Castanea … From Blight to Backcross Breeding

Photographs of ravaged trees across the state were taken by the Pennsylvania Chestnut Tree Blight Disease Commission to study the problem. This large stand near Oxford, Chester County, was photographed in November 1912.
Pennsylvania State Archives/RG-25

Not far from my home at the base of the South Mountain in Cumberland County, there is a wide spot in the road where you can park a couple cars at the edge of a block of public land. From that place, the visible but overgrown bed of a Colonial-era haul road ascends the mountain. Scattered along the road are the small, flat and circular remains of 18th- and 19th-century charcoal kilns that supplied the great Carlisle Iron Works at Boiling Springs. It’s likely the charcoal produced at these hearths came off the mountain in carts on this very road. A walk up this steep and eroded old trace will bring you face-to-face with ghosts. Maybe halfway between the foot of the mountain and the crest, the first chestnuts appear.

They are small, spindly saplings, springing out of the rocky old bones of the mountain from the still-living rootstock of giants. The great mother trees that produced these saplings died nearly a century ago, victims of a pernicious fungal infection that produced a historically epic change in Appalachian forests and all but destroyed one of the iconic species of Penn’s Woods. These little trees may survive until they are 20 feet tall or so, they may even flower, but then their bark will split and the fungus will strangle them, and in short order they will stand leafless and skeletal. Even as they wither in their death throes, however, shoots will erupt from the thin soil at their feet.

Like many things in Appalachia, the trees are tough and persistent. The American chestnut (Castanea dentata) simply refuses to die.

Burrs from backcrossed chestnuts, opened to reveal the nuts inside.
Penn State University/Photo by Steve Hoy

The Tree

Chestnuts are members of the same nut-bearing tree family as oaks and beeches (family Fagaceae). The 12 or so species of chestnuts (genus Castanea) are distributed worldwide through-out the temperate climate zones. DNA and fossil evidence suggest that the genus is ancient, dating back 60 million to 85 million years before the present, and was at one time much more widely distributed. Isolated from its relatives in Asia and Europe, the North American trees evolved into a distinct species. Castanea dentata is most readily distinguished from its Asian and European cousins by the shape and appearance of its leaves. American chestnut leaves are elongated in comparison to their width, the teeth on the edges of the leaf curve inward, and the stems usually have a reddish color. American chestnuts settled into what we think of as their historic range in the Appalachian Mountains during and after the Pleistocene Epoch. Fossilized pollen data indicate that the trees were limited to the Southern Appalachians during the height of the glaciations and began extending their range northward as the ice began to recede some 14,000 years ago, eventually reaching southern New England some 2,000 years ago.

Chestnuts were undoubtedly an important food source for many forest animals and birds and very likely for Native Americans as well. Interestingly, while preserved carbonized hickory and acorn shell fragments are common in archaeological investigations of indigenous campsites in Pennsylvania, chestnut shells are not. One reason may be that chestnut hulls are thin and may simply not preserve very well.

Another reason may be that chestnuts may have been less common for much of Pre-Columbian history than they were later. Despite claims in many sources that Castanea comprised 30 percent or even 50 percent of aboriginal Appalachian forests, recent research shows that the tree has fairly specific soil, moisture and climate restrictions that allow it to flourish where conditions permit, but greatly restrict it elsewhere. There’s emerging evidence from northwestern Pennsylvania that gradually intensifying Native American land use, especially intentional burning of the forest to provide animal habitat and to clear land for agriculture, may have also allowed the gradual expansion of stands of chestnut in the last millennium or so before the arrival of the Europeans. A study on Allegheny National Forest land found a clear correlation between historically known stands of chestnut and the locations of Native American settlements.

