Chestnut canker
Source: Robert L. Anderson (USDA Forest Service, Bugwood.org)
Four different options have been pursued for chestnut restoration:
finding the few naturally-resistant wild-type trees still surviving in the woods with just American chestnut genes, then interbreeding for natural blight resistance and growing new stock from nuts/grafts to transmit that natural resistance to future generations
using the latest laboratory techniques to insert blight-resistant genes into the Castanea dentata genome, developing a genetically-modified organism (GMO) that could live in the forest and survive the fungus
breeding a blight-resistant hybrid tree, mixing genes of the Chinese chestnut species (Castanea mollissima) with the American chestnut to create a hybrid with just enough Chinese chestnut genes to be blight-resistant
biocontrol by introducing a virus, bacteria, or other fungi that weakens the Cryphonectria parasitica fungus, allowing chestnuts to resist the attack
First Option: Finding Naturally-Resistant "All American" Trees
There are still millions of chestnut trees growing in the wild. Nearly all are young stump sprouts with a trunk less than three inches in diameter. Only a few manage to grw large enough to flower and produce nuts before being killed by the blight. It is very rare for a chestnut sprout to grow tall enough to compete with other species for light. A century after the blight swept through Virginia, the remaining chestnuts are trapped in the understory and no longer function as the dominant or co-dominant tree in any natural setting.
The survival of some trees incentivized an effort to find an all-American tree, naturally resistant to the blight. The American Chestnut Cooperators' Foundation, based at Virginia Tech in Blacksburg. has taken the lead in supporting the "find resistant native All-American chestnuts" approach. The vision is to use 100% American chestnut genes when replanting trees in the wild.
Seeds and grafts from relatively-healthy trees throughout the natural range of the chestnut have been planted near Mountain Lake, at the Blacksburg Airport, in Lesesne State Forest (Nelson County) and in other locations near Virginia Tech. In 2012, the foundation had growers in 28 states and Canada.
In contrast to the restoration approach of The American Chestnut Foundation (TACF):1
...the American Chestnut Cooperators' Foundation (ACCF) is not using Oriental genes for blight resistance, but intercrossing among American chestnuts selected for native resistance to the blight.
However, it appears that no 100% American chestnut trees still surviving have the ability to pass along their resistance to future generations. Rapid harvest of the infected trees before World War II may have removed most of the individuals with natural genetic resistance to the introduced fungus.
The largest "champion" chestnut in Virginia, located on a pasture fenceline in Amherst County, is one of the survivors that lacked the ability to transmit its good fortune to its offspring. The remaining healthy wild-type trees may have been located in areas that received fewer-than-average fungal spores, perhaps because of wind patterns. If that is the case, then trees may not be resistant. Those few wild-type trees may be survivors because, just by chance, they were not heavily infected.
An alternative explanation: the trees may have been infected by a hypovirulent (weakened) form of the fungus, so natural resistance was sufficient to fight off the blight. A virus does affect the fungus and weaken it. Chestnuts in Europe have recovered somewhat, as a result of this pattern.2
Yet another possibility is that the healthy trees have some genetic pattern that provides resistance to the blight, but the genes are not expressed fully in the offspring (F1) generation. In that case, breeding healthy trees to each other will not create a new generation of blight-resistant chestnuts.
The first option does not offer a viable way to re-create a natural chestnut population in the wild, because no one has found naturally-resistant trees which can pass their resistance along to the next generation. The search still continues, however. In 2019, a healthy 50-year-old chestnut tree was found in Delaware in an isolated part of the Delaware Nature Society's Coverdale Farm Preserve.3
old chestnut tree, in full growth form
Source: US Department of Agriculture, PLANTS Database (Image by John Foley, provided by National Agricultural Library. Originally from US Forest Service. United States, MD. 1901)
deer fences protect trees planted at The American Chestnut Foundation's Meadowview Research Farms
Second Option: Breeding a Chinese/American Hybrid
Botanists began cross-breeding American chestnuts with other Castenea species in 1894, before the arrival of the chestnut blight was recognized a decade later. Efforts by the US Department of Agriculture to produce a blight-resistant hybrid that would be "true to type" (i.e., a tree with all the characteristics of the American chestnut plus genetic resistance to the fungus) ended in 1960. The Connecticut Agricultural Experiment Station continued research into creating hybrids, partnering with what is now Virginia Tech.
