Restoration of the American Chestnut
J. Hill Craddock1, Leah Florence2 and Forrest MacGregor2
1Department of Biological and Environmental Sciences, The University of Tennessee at Chattanooga, 615 McCallie Avenue, Chattanooga TN 37403-2598 (Hill-Craddock@utc.edu)
2The American Chestnut Foundation, Asheville Office, 46 Haywood Street, Suite 213, Asheville, NC 28801
Originally published as: Craddock, J.H., Florence, L. and MacGregor, F. (2000) Restoration of the American Chestnut (abstract and poster). Appalachian Studies Association Conference, Knoxville, TN. 24-26 March 2000.
Abstract. The major focus of chestnut research in North America today is the restoration of Castanea dentata to its former position as a component of the Appalachian hardwood forest ecosystem. The return of the American chestnut requires a multidisciplinary effort. The American Chestnut Foundation (TACF) is actively engaged in all aspects of this major restoration project through its own breeding program and state chapter affiliate programs and by research grants to investigators and cooperators throughout the United States. The goal of the TACF is to breed genetically diverse blight resistant nuts for initial distribution in 2006. We plan to reintroduce the trees into the forest in an ecologically acceptable manner. TACF hopes to begin large-scale reforestation, using truly blight-resistant American-type trees, before the end of this decade. Recent advances in plant pathology and molecular biology, especially new recombinant DNA technologies allow us to confidently predict a successful outcome for our endeavor. As breeding tools, molecular markers will greatly facilitate selection of resistant progeny. The Foundation’s strategic plan calls for the development of funds and other assets from a variety of sources. We maintain an archive of folklore, historical, artistic and scientific material on American chestnut at our Asheville office that is open to scholars and researchers. We strive to educate the general public and hope to contribute to scientific knowledge by conducting research, fostering science-based learning and sharing among disciplines.
Introduction . The American chestnut was one of the most important timber and nut-producing trees of the eastern United States (Smith, 1950). In the early 20th century, it was completely eliminated from its ecological niche by the introduced fungal pathogen Endothia parasitica (Roane et al., 1986). The return of the chestnut to its place in the Appalachian forest canopy requires the concerted efforts of Government, University, and Private Foundation researchers and the work of dedicated volunteer enthusiasts. The TACF multi-part plan of action involves research on biological control of the chestnut blight disease, breeding the trees for resistance to the fungus and supporting research on the genetics and ecology of restoration. Soon after blight struck, USDA researchers introduced blight-resistant chestnut species from China and Japan into the US in an attempt to replace the American chestnut (Graves, 1950). The breeding strategies employed by TACF aim to transfer the genes for blight resistance from the Asian species into the American chestnut while conserving as much as possible of the native species’ genetic diversity. Great progress has been made towards a hybrid tree combining the timber form of the American with the blight resistance of the Chinese chestnut. The most promising TACF hybrids to date are the result of a backcross breeding program. Third backcrosses (B3s) are nearly indistinguishable from the American chestnut (Burnham et al., 1986; Hebard, 1994; Kubisiak et al., 1997).
Hypovirulence and Biological Control of Chestnut Blight
Biological control is based on “hypovirulence,” a phenomenon marked by the reduced virulence of the pathogenic fungus, making it less dangerous for its host. Hypovirulence is transmitted by a virus. The viral RNA can transform “lethal” cankers into slower-growing superficial bark cankers that do not kill the tree (Figure 1 shows effective biocontrol at work in the Italian Alps on European chestnut). Slowing the growth of the fungus allows the tree to live and bear fruit.
Essentially pure stands of second growth chestnut timber were not uncommon in the early 1900s across vast areas of the Appalachian mountains (Figure 2).
Materials and Methods
The American Chestnut Foundation employs a small but dedicated staff of scientists and technicians in pathology, genetics, forestry, plant breeding, development and administration. Our goal is to put the American chestnut, king of the Eastern Forests, back on its throne. The tree in Figure 2b was cut in 1905 as part of a timber volume study of chestnut second-growth forest in Connecticut. On many upland sites, American chestnut was by far the fastest-growing and most valuable hardwood species. (Photograph: Connecticut Agricultural Experiment Station).
The rot-resistant remains of fallen giant American chestnut trees may still be found throughout the Appalachian Mountain and Cumberland Plateau regions of Eastern North America. The photograph in Figure 3 was taken near the village of Chestnut Grove, in Warren County, Tennessee.
