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What is Killing Our Trees?

Butternut Trees in Trouble

This is the fourth in a six-article series called "What Is Killing Our Trees." The series will explore some of the lesser-known diseases that affect Michigan trees and potentially impact wildlife.

Trees are seldom added to lists of endangered plant species. But the butternut tree (Juglans cinera) is declining so dramatically that Canada placed it on the endangered species list in 2003 and the U.S. Fish and Wildlife Service classifies the butternut as a species of federal concern. Minnesota no longer allows butternut to be cut on state-owned lands.

Over the last 40 years, losses of butternut have been severe throughout its range. The culprit is butternut canker disease caused by the fungus Sirococcus clavgigenti-juglandacearum. The fungus was probably introduced to the Southeastern U.S. from Asia via Europe, but some scientists think it might be a native form that mutated into a virulent strain. The disease was first described in detail in the early 1920s, but some reports suggest it may have been around as early as the late 1800s. The U.S. Forest Service estimated that by 1995 nearly 80 percent of the butternuts in the Southeast had been killed by the disease. It has spread north and west, and now more than two-thirds of the butternuts in Wisconsin and Michigan show the tell-tale discolored trunks and dead branches of the affliction.

Butternut is closely related to the black walnut tree (Juglans nigra), but tolerates colder climates and rockier soil. It is found on relatively fertile, well-drained soils (including stream banks) throughout Southern Michigan and in Delta and Menominee counties in the Upper Peninsula. Butternut is shorter-lived than black walnut, seldom exceeding 75 years old and 60 feet in height. It is commonly called ‘white walnut’ because its wood is lighter in color. In the 1800s, butternuts were often planted around farmhouses because the sweet, oily nuts were eaten individually or used in baking. The husks could be used to dye fabrics.

Despite such occasional plantings, butternut was probably never abundant anywhere in Michigan, but rather, scattered as individual trees. It is seldom found in groves, and this isolation would seem to provide some protection against the spread of a fungal disease. Yet, no area seems to be free of the canker disease.

Bill Botti, owner of Clinton Trail Tree Farm in Eaton County and President of the Michigan Forest Association, says he finds a few butternut trees in about two-thirds of forest stands in Southern Michigan. "Ninety-percent of the butternuts are infected with canker disease," he notes.

"We are hard-pressed to find more than one or two butternuts per 40 acres in the Central Upper Peninsula and healthy ones are even scarcer," says retired DNR forester, Mike Zuidema, of Escanaba. "Most people don’t notice it in a forest setting, but eventually the canker disease causes a loss in diversity and mast for squirrels, deer, bear, and other wildlife."

Spores enter butternut trees through a leaf scar or bark injury. Rain splash moves the fungi down to the trunk and that causes the tree to form cankers. Eventually, so many large cankers form that the tree is self-girded and literally chokes to death. This can take just a few years or in some cases two decades or more.

Treating diseased trees is virtually impossible. Researchers are attempting to identify individual trees that are resistant to butternut canker disease and to better understand how the fungus spreads. Scientists suspect insects and/or birds carry the fungus long distances but the specifics are unknown. Management of the disease is made particularly difficult because butternut canker affects fairly young trees as well as older ones, and butternut regeneration is limited in forest stands. The species is shade-intolerant and the seeds only survive about two years in the soil. So, unless part of the surrounding canopy is removed, dead trees are not naturally replaced.

There is considerable interest among foresters in hybrid butternuts that may prove resistant to canker disease. Black walnut can be infected by canker disease, but is not impacted much. However, butternut does not hybridize with black walnut. Most of the related research is focusing on crosses of butternut with English walnut (Juglands vegia) and Japanese walnut (Juglans ailantifolia). Unfortunately, research is hampered by the prevalence since World War II of butternut hybrids of unknown origin. Such trees are found throughout the butternut’s range, confounding the search for native butternuts that might have some resistance to canker disease.

Bill Botti and the Michigan Forest Association are trying to help researchers find seed-bearing, healthy butternuts. If you know of such trees, you can send information to the Association at 6120 South Clinton Trail, Eaton Rapids, MI 48827 or contact Mr. Botti by phone at 517-663-3423.

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