When the first European settlers arrived in the new world, the American Chestnut (Castanea dentata) was the dominant tree species in the area that would later be the eastern United States. Ranging from the panhandle area of Florida to the forests of Maine, the American Chestnut accounted for one in four trees in the forests it inhabited. Tall, fast growing, and resistant to drought and shade, the Chestnut competed very successfully with the other trees, such as Oaks, that shared its range. With a precolumbian population of 4.5 billion trees, (my estimate) many of them over 100 feet tall with a diameter of up to seven feet, the nut-bearing American Chestnut was a critical part of the ecology of the eastern Unites States. Chestnuts bear nuts reliably -- unlike Oaks which only bear acorns abundantly when the weather is suitable -- and, in their day, Chestnuts were the primary food source for a wide variety of wildlife species.Strong, light, fine-grained and straight, the American Chestnut was used for everything the early settlers made out of wood. It was especially prized for use as masts for ships and, since the wood is highly rot-resistant, it was used for all applications where wood was in contact with the ground, such as fence-posts and foundation timbers. The nuts were eaten, used to fatten livestock, and were a significant export product for the colonies. Tannin from the wood and bark was used to make leather. The Chestnut was as central to the economy of the colonists as it was to the ecology of the area.
The first blight to hit the American Chestnut was the introduction of Phytophthora cinnamomi, an organism that causes root rot, originally from the far east. Phytophthora cinnamomi affects an large number of plant species, among them the American Chestnut. (In the Chestnut it is also called Ink Disease because of the discoloration it causes to the roots.) It is impossible to know exactly when or how it was introduced, but it probably came with a shipment of goods or ornamental plants, and a good guess of when would be around the time of George Washington's presidency. By 1825, when the problem of root rot in agricultural planting began to be discussed, the infestation was widespread.
Phytophthora cinnamomi is most damaging in warm, moist areas at lower elevations and it attacked Chestnuts in the lowlands of Georgia and the Piedmont area of the Carolinas and Virginia. Phytophthora attacks the roots of the tree and the whole tree dies. By the time of the Civil War the Chestnut was largely extinct in the low lying areas of the southern states but it continued to thrive in the mountains and the northern states where the altitude or the cooler weather offered protection. At the turn of the twentieth century it was still the dominant tree of the Eastern Unites States but its range had been reduced at the southern end and its population had dropped to a mere 3.5 billion trees.
The second blight hit a century after the first. In 1904 a second fungus affecting Chestnuts was discovered in the Bronx Zoo of New York City. Cryphonectria parasitica, also an import from Asia, spread quickly, carried by wind and animals, and over the next 40 years American Chestnuts across the entire range of the species were infected with the blight and died back to the roots. The spread of the Chestnut blight fungus to the US was one of the greatest environmental disasters in recorded history. Its impact to the ecology of the area cannot be overstated.
The blight fungus cannot live in contact with the soil and does not attack the roots. Today, the American Chestnut is still quite common in easterm forests where it is an understory species that sprouts from the roots of trees that were killed three quarters of a century ago. The sprouts grow for a year or two and then are killed by the blight. There are a very few flowering adult American Chestnut trees that, through luck or some slight resistance to the blight, grow large enough to produce nuts. These surviving "mother" trees are the key to the efforts to reintroduce the species.
Selective Breeding of these surviving American Chestnuts is one approach to to restoration. The American Chestnut Cooperators' Foundation (ACCF) is an organization dedicated to restoring the American Chestnut by selectively breeding resistant, pure-American trees. Since their founding twenty years ago they have made some progress in developing methods to keep the trees alive using a combination of selection for resistance, physical management of the trees and biological management of the blight (which will be discussed later). They remain hopeful but are far from having a tree that can be used to restore the wild population. Sadly, there is little reason to hope that the genes for substantial resistance can be found in the pure-American gene pool.