The American Chestnut Foundation, founded in 1983, in now the leader in efforts to cross-breed the Chinese chestnut (Castanea mollissima) with the American chestnut (Castanea dentata). The Wagner Research Farm was started at Meadowview, Virginia in 1989. There are now four sites totaling 160 acres that form the Meadowview Research Farms. The second farm was donated in 1995, and two others were purchased in 2002 and 2006.
Two parent trees (named "Clappper" and "Graves") located at the Connecticut Agricultural Research Station have been the primary sources of American chestnut pollen. Other parents (including "Nanking" and "Mahogany") are also involved in testing.4
inside the Glenn C. Price Laboratory of The American Chestnut Foundation, at Meadowview Experimental Farms in Washington County
Over seven generations of chestnut hybrids have been raised since the 1980's. To start, an F1 hybrid of 50% Chinese and 50% American trees was backcrossed with an American chestnut to create a B1 generation, then backcrossed again to create a B2 generation. The backcrossing increased the percentage of American genes while theoretically retaining the blight-resistant genes of the Chinse chestnut parent. The F1 hybrids were roughly half the height of the 100% American chestnuts. Though B1 and B2 hybrids had more American genes, their lack of disease resistance stunted their growth.
The B3 trees chosen for subsequent intercrossing were selected based on their morphology and blight resistance, in hopes of creating a population that grew as tall and straight as the American chestnut but carried the genes from Castanea mollissima to resist the blight. Cross-bred trees that displayed cankers were common. Vertical cankers indicated the wound tissue was having more success at isolating damaged tissue, compared to cankers that quickly girdled a trunk or branch. The longer a hybrid survived, the more likely they would be selected for the next stage of breeding.
In 2023, the American Chestnut Foundation reported:5
Thirty years of backcross breeding to capture the natural blight resistance of Chinese chestnut, involving over 50,000 trees, has resulted in approximately 500 trees that contain on average only 12% Chinese chestnut DNA but have inherited at least some blight resistance, and some of these advanced hybrids also exhibit the desired character typical of American chestnuts.
vertical chestnut cankers indicate resistance, as wound tissue isolates the infection
To breed chestnuts, volunteers and staff working for The American Chestnut Foundation get a lift in a bucket from a utility truck (or climb mature trees) to gather the male pollen from specific trees with desirable genes. Pollen can be stored pollen stored at -80C°, but can freeze and thaw only once to remain fertile. Before maturing, female flowers of key trees in the American Chestnut Foundation orchard are covered with bags ("chastity belts") to isolate them from wild pollen. That allows researchers to fertilize with pollen from a specific father with the desired genes.
Volunteers go back up the trees to pollinate female flowers with selected pollen so the tree will grow a nut with genes from pre-determined parents. After a summer of growth, the nuts are harvested by volunteers/staff who go up the tree a third time - ideally, before squirrels decide the chestnuts are ripe.
The American Chestnut Foundation (TACF) breeds chestnuts to be resistant, but the blight still kills them
By intercrossing offspring of Clapper and Graves with various American chestnut parents, the American Chestnut Foundation has produced a final genome that is 15/16ths American and 1/16th Chinese. The hybrids include at least some of the all-important genes for blight resistance from the Chinese ancestors, but showing primarily characteristics (such as height at maturity) from the American ancestors. Breeding to create blight-resistant populations that are also resistant to Phytophthora pathogen is also underway.
Breeding a Chinese/American hybrid is a long-term (perhaps 100-year) project, requiring continued private support and cooperation from universities and government agencies. Creating the BC3F2 generation that is 15/16ths American and 1/16th Chinese took until 2005, when the "restoration chestnut" was finally born. Further efforts included two more generations, one in 2014 and another in 2021, before reintroduction on a large scale was planned to start in 2028.
Reintroduction plans include establishing numerous stands of "new" chestnuts widely throughout the Appalachians, planting trees in each of the areas defined by the US Geological Survey 1:24,000 scale quadrangles. Planting of test plots began in 2008, with expansion to 14,000 trees planted on Appalachian Strip mines in 2014. Details of the genetics of planted samples are carefully planned and tracked. Wide distribution may lead to natural escape and growth of chestnut trees with American characteristics and blight resistance.6
greenhouses at the Meadowview Research Farms
The American Chestnut Foundation has distributed pollen and nuts to cooperators throughout the eastern United States. The scientific focus is on growing genetically-resistant trees. There, the Meadowview Research Farms are stocked with thousands of chestnut trees that are watered, weeded, fertilized, studied. In many cases, the trees that have been planted are destroyed and replaced, once the evidence is clear that a particular mix of genes is not a success.