Male chestnut flowers.
Penn State University/Photo by Steve Hoy

Witness trees are landmark trees mentioned in old deeds and land surveys that often served as property corners or boundary markers. The documents typically noted the type of tree, so witness trees provide a snapshot of the species growing in local forests at the time the documents were produced. Careful analysis of witness tree data indicates chestnuts were not terribly common in the 18th century when the earliest land patents were filed. Oddly, one of the factors that promote the expansion of chestnut trees is deforestation. Chestnuts can regenerate from stumps and live rootstock, and they thrive in the bright sunlight of open areas, so early Euro-American timbering in the 19th century may have allowed them to greatly increase their range and numbers.

In any case, by the mid-19th century, the American chestnut was one of Pennsylvania’s most common forest trees. In some places, when the trees flowered in late June, entire mountainsides turned white.

Although there’s some debate about the numbers and distribution of American chestnuts and how that may have changed over time, there is no debate about the tree’s value throughout Pennsylvania’s history. The nuts are delicious, plentiful in season, and highly nutritious. They were harvested for family and livestock use in all of rural Pennsylvania. They were an important commercial product as well, and they were shipped by the barrel to markets in every major commonwealth city. The wood is light, easy to work, and extraordinarily beautiful. It is also very high in tannic acid and thus naturally resistant to decay, as many very old chestnut barns, cabins and fences amply testify. Since chestnuts can grow to enormous size (150 feet tall and up to 6 feet in diameter), the lumber was once plentiful in all dimensions. Chestnut came to be widely employed as telegraph poles and railroad ties. As interests in landscaping and landscape architecture grew in and near American cities in the late 19th century, the chestnut became a popular shade tree and a component of many botanical and public gardens and parks. In a sad twist of circumstance, its importance as a landscaping tree played a role in its undoing.

An American chestnut specimen photographed in 1911 by the Pennsylvania Department of Forestry exhibits cankers caused by the blight.
Pennsylvania State Archives/RG-6

The Blight

Around the turn of the 20th century, according to the accepted origin story, an unwanted visitor arrived with a shipment of Japanese chestnut rootstock delivered to the New York Zoological Gardens. In one of the first—and to this day one of the worst — examples of the unwitting importation of exotic organisms, the young trees arrived infested with a fungus that is endemic in the Old World, Cryphonectria parasitica, the Chestnut Blight. Asian and European species of chestnut are affected by the fungus but are not usually killed by it. American trees proved to have little resistance. The fungus spores are transported by wind, rain, birds and animals and enter mature trees through natural cracks in the bark. The fungus grows in the cambium, the layer of cells beneath the bark that conduct water and nutrients from the roots to the branches. In short order, the invader begins to strangle the host tree.

By 1905 the staff at the zoo noticed their chestnut trees were suddenly sick and dying. William Murrill of the New York Botanical Garden investigated and identified the fungus, but it was already too late. The blight spread like wildfire.

The epidemic roared through Appalachia, and Pennsylvania stood in the middle of the whirlwind. In 1909, just a few years after the blight’s initial identification in New York, John Mickleborough of Pennsylvania’s new Department of Forestry noted that the blight already extended “from near the northern boundary of Maryland, through southeastern Pennsylvania, across New Jersey and New York.” In 1911 the Pennsylvania General Assembly established the Pennsylvania Chestnut Tree Blight Disease Commission and appropriated more than a quarter million dollars, an enormous sum at the time, to study the disease and propose solutions. Congress passed the Plant Quarantine Act in 1912 to try to limit unwitting importation of plant diseases and pests. Despite rigorous research, substantial public and private support, quarantines, cutting of buffers, and experiments with a variety of controls, the pandemic traveled on the wind and could not be stopped or even slowed. Financial loss for the year of 1912 from Pennsylvania, South Carolina and West Virginia was estimated to be $82.5 million. The blight was present throughout the chestnut’s range within a decade of its outbreak. By 1950 the American chestnut, one of the most common and important trees in Pennsylvania and everywhere in Appalachia, was functionally extinct.

