Oc-j). The fungus is found mostly on butternut (Juglans cinerea L.) trees—hence its name—but other members of the walnut (Juglandaceae) family may also be affected.
It can be difficult to diagnose the cause of a canker on a butternut tree, since these wounds on a tree may be caused not only by a pathogenic fungus but also by a microbial infection stemming from winter frost, physical injury or the presence of insect pests. However, the symptoms associated with the Oc-j fungus have certain characteristics that scientists can recognize by carefully examining each part of the tree and performing cultures from twigs.
The small, elongated and sunken black blemishes patches indicative of infection by Oc-j are often found under the bark of butternut twigs and sometimes under the bark of branches. In the spring and early summer, cracks in the bark of the branches and main stem can even leak a blackish fluid, which is a characteristic sign of the infection beginning. Infection usually occurs in the lower crown of the tree first, and then spreads downward as fungal spores are washed by rain along the branches and down the main stem. In the summer, a whitish margin sometimes forms around the sooty black cankers. Since infected trees try to heal the cankers by growing bark over them, symptoms may only be visible after the bark has been peeled from a branch or twig. If the exposed cambium layer (the living part of the tree) under and around the canker is dark brown or black in colour, the cambium layer is dead.
As the disease intensifies, multiple cankers form on the branches, stem and roots. The infected tree stops producing nuts. The cankers grow and join together, killing the affected branches. If the cankers girdle the stem, the tree dies. Cankers also serve as entry points for other pathogenic and decay organisms. The fungus is carried from tree to tree by wind and by a variety of insects.
Butternut canker has been found across the entire geographic range of butternut. It has killed up to 90% of the butternut population in some areas of the United States. In Canada, the fungus was first reported in 1990 in Quebec, 1991 in Ontario, and 1997 in New Brunswick. However, it is not known how long the Oc-j fungus has been present in the deciduous forests of North America. Damage caused by the fungus was first noted in Wisconsin in 1967, but it was not until 1979 that the fungus itself was described. Given the low genetic variability and the high level of virulence characterizing this fungus, scientists believe that it was introduced to North America.
The continuing spread of butternut canker is a major threat to the survival of this species. Canadian Forest Service researchers recommend various measures for controlling the disease.
Once a tree has become infected, it is difficult to halt the spread of the disease. Control measures therefore focus on protecting healthy trees by promoting vigorous growth and seed production. One recommendation for promoting the health of butternut trees is to increase the amount of sunlight available to them by removing some of the competing species around them. Reproduction can be supported by maintaining a density of at least 10 butternut trees per hectare and by creating stand openings of a size equal to twice the height of the surrounding trees to promote germination.
In a forest stand or managed woodlot, infected trees should be removed as quickly as possible to limit the spread of the disease. Any tree with at least 25% crown dieback and at least 20% of the circumference of the main stem affected by cankers should be removed, as should trees with more than 50% crown dieback, even if the stems are canker-free. High-value trees that are severely infected can be conserved by pruning the affected branches and excising trunk cankers.
Another way to promote the survival of this species is to take advantage of the genetic resistance to canker that is believed to exist in some butternut trees. It is therefore important to conserve all butternut trees with a canker-free stem and less than 50% crown dieback, as well as those with less than 20% crown dieback and less than 25% of the circumference of the main stem affected by cankers.
Protecting butternut, which is prized for its wood and nuts and for being an integral part of the biodiversity of our forests, and which has been protected under the Species at Risk Act since 2003, deserves all the efforts that can be marshalled to save it.