Starting in 2009, crossbred chestnuts were planted in National Forests in Virginia, Tennessee, and North Carolina. Though the locations are not made public in order to protect the planted seedlings, it is possible to guess at one character of each site: they are protected from deer. Excessive deer browsing will kill chestnuts faster than the blight, so it is likely that restocking efforts will be concentrated on rocky outcrops and fenced areas.
On October 12, 2014, the Forest Service signed a Memorandum of Understanding with The American Chestnut Foundation to govern the restoration in the forests. The Federal agency highlighted the wildlife benefits rather than potential timber production:7
One of the greatest benefits of restoring the American chestnut will be a food source to wildlife because of its capacity for large and plentiful nut production.
If plantings are successful over the next 50 years and the chestnut is able to regain its once-dominant position in the Appalachian forest, then there will be substantial ecological effects. Restoration may be a good thing for the chestnut species in particular and perhaps for the southern forest ecosystem as a whole, but other species will be diminished in their significance. One concern is that a genetically-improved chestnut would become an invasive species comparable to the Bradford pear, or some insects that normally feed on chestnuts may have a toxic response to the hybrid Chinese/American chestnut trees.
The American Chestnut Foundation (TACF) relies upon volunteers to plant chestnuts at its nursery on Blandy Experimental Farm, the State Arboretum of Virginia
Third Option: Creating a New Genome
The third option - direct intervention in the genetic pattern of the chestnut to insert a new gene - is being pursued at the State University of New York, College of Environmental Science and Forestry and with numerous cooperators. National Science Foundation grants are supporting the research, with the expectation that the results will assist in tackling other diseases in other species within the Fagaceae plant family, including beeches and oaks.
Genetic modification requires far less time than standard plant breeding practices. It naturally takes about seven years to grow a chestnut to maturity and collect pollen/nuts for a new generation.
One early effort to speed the process used radiation-induced genetic modification. Nuts were exposed to a radiation source at Blandy Farm, now the State Arboretum of Virginia, but without success. Today, The American Chestnut Foundation exposes seedlings to 16 hours/day of full spectrum artificial light in a room in the Glenn C. Price Laboratory, completed in 2010 at Meadowview Research Farms. The extra light speeds up the maturation process so pollen can be created within one year.8
16 hours of full spectrum light per day at the Glenn C. Price Laboratory causes chestnuts to mature early
The chestnut tree turned out to be a difficult species to manage in a laboratory setting. It is relatively challenging to generate a seedling from a mass of chestnut cells, genetically modified or not.
Isolating the resistance genes in Chinese chestnuts and then transferring them into American chestnuts, creating a Genetically Modified Organism (GMO) in the laboratory and then planting nuts outdoors, is an option. However, the genes located naturally within the chestnut nucleus that allow Asian trees to resist the fungus are still not well understood. Multiple genes interact in order to confer resistance; blight resistance is not controlled by a single factor. To use modern tools such as CRISPER, geneticists need a better understanding of which nucleotides interact to create resistance before they can alter specific DNA elements.
Instead of transferring Asian genes into the American species through traditional Mendelian breeding or through CRISPER techniques, chestnut research is focused now on transferring a well-understood natural gene from wheat into the chestnut genome. The specific wheat gene produces the oxalate oxidase (OxO) enzyme. Any species that eats wheat (including humans) is already are well-adapted to the enzyme.
The oxalate oxidase enzyme disarms the blight fungus by converting its oxalate into carbon dioxide and hydrogen peroxide. Thanks to the OxO enyxme, the fungus shifts from being a deadly pathogen which attacks live tissue to a saprophyte which lives only on already-dead tissue.
Geneticists can achieve desired results through gene transfer without controlling where the gene will end up in a specific chestnut chromosome. So long as the gene is expressed and manufactures the proteins to form the oxalate oxidase (OxO) enzyme, the chestnuts acquire resistance to the blight.
Slowing the rapid growth of the Cryphonectria parasitica fungus allows an infected chestnut tree enough time to create wound tissue that blocks further invasion of the tree by fungal hyphae. Several transgenic American chestnut lines have been developed at the State University of New York, College of Environmental Science and Forestry. A new assay process, cutting a small segment of a stem from a young tree in a greenhouse and exposing that segment to the fungus to measure speed and extent of infection, allowed scientists to identify blight-resistant trees within one year of growth.
