The plight of butternut

Stem of a butternut tree
Butternut trees have flat ridges of ash-gray bark

Butternut trees, Juglans cinerea, are beautiful relatives of the more common black walnut. They get the name ‘butternut’ from the rich, fatty nuts, which taste better than walnuts. Another name for the tree is ‘white walnut’ because the wood is lighter in color than black walnut wood.  You may already know butternut from eating its nuts or working with its wood, but the tree is disappearing before our eyes.

Butternut trees were very common in New York and Wisconsin when I was in school. In Wisconsin, forested  slopes often had many butternuts mixed with other upland hardwoods. Beginning in 1967, though, butternut canker disease has been spreading through the entire range of butternut. In some areas, more than 80% of butternut trees have died from the disease. In Central Kentucky, most of the trees that I saw in the woods in the early 1980s are gone, and the ones that are left are mostly infected.

Butternut canker is caused by a fungus with the unwieldy name of Ophiognomonia clavigignenti-juglandacearum (Oc-j for short). The fungus infects butternut trees through small cracks or wounds and causes a canker – a dead area of tissue under the bark. The tree fights against the infection by creating a chemical compartment around the infected site and growing new tissue around the compartment. However, the fungus can create multiple cankers. Once these cankers encircle the stem, death is certain.  Oc-j can also infect other Juglans species, especially heartnut, but black walnut is quite resistant.

Butternut tree
Butternut, Indian Creek, Daniel Boone National Forest with Barbara Morgan. The tree had canker in 2013 and is now almost dead.

Where did this fungus come from? Nobody knows for sure. The rapid spread of the disease and the high rate of death of butternut suggest that this is an introduced disease. Genetic analysis of Oc-j shows very little genetic variation. Apparently, the fungus consists of three genotypes, or clones, that reproduce asexually. The sexual form of the fungus is not known. The low genetic variation of the fungus is also evidence that this is an introduced fungus.

Without action, butternut trees may disappear. Although butternut has been proposed for listing as a threatened or endangered species under the Endangered Species Act, the Fish and Wildlife Service has taken no action. Listing of the tree as threatened or endangered would trigger some federal and state action and may provide financial support for finding resistant trees.

Several universities are trying to identify resistant butternuts. The University of Tennessee has an active program to collect butternut seeds and plant them in plots underneath infected trees. The seedlings that survive under these conditions may be resistant, or partly resistant. Once resistant seedlings have been selected, they can be crossed to create a new generation of trees which can then be tested for resistance. As you might imagine, this is a process that takes a very long time and with an uncertain outcome.

The plight of butternut illustrates a major problem in trees all over the world. International trade, on the rise for hundreds of years, is bringing trees into contact with insects and diseases that they have never encountered before. You have heard of the big ones – chestnut blight, Dutch elm disease, emerald ash borer – but you may not have heard of all the hundreds of other foreign organisms that trees are being exposed to. And it’s not just in North America – new diseases are cropping up throughout Europe as well.

We can’t stop international trade, though we can do a better job of requiring treatment of imported wood and plants. What we need to do, then, is to ramp up our ability to respond more quickly to threats, and to begin breeding and selection programs for resistance. We don’t seem to learn this very well – our response to emerald ash borer has been too little too late.

Butternut canker
Symptoms of butternut canker on a young stem. The brown tissue has been killed by the fungus. Manfred Mielke, USDA Forest Service,

I am optimistic that as long as funding is available for butternut canker disease resistance programs, the species will survive. But its glory days are behind it.