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William L. MacDonald, Professor of Plant Pathology
Division of Plant and Soil Sciences
401 Brooks Hall, PO Box 6057
West Virginia University
Morgantown, WV 26506-6057
At the turn of the 20th century, the American chestnut tree was an integral part of the heritage of eastern North America. Chestnut wood played an important role in almost everyone's life from the time they were rocked in chestnut cradles until they were buried in chestnut coffins. More than one-forth of all the hardwood timber cut in the Appalachians was chestnut. The tree was known for its straight bole, highly durable wood and a sweet flavorful nut. The chestnut grew from southern Maine and Ontario to northern Georgia and Alabama. Chestnut was the backbone of the forest economy in the Appalachians as no other species exceeded the volume of chestnut wood cut. Sale of nuts contributed significantly to many local economies and extracts of tannins from bark kept numerous leather tanneries in business.
The American chestnut was destroyed in about 50 years by Cryphonectria parasitica, a fungus introduced from the Orient and discovered in New York City in the early 1900's. This fungus initiated one of the greatest natural disasters in the annals of forest biology. Despite early attempts at control, the fungus spread in ever increasing waves of approximately 25 miles per year from the leading edge. Local spread occurred as a result of wound infections initiated by wind and rain disseminated spores, but longer distance dispersal probably occurred via birds or through the movement of infected wood. Fortunately, chestnut has survived principally as shoots produced from living root systems that continue to sprout. Unfortunately, these shoots become infected when they are 1-12 years old, perpetuating the cycle of blight.
As the disease progressed unabated in North America, efforts shifted to the only hope for control, breeding blight resistant chestnut trees. Early breeding programs were designed to preserve the best traits of the American chestnut while incorporating resistant germplasm from the Chinese or Japanese chestnut. This approach relied almost entirely on making large numbers of crosses. Few second generation trees were grown from first generation hybrids and most F1 hybrids were backcrossed to a resistant parent, typically one that laced the desired traits of the American chestnut. These undertakings met with limited success and were never designed to return the American chestnut to the forests of eastern North America.
Currently, there are two avenues of control being pursued and both can be considered biological. The first approach once again involves traditional plant breeding. The renewed interest in breeding blight resistant trees came over 10 years ago with the realization that earlier breeding efforts were haphazard. The current breeding program tests the hypothesis that the well established backcross plant breeding method is valid for chestnut. With this method, American chestnuts that are blight susceptible are crossed with resistant species. The first-generation hybrids then are backcrossed to American chestnut rather than to the resistant parent. Resistant plants are then selected by screening the backcross progeny. Further backcrosses to American chestnut are made with progeny that express high levels of resistance. With this approach, it should be possible to develop genotypically American trees that contain the resistance genes of the Asian species. Molecular biological studies also are underway to aid in the early identification of progeny that carry the appropriate genes for resistance. The breeding effort is being sponsored by The American Chestnut Foundation, a privately funded non-profit organization whose goal is the restoration of the American chestnut.
A second approach to disease control became possible when Italian and French scientists observed non-lethal chestnut blight cankers on European chestnut growing in Italy. They observed that strains of the fungus associated with such infections produced colonies that were abnormally pigmented and shaped. They further demonstrated that these strains contained some "contagious factor" that was responsible for their inability to produce lethal infections. We now know that the factor responsible for the debilitation represents a new class of viruses called "hypoviruses". Other hypoviruses have since been found associated with C. parasitica. Researchers working with chestnut blight in North America were particularly encouraged when hypovirus-infected stains were found in stands of American chestnut recovering from blight in Michigan.
With the discovery of hypoviruses comes renewed hope that biological control of chestnut blight may be possible within the natural range of chestnut. Yet, major obstacles appear to limit the potential of hypoviruses. Laboratory and field tests have revealed the presence of many genetically different strains of C. parasitica in the Appalachians that are incompatible with one another. When strains are incompatible, their hyphal filaments often fail to fuse so that hypovirus transmission is prevented. In areas where hypoviruses have effectively controlled C. parasitica, strains often are compatible with each other. The task then is to devise methods that will allow us to bridge this system of incompatibility. Two approaches currently are being investigated. The first requires knowledge of the genetics of the compatibility system. We now know that some strains are inherently better transmitters of hypovirus than others. By understanding the genes that regulate compatibility, we believe strains can be chosen for hypovirus introduction that are more capable of interacting with large numbers of compatibility types.
A second approach has employed molecular biology techniques to integrate the hypovirus into the nucleus of C. parasitica (most naturally occurring hypoviruses are carried cytoplasmically). Nuclear integration provides an important gain for the hypovirus as it allows transmission to occur during sexual reproduction. The barriers of incompatibility do not exist during sexual reproduction, therefore, when normal strains mate with hose carrying hypoviruses, about one-half of the wind-borne sexual spores that are produced carry hypovirus. A further advantage of the nuclear integration is that the hypoviruses are passed to a variety of compatibility types, a step that should further aide their distribution to the numerous strains that exist.
Ultimately, the answer to chestnut blight control may rest with a marriage of biological control technologies. Because no species has adequately filled the niche once held by the American chestnut, its return would improve the balance of many eastern forest ecosystems.