There are multiple strains of Cryphonectria parasitica, after 12 million years of evolution. At different research stations, three strains are used so results of resistance tests can be evaluated consistently. The "ep155" strain is the most infectious version used, while the "SG2-3" is the weakest.9
three strains of the chestnut blight fungus, Cryphonectria parasitica, are used to test resistance at Meadowview Research Farms
The oxalate oxidase (OxO) enzyme is a natural product in an unnatural place. It is not a pesticide. However, the transgenic process does not create a 100% American chestnut. It creates a Genetically Modified Organism that is Federally regulated.
the fungal hyphae of the blight grow faster than wound tissue, but adding genes to create the OxO enzyme might allow chestnuts to survive an infection
Source: Joseph OBrien, USDA Forest Service, chestnut blight or canker (Cryphonectria parasitica)
Other research is based on an artificially-created gene, mimicking the capability of frogs to survive despite all the fungi surrounding them in their moist habitats. The gene manufactures antimicrobial peptides, small proteins that kill the fungus.10
Though the transgenic approach produces a genetically-modified chestnut that resists the blight, there is still a major constraint in planting this new version of the chestnut in the wild. If all the laboratory products shared a common genome, any restoration effort would lack much of the natural genetic diversity that allows a forest to survive attacks by other microbes. One technique to increase diversity is to incorporate the OXO gene in different backcross lines that are more resistant than average.
In a natural setting, genetic variations provide varying resistance to a number of threats. Some trees in a forest survive different attacks, while other trees succumb - but there are survivors. If chestnuts from the laboratory shared resistance to blight and almost all their other genes too, then some pathogen other than Cryphonectria parasitica could wipe out all of the chestnuts at once. Restoration in the wild, once approved by Federal regulators, will require inserting the gene into different haplotypes and intercrosses.
In 2013, the Forest Health Initiative planted three experimental patches of chestnuts in the wild, including one location in Virginia. The three-year experiment assessed the resistance of the genetically modified chestnuts and the response of the ecosystem to the reintroduction. Experimenters speculated that using chestnuts that are all-American, without Asian genes, would minimize surprises that might develop during a wide-scale introduction where potential impacts of "alien genes" from Asia might create problematic interactions with other species in an American forest.
In the Darling 54 and Darling 58 transgenic lines, the OxO gene was located at two different places on different chromosomes. Both demonstrated resistance. By 2019, New York University was working with the American Chestnut Foundation to grow 10,000 trees. Transgenic pollen was placed on the flowers of blight-susceptible mother trees, with the expectation that perhaps half of the offspring would inherit the resistance. That mixed the genes and minimized the risk of planting a monoculture of new chestnuts with the same genome, so all the trees could be susceptible to a different pest or disease other than Cryphonectria parasitica.
In 2020, the State University of New York College of Environmental Science and Forestry sought Federal approval to plant transgenic trees from the Darling 58 line in the wild. The petition asked the US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) to declare that the blight-resistant plants did not have to be regulated, because the genetic engineering to add the capacity to produce the oxalate oxidase enzyme posed no risk. The American Chestnut Foundation had developed the dentataBase to track the location of all trees distributed for research.
The Animal and Plant Health Inspection Service included in its Notice of Intent to conduct an Environmental Impact Statement that the "preferred alternative" is to allow planting of Darling 58 transgenic trees in the wild, without regulations.11
A new line of transgenic, disease-resistant trees was announced in 2022. The "DarWin" line had a different version of the gene that produced oxalate oxidase (OxO), with the gene activated when blight infections occur. Getting Federal approval to plant DarWin trees in the wild will require submitting a separate application to the U.S. Department of Agriculture Animal and Plant Health Inspection Service; the request for approval of the Darling 58 line continued without modification.
Researchers planned to obtain approval to plant chestnuts with the DarWin trait, and were examining the potential to genetically modify chinkapin, elm, and ash trees to resist diseases better. The expectation was that Federal approval for the DarWin line would be swift, once the Darling 58 approval was granted, because:12
...the USDA will already be familiar with OxO expressing American chestnut trees, and therefore likely only go through step one of the review process. It is unlikely they
would find a higher plant pest risk than Darling 58, since the "DarWin" trees are producing less and more regulated OxO production. So, the time for completion is expected to be 180
days post Darling 58.