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The scale of the devastation is hard to fully grasp. The trees died in the millions. Mortality estimates throughout the American chestnut’s range ran between 3.5 billion and 4 billion trees. Across Pennsylvania, and everywhere in Appalachia, great swaths of bare dead limbs rattled in the wind like supplicating arms lifted toward heaven. To this day, the Chestnut Blight remains one of the most virulent, destructive and profound ecological disasters in history.

The Future

If the blight pandemic is an apocalyptic tale, it may prove to yet have a happy ending, and again Pennsylvania is playing a critical role in the story.

Biologically, American chestnut has a couple characteristics that contribute to the possibility of redemption. The rootstock of the tree is unimaginably persistent, and the roots have continued to sprout and send up hopeful shoots for decades. This has preserved the species, albeit in a greatly diminished form, since the blight took hold. American chestnut also hybridizes readily with other species of its genus. This allows for the possibility of genetic exchanges with other kinds of chestnuts that are more resistant to the blight

I reached the American Chestnut Foundation’s Director of Restoration, North Central Regional Science Coordinator, and Regional Science Coordinator Supervisor Sara Fitzsimmons at her office at the Arboretum at Penn State and asked her when efforts to try to repair the devastation of the blight began. “Actually, in the mid- to late teens the USDA got involved and went to China looking for a replacement for the American species.” The Asian trees played a role in 1930s breeding programs done as cooperative ventures with agricultural extension offices and universities in West Virginia, Connecticut and Illinois, and at the Pennsylvania State Forest Academy in Mont Alto, Franklin County, one of the first forestry schools in the nation (now Penn State Mont Alto). Sara noted, “There are still some plantings around from those early hybridization experiments near Mont Alto and a few other places. These are mostly pure Chinese trees, or first generation hybrids or backcrosses.” Unfortunately, while these early experiments did demonstrate some potential, they met with only spotty success.

Federal funding for the research began to dry up in the 1960s and ’70s, and as Sara noted, “everybody sort of gave up.” But not quite everybody. In the 1980s researchers who had been working on chestnut restoration combined their efforts with interested landowners and volunteers and formed  a nonprofit organization. The American Chestnut Foundation was born in 1983. Much of the 1980s was spent in fundraising and outreach, but in 1989 the foundation established its first nursery farm in Virginia. The Pennsylvania Chapter was established in 1994. Sara was hired in 2003 and the chapter’s breeding program became affiliated with the Arboretum at Penn State in 2005. By 2014 the breeding program hired a full-time orchard manager, and within the last 10 or 15 years, a number of faculty members from Penn State’s renowned forestry program became involved with chestnut research.

Planting seedlings at The Arboretum at Penn State. Penn State University/Photo by Steve Hoy

The American Chestnut Foundation’s efforts to restore the species have focused on three areas. Biotechnology research, conducted primarily in New York, has focused on genetic engineering aimed at producing blight resistance. Biocontrol research, carried out in Connecticut, West Virginia and Maryland, concentrates on finding and propagating less virulent strains of the blight fungus. The Pennsylvania Chapter’s research has focused on the oldest and so far most successful branch of restoration research, backcross breeding.

First proposed in the early 1980s by Dr. Charles Burnham, a plant geneticist at the University of Minnesota, backcross breeding takes advantage of the American chestnut’s propensity for hybridizing with other kinds of chestnuts and the natural blight resistance of Chinese chestnuts (Castanea mollissima). The approach begins with hybrid Chinese and American chestnut trees. These trees are then backcrossed to the American species. Each generation is inoculated with the blight fungus and only those trees with the highest resistance are used to breed further generations. This process continues over multiple generations to produce a tree that retains no Chinese characteristics other than blight resistance and is to all visual appearances an American chestnut. The backcross program has been implemented in Pennsylvania and at 16 other American Chestnut Foundation state chapter orchards, and thousands of the backcrossed trees have been produced.

