Pest of the Month: Butternut Canker

By Richard Rees, Ph.D.

What does it look like?

Butternut canker is a fungus that causes a lethal canker disease of Juglans cinerea (butternut) trees. The fungus forms pegs under the bark that break through to the surface and expose the spores. The elongated, sunken areas of young cankers often originate at leaf scars and buds, and are best identified by their inky black center and whitish margin. Hidden by bark and bordered by successive callus layers, older, perennial cankers occur in bark fissures. Those covered by bark can be identified by the build-up of successive callous layers.

Host material and range

Butternut (also called white walnut and oil nut) is a small- to medium-size tree that matures around 75 years of age. Although cankers develop throughout a butternut tree, they frequently occur on the main stem, at the base of trees, and on exposed roots. Butternut is the only natural host known to be killed by the disease; however, the fungus oftentimes goes largely unnoticed due to the fact that butternut trees are scattered and death from the disease is generally very slow. Butternut canker can be spread in a variety of ways — most often by rain-splashed spores or insects and birds. The first notice of the disease was described in Wisconsin in 1967. However, it was not until 1979 that the fungus itself was described and reported in Ontario, Canada.

Current threat

The severity of the disease has prompted the United States to consider butternut a “species at risk.” The rapid movement of the fungus, along with its extreme virulence and lack of genetic diversity, makes the disease a disastrous threat to butternut populations. It has been found in 55 counties in the southern United States, where more than 75 percent of butternut trees have been killed by the canker. Because of the limited resistance displayed by the host, the fungus kills up to 80 percent of the population in some areas of the country. As a result, butternut trees are featured on the list of Endangered and Threatened Plants under the Endangered Species Act.

Prevention tips

Prevention is the key to management of canker diseases. Prevention methods are of high importance, because canker pathogens are extremely difficult to eradicate once the infection has begun. Most importantly, avoid planting trees species that are genetically susceptible to cankers. Optimize conditions that allow for tree growth, and be aware of environmental stresses such as drought that predispose trees to cankers. This is especially important on newly transplanted trees since they are at a higher risk to cankers. In addition, utilize tree wraps to protect young, thin-barked trees from sun damage to the trunk. Maintain soil moisture throughout the season, but be conscious not to overwater.

Treatment tips

Currently there are no known butternut strains with canker resistance, and there is no known cure for trees infected with the canker. Studies are underway across the range of butternut to locate disease-resistant trees.

What can you do?

It is important to first learn to identify the invasive species in your area. Report any sightings to your county extension agent, because the sooner invasive species are detected, the easier and cheaper it is to control them. Make sure to always clean your boots, gear and tires after working a site to prevent the spreading of seeds, insects or spores to a new location.

Richard Rees, Ph.D., is product development manager – fungicides at Bayer Environmental Science, a business group of Bayer CropScience LP.