However, at the end of 2023 The American Chestnut Foundation announced it was withdrawing its support for Federal approval and cancelling all further research involving the Darling 58 American chestnut. Field and greenhouse tests showed significant performance limitations, with cankers expanding after an initial delay. What had been interpreted as successful resistance was apparently only temporary resistance. Darling progeny which inherited the gene which produced oxalate oxidase (OxO positive trees) showed reduced growth and survival, compared to OxO negative trees. The genetically modified trees were significantly shorter, so their potential for survival when planted in the woods was lower.
chestnut cankers appeared later in Darling 58 trees planted at Virginia Tech's Kentland Farm
Source: The American Chestnut Foundation (TACF), Darling Performance
The foundation also discovered that pollen with the OxO gene had come from a Darling 54 tree, not a Darling 58 tree. There had been a "switched at birth" pollen mix-up error at the beginning of the research, perhaps due to a swapped label on a vial of pollen in 2016. In Darling 58 trees, the OxO gene was inserted into chromosome 7; in Darling 54 trees, the gene was inserted into chromosome 4. Only in 2023 did scientists discover they had been growing Darling 54 trees, not Darling 58 trees.
Inserting the OxO gene on chromosome 4 deleted 1,069 base pairs in a native chestnut gene (SAL1) that was essential for salt tolerance. Disruption of the SAL1 gene may have been the reason that the Darling 54 trees (which had been thought to be Darling 58 trees) were relatively short. A Darling 58/54 tree that inherited both copies of the OxO gene on chromosome 4, creating a homozygous genome, did not survive.
The American Chestnut Foundation discovered that there were very few actual Darling 58 trees, and perhaps as little as just one. Rather than restart the research using the gene on chromosome 7, scientists chose to create a different genetic modification which would be expressed only in wound tissue. Using something other than the 35s promoter for gene expression would be required.
Finding a way to limit the effects of the OxO gene, rather than have constant expression of the gene, might minimize deleterious side effects such as reduced height. Constant or "constitutive" expression created by the promoter may give a tree the equivalent of a continuous fever, while an "inducible" expression would limit the impacts to just wound tissue.
the 35s promoter caused the OxO gene to always be expressed ("turned on"), while a different promoter might induce expression only in tissue exposed to the blight
Source: The American Chestnut Foundation (TACF), Chestnut Chat (December 15, 2023)
The transgenics approach was still viewed as both safe and promising, but the Darling line was assumed to be ineffective. The benefits of the inserted OxO gene were not expected to continue through a normal lifetime of a tree or be transferred successfully to subsequent generations due to "gene silencing," but the negative impacts from the disrupted SAL1 gene were being passed along. If introduced into the wild, Darling 54/58 trees would be shorter than average and would not compete successfully for light in a forest setting.
The foundation announced to donors in late 2023 that it would pursue different research strategies, and that it would not support further distribution of Darling 58/54 trees. The existing Darling 58/54 trees, such as the seedlings growing at Meadowview Research Farms in the greenhouse, would continue to be used for research.
The various types of chestnut trees previously distributed to foundation donors were backcrosses without the transgenic modification, since trees with the inserted OxO gene could be ground only in the few places authorized by Federal permit. The American Chestnut Foundation encouraged everyone with a chestnut tree to continue to care for it. The decision to stop researching the Darling 58/54 line did not create any reason to cut down chestnuts that did not have the inserted OxO gene.
The foundation announced it would not provide "leftover" Darling 54/58 research trees to donors or distribute them widely to orchards already established across the eastern United States:13
TACF does not support further development or deregulation of the Darling line of American chestnuts because, in our opinion, they have unacceptable performance and underlying genetic issues that make them unsuitable for release and restoration... TACF's position is that the Darling line is ineffective as a restoration tree, and may compromise future generations of disease-resistant American chestnut populations.
However, the foundation's scientific partner was more optimistic. The State University of New York College of Environmental Science and Forestry (SUNY ESF) decided to continue with the application for Federal approval to plant the genetically-modified Darling 54/58 trees in the wild.
The American Chestnut Foundation declined to support its former partner. The relationship was damaged in part because the foundation had been notfied about the mistaken identity of the Darling 54/58 trees by a separate lab in Maine, not by SUNY ESF.
