Chestnut blight is a fungus that has virtually wiped out the American chestnut, as a commercial species, from eastern hardwood forests. Although roots from trees cut or killed many years ago continue to produce sprouts that survive to the sapling stage before being killed, there is no indication that a cure for this disease will be found, however breeding of a disease resistant tree is currently being tested. (More on this below.) The fungus is widespread and continues to survive as a nonlethal parasite on chinkapin, Spanish chestnut, and post oak.
A chestnut disease was first introduced to North America from an exported tree to New York City in 1904. This American chestnut blight, caused by the chestnut blight fungus and presumably brought in from eastern Asia, was first found in only a few trees in the New York Zoological Garden. The blight spread with a vengeance and in its wake left only dead and dying stems in what was a healthy forest.
By 1950, American chestnut had tragically disappeared except for shrubby root sprouts the species still continually produces (and which also quickly become infected). Like many other pest introductions, blight had quickly spread into its new - and defenseless - host causing wholesale destruction throughout the entire range of the chestnut. But with these sprouts brings hope of reestablishing American chestnut.
For decades, plant pathologists and breeders have tried to create a blight-resistant tree by crossing our own species with the resistant Chinese chestnut and other chestnut species from Asia, but always with unsatisfactory results. Now, advances in our understanding of genetics have shown us where those early researchers went wrong.
We are on the Attack of American Chestnut Blight and Restoring the American Chestnut!
Advances in genetics have shown us where those early researchers went wrong in working with the American chestnut. The American Chestnut Foundation (TACF) is confident that "we now know we can have this precious tree back." However, it still is a very complex process and will take years of work. Advances in genetics have shown us where those early researchers went wrong in working with the chestnut. The American Chestnut Foundation (TACF) is confident that "we now know we can have this precious tree back." However, it still is a very complex process and will take years of work.
In 1989, The American Chestnut Foundation established the Wagner Research Farm in Meadowview, Virginia. The purpose of the farm was to continue a breeding program.
Under the supervision of a full-time researcher, chestnut trees have been planted at the farm, crossed, and grown over the last eight years. In 1995, the farm was filled to capacity with over 5,800 chestnut trees at various stages of genetic manipulation.
TACF's breeding program is designed to do two things:
Introduce into the American chestnut the genetic material responsible for blight resistance.
Preserve the genetic heritage of the American species.
Preserving the genetic heritage of the American species is the hard part.
Old science told us that resistance is controlled by numerous genes running a very complex system. Scientists simply flooded chestnut progeny with Chinese chestnut genes by crossing their Chinese-American hybrids with other promising Chinese-American hybrids. The result was consistently a blight-resistant but very Chinese chestnut-like chestnut tree.
New techniques are now being used. By an elaborate and time consuming system of backcrossing and intercrossing, TACF's breeding program is attempting to develop a chestnut that will exhibit virtually every American characteristic. The desired tree is one that is fully resistant and when crossed, the resistant parents will breed true for resistance.
The method of breeding entails crossing the Chinese and American trees to obtain a hybrid which is one-half American and one-half Chinese. The hybrid is backcrossed to another American chestnut to obtain a tree which is three-fourths American and one-fourth Chinese, on average. Each further cycle of backcrossing reduces the Chinese fraction by a factor of one-half.
The idea is to dilute out all of the Chinese characteristics except for blight resistance down to where trees are fifteen-sixteenths American, one-sixteenth Chinese. At that point of dilution, most trees will be indistinguishable by experts from pure American chestnut trees.
TACF indicates that "the process of producing seeds and testing seeds for blight resistance now requires about six years for each backcross generation and five years for intercross generations.
The American chestnut may just be back in the near future...
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