Cross-breeding with more resistant species is another approach to restoration. The American Chestnut has a number of relatives with which it is interfertile and several of them are highly resistant to the blight. In the 1940s and 1950s the federal government sponsored a program to produce a blight-resistant hybrid of the American and Chinese Chestnuts that would have the blight resistance of the Chinese and the vigorous and straight growth characteristics of the American. This program was, by and large, a failure; Strong blight resistance requires the combined effects of several genes and most of the trees with enough Chineese genes to resist the blight also shared other Chinese characteristics such as crooked growth and short life.
About the time it was becoming apparent that the cross-breeding approach was more difficult than previously hoped, another possibility presented itself: A virus was discovered that infected the blight fungus, rendering it less damaging. This virus, called hypovirulent because of its tendancy to reduce the damage done by the fungus, was shown in many cases to allow blighted Chestnut trees to heal the lesions caused by the fungus. Given the poor results of the hybridization efforts and the promise offered by combatting the blight with the virus, the federal cross-breeding program was abandoned and the government effort was focused on the use of the virus.
The work on controlling the blight using hypovirulence continues today, as does a degree of federal sponsorship. The ACCF and researchers at Virginia Tech are working to control the blight with hypovirulence. In Europe the slightly more blight-resistant European Chestnut is showing some recovery, in large part because of the spread of hypovirulence. In the United States the results to date have been less encouraging. Researchers have shown that, by inocculating trees with a combination of several strains of hypovirulence, individual trees can be made somewhat resistant to the blight but the resistance does not seem to spread to adjoining stands, as it does to a greater extent in Europe. You can read more about hypovirulence, including considerable information that I have simplified or omitted, in a master's thesis that I found online: (Nancy Robbins, VPI 1997) PDF.
The American Chestnut Foundation (TACF) is a private organization that was formed in 1983 to pick up the cross-breeding effort, more or less where the government program left off. Looking at the results of the initial effort they re-evaluated the genetics of blight resistance. From the lower than expected number of blight resistant offspring in the American/Chinese hybrids they concluded that blight resistance requires the combined effect of more than one gene, probably three, and that the resistance was "partially dominant." Partial dominance means that a tree that received all three resistance genes from one parent, but none from the other, would exhibit some resistance to the blight but much less than the fully blight-resistant (Chinese) ancestor.
Armed with this new understanding of the genetics they developed a multi-generational back-cross method that would result in a tree that was indistinguishable from its American ancestors except that it was highly resistant to the blight. To accomplish this multiple generations of hybrids are produced with one of the parents in each generation being American.
The first generation is half Chinese. These trees are infected with the blight (Cryphonectria parasitica) and the ones that live (about one in eight) will be the ones that received all of the resistance genes from the Chinese parent. These trees are then crossed back to pure American stock giving offspring that are one quarter Chinese. Again they are infected with the blight and only those trees with the genes for blight resistance will live. This process is repeated twice more giving trees that are fifteen sixteenths American but still have the resistance genes from their Chinese great-great-grandparents.
These trees should have very American characteristics but will only be partially blight-resistant since the resistance is only partially dominant, and one parent did not have the genes. These trees are then crossed with other trees from their generation. Some of their offspring will get resistance genes from both parents and should be as resistant to the blight as their Chinese great-great-great grandparents. This intercross is repeated one more time, selecting for trees with strong blight resistance and the fewest Chinese characteristics, and the resulting trees should be true-breeding, highly resistant, American Chestnut trees, for all practical purposes.
Since each generation takes slightly less than six years, and since the entire plan requires six generations, the expected time to complete the plan from the start is approximately 35 years. The good news is that the initial breeding effort produced trees that could be used for the second (one-quarter-chinese) generation so when TACF started in 1983 they could skip the first twelve years. If you do the math, that suggests they should have nuts ready to start reintroduction very soon now. They appear to be on schedule and they hope to have nuts for seed to start the reintroduction in the next few years. The last intermediate generation is being planted now.
But, we may not have to wait... A number of researchers are working on the American Chestnut problem using a more modern, “genetic engineering” approach. From the results obtained in the classical genetics breeding effort they know that the resistance to the blight requires three genes. A number of researchers are trying to map their locations on the chromosomes. Once the exact genes that confer blight-resistance are known a number of things become possible. The most obvious step is to speed up the hybridization effort already underway. It takes a long time and a lot of effort to raise trees to the age where they can be screened for blight resistance by inoculating them with the blight fungus. With genetic screening a much larger number of offspring could be evaluated in a much shorter time. Time and effort would not need to be wasted on trees that would die when inoculated with the blight.