The director of chestnut restoration at SUNY ESF noted that the Darling 58/54 trees remained under Federal permit, and there was no evidence that the trees posed any risk. After so much investment in the research, he contended that further effort was justified:14
It might bring up a question of whether it's better to have short trees or dead trees.... If the trees are resistant to blight and can keep growing, then the height might not matter as much.
The American Chestnut Foundation planned to pursue multiple other research paths, after cancelling further Darling 54/58 research with a constitutive promoter
Source: The American Chestnut Foundation (TACF), Chestnut Chat (December 15, 2023)
Fourth Option: Biocontrol by Weakening the Blight Fungus
If a virus, bacteria, or other fungi weaken the Cryphonectria parasitica, then a chestnut tree might constrain canker growth and survive into maturity. Creating a "hypovirulent" fungus would alter its lethal character. At a minimum, such a virus could be introduced in breeding orchards to facilitate growing trees which might be resistant due to genetic engineering or cross-breeding.
One dream of chestnut restoration specialists is that a hypovirulent form of the Cryphonectria parasitica fungus could out-compete the current form in the wild. A weakened fungus would allow American chestnuts with pre-1904 genes enough time to grow wound tissue and resist the girdling which kills the tree. In such a scenario, the native chestnut could recover naturally without a massive re-introduction effort.15
Today, The American Chestnut Foundation is pursuing the last three strategies to breed a blight-resistant tree in its "3BUR: Breeding, Biotechnology and Biocontrol United for Restoration" program.
Because planting a transgenic tree in the wild could have environmental impacts, the U.S. Department of Agriculture Animal and Plant Health Inspection Service (APHIS) had to complete an Environmental Impact Statement (EIS). The State University of New York - College on Environmental Science and Forestry (ESF) triggered the process in 2020 by filing a petition to deregulate the blight-tolerant Darling 58 American chestnut.
Federal regulations allow using transgenic pollen on American chestnut flowers, but require strong protection to prevent accidental release
Chestnuts are not the first tree species to go through the Federal regulatory approval process. In 2015 the Federal government approved planting genetically-modified apple trees, but only after the public review process stimulated 175,000 comments. An enzyme was modified so the "Arctic apples" would be less likely to turn brown when bruised or sliced open.
Those opposed to introducing the genetically-modified trees into the wild argued that the potential risks were not understood well enough. The preferred alternative was to focus on restoration using offspring of surviving pure American chestnuts.
Opposition was not limited to just the Darling 58 chestnuts. The Global Justice Ecology Project objected to plans by Living Carbon to grow poplars that had genes inserted to enhance photosynthesis and thus sequester carbon faster.
Living Carbon, a public benefit corporation, bypassed the review requirements of the US Department of Agriculture by using the "gene gun method" to incorporate into poplar chromosomes a trait for faster growth. The first genetically modified poplar trees were planted in a Georgia forest in 2023.
There was also opposition to a proposal to grow genetically-modified eucalyptus trees in Brazil, but by 2004 China had already planted more than a million genetically modified trees.16
Though development of chestnuts with a blight-resistance genome is a great challenge to reintroduction, ink disease caused by Phytophthora cinnamomi could still limit the ability to restore the chestnut as a major component of the Eastern deciduous forest. Modifying the complex genetic makeup of the chestnut species to address both internal threats is not yet accomplished.
One external factor is probably the most significant constraint on replanting chestnuts in the 21st Century. Deer browse so heavily on planted trees that the genome of the seedlings may not matter. If deer eat all the planted chestnuts, there will be no opportunity for new trees to demonstrate resistance to blight or ink disease.17
After more than 30 years of crossbreeding and intensive genetic research, no one is able to identify when chestnut restoration might revise the ecology of forests within the traditional range of Castanea dentata. Back in 1989, when 20 acres was first made available to The American Chestnut Foundation at Meadowview, the chief of research for the Virginia Department of Forestry made clear that overcoming the challenge required a long term perspective:18
non-resistant chestnut seedlings were planted on Bull Run Mountain in Prince William County in 2011-12 to test ability to survive deer, rodents, and storms
Chinese chestnut trees have multiple trunks and grow only to 50' high
chestnut burs protect multiple nuts
a grove of F1 crosses, with 50% American and 50% Chinese chestnut genes, at Meadowview Farms
B1 and B2 backcrosses have a higher percentage of American chestnut genes, and are more susceptible to blight