Further testing of their resistance and viability continues, but the results to date have been very encouraging. Experimental reintroductions began in 2011 in Indiana, Tennessee, West Virginia and Vermont, and these experiments are being monitored closely. Reintroduction in Pennsylvania holds great promise because of the tree’s preferred habitat. Sara Fitzsimmons noted that “chestnuts love dry, rocky, acidic and well-drained soils, so they’re ideal for the reclamation of old surface coal mining sites.” If the backcrossed trees fulfill their promise in Pennsylvania, they may not only restore an important component of our forests but also help the commonwealth recover from another historic environmental scourge by restoring landscapes damaged by coal extraction. In Pennsylvania and the other Appalachian states, it’s entirely possible that the next generation of kids will grow into an adulthood in which wild chestnuts abound in the woods and where mountainsides again turn white with their flowers.

The Sentinel

Based on a tip I got from Sara Fitzsimmons, I spent part of a late afternoon in the early spring navigating a couple of narrow country roads in York County. I finally found the property I was looking for and met the owner, Cathy, at her back door. We took a short walk across her property and there, alone in the middle of an open field and framed against the sky, was what I was looking for, something I have never seen in my 60 years on the earth.

The old tree was an original: a survivor. She stood gnarled, blight-damaged and weather-beaten, but the swelling buds on her branches and the dense litter of last year’s hulls lying on the ground were proof that she was very much alive. This old tree stands near a property corner and is likely a witness tree on an old deed. That and the kindness of a long line of landowners is what probably saved her from the sawyer. It’s one of a remarkable few surviving trees still living in various corners of the commonwealth and other Appalachian and Midwestern states that somehow survived the horrific pandemic. Possibly a small variation in its genetic makeup conveyed some degree of blight resistance, and the species’ general toughness and persistence certainly served it well. The American Chestnut Foundation has been working with Cathy to gather nuts from the old survivor for years, and those nuts have played a role in the backcrossing experiments conducted at Penn State.

A rare survivor, this American chestnut produces nuts that have been used in successful backcross breeding efforts at Penn State. PHMC/Photo by Don Giles

Cathy and her family view the old tree with reverence, and they see themselves as stewards entrusted with something precious and important. As we walk around the old tree, Cathy smiles and tells me this: “She’s a good old girl. Produces for us every year. It’s just such a thrill to see her standing up here.” Then she says she has something else to show me. Growing next to one of her storage sheds is a young chestnut. It’s a backcross tree propagated from a nut given to her by the American Chestnut Foundation, and it contains within it the genetics of the old survivor tree itself. It has the telltale cracks and blisters of the blight on it, but it stands 15 or 20 feet tall, is growing vigorously, and last year produced a crop of nuts. It’s the old tree’s first living offspring in many decades.

The old tree stands framed against the sky and the late afternoon sun. Despite all the damage and stress it’s endured, it’s beautiful in the same way an old half-ruined barn is beautiful. It conveys its own history. Like the folk music and the many old stories that echo from this landscape, the chestnut’s tale has a dark and tragic turn, but in the end it’s all about tenacity, grace and hope. The roots go deep into Pennsylvania’s landscape, all the way to bedrock.

For More Information

There is a rich literature on the American chestnut and the blight. One of the best and most recent books is Susan Freinkel’s American Chestnut: The Life, Death, and Rebirth of a Perfect Tree (University of California Press, 2007).

Many of the original federal technical papers and studies on the disease have been published as Chestnut Blight (U.S. Department of Agriculture, 2012). Proceedings from the Pennsylvania Chestnut Tree Blight Commission, published in 11 volumes by the state between 1912 and 1914, provide details on the commission’s research and work.

For information on efforts to restore the American chestnut, visit the website of the American Chestnut Foundation at acf.org.

Joe Baker is an archaeologist, writer and editor, currently at the Pennsylvania Department of Transportation’s central office in Harrisburg, Dauphin County. He has been working in this capacity at various times for three federal agencies and three state agencies since 1979.