Chestnuts are not extinct. They continue to sprout from the bases of blight-killed stems. Chestnut grows very slowly in the shade of the oak-hickory and oak-pine forest that has largely replaced the chestnut forest. Jonathan Pewitt, a recent graduate of the University of Tennessee at Chattanooga, (Figure 4, in a 1998 photo) cataloged chestnut sprouts on the Cumberland Plateau near the Tennessee River Gorge. The “saplings” in Figure 4 were already growing when blight swept through the area in the 1930s and survive now only precariously. Survival of the species varies greatly from site to site, (Griffin et al., 1991).
Large Surviving Chestnuts.
Many of the surviving specimens appear as clumps of multiple stems arising from a common root system (clones), although the connections are not always clearly evident. Larger stems may bloom. But the largest stems are often heavily cankered with blight and usually die before nuts are produced. The surviving American chestnut trees such as the clump seen in Figure 5, near the TN-NC border are valuable for many reasons. Their potential as “Mother Trees” in the TACF backcross breeding program will confer local adaptation to the hybrids and ensure conservation of the native species’ genetic diversity.
Chestnut blight is caused by a fungus, an orange mold (Cryphonectria parasitica), that grows in and kills the inner bark of the American chestnut (Figure 6). Christine Bock, chestnut enthusiast and lead horticulturist at the Tennessee Aquarium, and Walter Norfleet, UTC student found this unusual swollen canker (Figure 5) that may harbor a hypovirulent strain of the blight fungus; evidence that biological control of chestnut blight may be possible in Appalachia as has occurred naturally in Europe.
Classical plant breeding methods are used to transfer pollen from one parent to another in the TACF backcross breeding program. State of the art biotechnologies, particularly the use of molecular markers, complement the traditional approach. In Figure 9a, a glass microscope slide is used to transfer TACF hybrid pollen to an American chestnut . In Figure 9b, the pollen is transferred directly form the male catkin to the pistillate flowers.
Chestnut flowers come in two types of inflorescence: the male catkin (Fig. 8a) is composed of hundreds of tiny pollen-producing staminate flowers; the bisexual catkin (Fig. 8b) has staminate flowers along most of its length with clusters of female, or pistillate, flowers at its base.
Paper bags are used to isolate the female flowers before and after hand pollination.
We have been breeding chestnuts for timber form and resistance to chestnut blight since 1989 at our research facility in Meadowview, Virginia. To date, the program has produced many thousands of hybrid seedlings advanced through the third backcross generation (BC3s). Intercrosses among the backcross trees (BC3F2s) segregate for blight resistance and other important characters (Hebard, 1994; Kubisiak et al., 1997). The scientific staff regularly publishes the results of TACF experiments in peer-reviewed journals (visit www.acf.org for a complete list of references). The TACF membership base has grown to more than 4000 chestnut enthusiasts nationwide. TACF chapters are active in New York, Pennsylvania, Maine, Indiana, Massachusetts, Connecticut, and the Carolinas. Kentucky and Tennessee have active cooperating members. Each state chapter has its own regional breeding program and other important activities. Our external grant program disburses some $15,000 per year to support a broad diversity of American chestnut-related research projects. We publish a serial journal, The Journal of the American Chestnut Foundation and a newsletter, The Bark (TACF).
The orchards at TACF’s research farm near Meadowview, Virginia (Figure 11), grow thousands of hybrid seedlings from which only a few will be selected. The rigorous screening process deliberately exposes the young trees to blight allowing only the most resistant American-type individuals to advance to the next generation.
The restoration of the American chestnut will be successful if the current rates of progress are maintained in research on biological control of chestnut blight and chestnut breeding. Efforts are underway to conserve threatened North American Castanea dentata genetic resources. Ecological and sylvicultural studies have been undertaken that will allow us to better plan the actual return of the chestnut to the mountains of Appalachia. Figure 12 shows the view southward across part of Cherokee National Forest in western North Carolina.
Literature Cited (For a complete list of references, visit J.Hill Craddock’s Chestnut Links at http://www.utc.edu/Faculty/Hill-Craddock/chestnutlinks.html and the TACF homepage at http://www.acf.org)
Griffin, G.J., Smith, H.C., Dietz, A, and Elkins, J.R. (1991) Importance of hardwood competition to American chestnut survival, growth, and blight development in forest clearcuts. Can. J. Bot. 69:1804-1809.
Kubisiak, T. L., F. V. Hebard, C. D. Nelson, J. Zhang, R. Bernatzky, H. Huang, S. L. Anagnostakis, and R. L. Doudrick. (1997) Mapping resistance to blight in an interspecific cross in the genus Castanea using morphological, isozyme, RFLP, and RAPD markers. Phytopathology 87(7): 751-760.
Smith, J.R. (1950) Tree crops - a permanent agriculture. Second Ed. Devin-Adair Co., New York. 408 pp. (First edition Harcourt, Brace, 1929). Reprinted: Island Press, Washington, DC., ISBN 0-933280-44-0