Another approach –- one that is more difficult but potentially offers a better result -- is to create a transgenic American Chestnut that contains only those Chinese genes needed to confer resistance. A tree so produced would be 99.999% American -- as opposed to the TACF hybrids that are targeted at about 94%. TACF hopes that their 6% Chinese trees will be indistinguishable from their American ancestors but that is not guaranteed. The transgenic trees would be almost certain to have an entirely American phenotype.
Yet another approach does not depend on identifying the three elusive genes that confer resistance in the Chinese Chestnut. Many genes tend to perform the same function in different species, and the genes that confer resistance in one species will often work in another. A great deal of research has been done on the genetics of food crops (corn, wheat, fruit trees, etc.) because the return on investment in such research tends to be large and immediate. A number of the genes in the "toolbox" that genetic-engineers use for working with other species come from this research on food crops. Researchers at the State Univeristy of New York are working on an American Chestnut that contains OXY, a gene from wheat that encodes oxalate oxidase. Oxalate oxidase breaks down oxalic acid, the compound exuded by Cryphonectria parasitica to kill cells. They hope to begin test plantings this fall. There is good reason to be hopeful about the SUNY effort because on Arbor Day last year they planted two transgenic Elm trees that they hope will be resistant to Dutch Elm disease.
With the TACF's blight-resistant near-American trees entering the last generation and SUNY's even-more-American transgenic trees to be planted in the fall we can reasonably expect to begin reintroduction of the American Chestnut in the next decade -- not long at all for breeders of trees.
Victory is at hand, except... There were two different blights, remember? The efforts we have examined so far only address the more recent (and more serious) blight. These new blight-resistant trees will still not grow in parts of the original range. In most of Georgia and in the Piedmont of the Carolinas and Virginia they will be killed by Phytophthora cinnamomi, root rot disease -- the first blight.
The Carolinas Chapter of TACF is working to solve that problem, too, but a bit more time will be needed. One of the more active members of the chapter was eager, a number of years ago, to participate in the TACF breeding effort. He has a 200+ acre farm near Clemson South Carolina that he wanted to use to help with the effort. The problem was that most of the trees he planted would die of the root rot before they grew large enough to be tested for blight resistance. He could grow Chinese and Japanese Chestnuts but Americans would die. He consulted with some geneticists and experts on plant diseases and discovered that the problem was a Phytophthora infestation.
Resistance to Phytophthora cinnamomi is also present in the Chinese Chestnut -- another two or three genes -- not the same genes that confer blight resistance. Working with a genericist at Clemson he came up with a method for screening for Phytophthora resistance. He has six big tubs out in front of his house and he plants hundreds of carefully labeled nuts in each one. He plants Pure Americans and nuts from a number of different TACF hybrids. When all the nuts have sprouted and seedlings are coming up he infects the tubs with Phytophthora cinnamomi, looking for resistant individuals.
He has not yet found any resistance in the latest generation of TACF trees. Since they are only one fifteenth Chinese and their ancestors were not selected for Phytophthora resistance it is highly unlikely that they would have the needed genes, and indeed they don't -- they all die. But he has identified a number of individual trees from a few generations back that do have good resistance to Phytophthora as well as to the blight.
In the same way that the TACF was able to use the work of the federal breeding effort to skip a generation, the Carolinas Chapter is hoping to use the national TACF effort to skip a few more generations in the effort to produce an American Chestnut that resists both Cryphonectria and Phytophthora. This requires selecting for a larger number of genes which means that a much smaller percentage of each generation will qualify but, since the Phytophthora screening can be done in bulk with seedlings just out of the nut it should be feasible. But it will take a while -- figure 2025 for true-breeding trees with resistance to both blights and a strongly American phenotype -- unless the generic engineers can find a gene for root rot, too.
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