Since the early 1900's a disease known as Chestnut Blight has infected many American Chestnut trees and causing their removal from forests. A greater look at the history of this fungus as well as the mechanisms of action will allow us to learn on how to preserve the American chestnut. At one point, the American chestnut was virtually eliminated. With the help of government acts and conservation agencies, the American chestnut is slowly growing back in population. Two methods of restoration of the chestnut include a hybridization and the use of hypovirulant strains. This issue shows a variety of interest from ecologists to those in the timber industry who cannot lumber Asian species of chestnut primarily because of their size.
In the 1880's a harmful fungus known as blight, inhabited the United States from imported Japanese chestnut trees. Blight quickly spread, killing chestnuts and chinquapins, which is another species of chestnut that produces 1 nut per bur. In 1904, Chestnut blight appeared infecting trees in New York City and spread at a rate of 20-50 miles per year. By 1906, W.A. Murrill reported that this disease is known to occur in New Jersey, Maryland, District of Columbia, and Virginia. In 1912, the Planet Quarantine Act was passed to reduce the chances of plant deterioration or devastation prevention. Chestnut Blight or Chestnut Bark Disease was originally found in 1904 and within 50 years, it spread across the eastern United States, from Maine to Georgia and as far west as the edge of Michigan. By 1950, the American chestnut was essentially eliminated as a forest tree. In 1972, importation from Italy gave a biological control in which a virus helped prevent the blight fungus from killing chestnut trees (Anagnostakis WWW). A long history indicates that this tree does have an important role in the ecosystem and industrially.
Chestnut wood is highly resistant to rot and used extensively for poles, fencing and building materials. Formerly known as Endothia parasitica, Cryphonectria parasitica enters the wound, grows in and under the bark and eventually kills the cambium all the way around the twig or bush. Blight does not affect the root system, so the ones that do survive, survive as shrubs. There is, however, no significant wild production from these shrubs. Breeding projects are underway to combine the nut quality and timber. (Anagnostakis WWW). Although currently imported Chinese chestnuts are healthy producers of good nuts, they cannot replace our native chestnut because the Chinese tree is an orchard tree, while the American chestnut creates huge tracts of big timber (ACF WWW). Overall, it is important to understand how blight works to further help protect the trees against it.
Blight is a Castanea Disease which has eliminated American Chestnut Trees from landscape. This disease causes cankers on the branches then moves into the trunk and thus kills the tree. Currently, there is no chemical control for this disease. Resistance to Endothia parasitica has come from a fungus which makes Asiatic chestnut trees resistant to blight (Castanea Disease WWW). Pycnidia are tiny, orange spore-cases that characterize spreading chestnut blight. This disease has killed 4 billion chestnuts. *
The fungus that causes chestnut blight invades a tree through different types of wounds. These wounds include beetle borings, claw marks, and broken branches. The fungus then multiplies beneath the bark to form a mat-like web which eventually develops into a killing canker. Defense by the tree is done by ringing the lesion with new bark tissue. However the scab is overcome repeatedly until the canker girdles the limb, and cuts off the flow of nutrients and water to the tree. On average, a chestnut tree dies 4 days after being infected with blight. Blight spreads through windborne spores and through sticky masses dispersed by insects and small animals (Anderson 1992). With this exact knowledge of blight, ecologists can now find ways of protecting the American chestnut from this fungus.
Breeding work in Connecticut has been high and focused on producing hybrids that were combination of species, looking for single ideal progeny that could be propagated clonally (Anagnostakis WWW). The Connecticut Agricultural Experiment Station has been working on planting blight resistant Asian chestnut species and crossing them with susceptible American trees (Anagnostakis WWW). Specific examples of hybridization are being done by different agencies with different species of Asiatic nature. Japanese chestnut trees are more resistant to blight than Chinese chestnuts which are both more superior to the American chestnut which shows no resistance. Hybrids of Asian/American chestnut were made to improve the orchard qualities of chestnut trees (Anagnostakis WWW). Arthur H. Graves of the Brooklyn Botanical Garden started planting chestnut trees and making hybrids in the early 1930's. There are 5 Castanea species which have 3 nuts per bur including the American chestnut (dentata), European (sativa), Japanese (crenata), Chinese (mollissima), and Chinese dwarf chestnut (seguinii). These five Castanea species have the ability to form hybrids but only the Asian species are viable due to their resistance to blight.
The American chestnut survival involves a test of integrated management plan which combines managed cutting with the introduction of improved American chestnuts and hypovirulent strains of blight fungus. Hypovirulence is a type of biocontrol which slows down and weakens disease progress (Griffin WWW). The Nature Conservancy's Harkness Preserve in Rockport, main are working on preservation of the American chestnut. Their weapon in their conservation efforts are strains of weakened hypovirulent fungus. The way in which this fungus works is that the tree fights a lesser of two evils. Peter Blanchard states, "the tree winds up fighting a low grade flu rather than life threatening pneumonia (Anderson 1992)." Many of the 43 trees located at the Harkness Preserve have healed cankers and only three have died because of blight. With this form of hypovirulence treatment, there are drawbacks. Hypovirulence may not spread as easily among American chestnuts because blight has differentiated into at least 250 strains (Anderson 1992). Other variables include peculiarities of each tree, weather, and site ecology. There are cases where some American chestnut trees have survived despite the fact being infected with blight.
An isolated incident occurred in 1992 in Pennsylvania with a small group of pure American chestnuts, where these trees demonstrated growth while still infected with blight. A particular tree found in Erie County, near Springfield, a 39 foot tall, 44 ft spread American chestnut tree stands with blight (Chamberlain WWW). Further analysis shows this tree is in a gradual state of decline. Killing and healing cankers are both present in the crown of this tree. The healing cankers are more dominant in the lower limb. The explanation for this phenomena, is perhaps due to hypovirulence. This hypothesis does, however, lack experimental evidence of any kind.
The American chestnut which was once almost eliminated from existence in the late 1950's has once again emerged thanks to conservation efforts. Chestnut blight, a deadly fungus, has the ability to kill chestnut trees. However, some chestnut species in Asia have resistance to blight. As a result, a method of conservation has been through hybridizing American species with Asian species. Another method of conservation has been through hypovirulence strains in the infection is reduced. Independently, this hypovirulence method may be a reason why some American chestnuts are surviving despite being infested with chestnut blight. This issue remains important to both the timber industry, although that isn't why chestnuts are being restored, and certain ecological organizations including the Nature Conservancy.