Written by Scotty Smith, Director of Land Conservation
The American chestnut tree, Castanea dentata (C. dentata) was once one of the most dominant tree species of deciduous forests in the eastern United States, comprising as much as 25% of the total timber volume in portions of Appalachia. There were an estimated 4 billion individual chestnut trees in these forests (TACF), making the American chestnut tree a vitally important source of lumber, food, and wildlife fodder in these areas. Having vigorous growth that creates monumental trees, wood that is resistant to rot, and nuts that are sweet and nutritious for both humans and wildlife, this species was essentially the perfect tree until everything changed in the early 1900s. The American chestnut survived for 40 million years, only to be nearly wiped out in only 50. This is one of the most, if not the most, impactful ecological disasters to take place in our forests in human history.
In 1904, Cryphonectria parasitica (C. parasitica), the fungus that causes chestnut blight, was first reported in New York. Within 50 years, the air-borne spores had spread across the chestnut’s natural range, killing almost every individual and reducing the trees to basal shoots, which eventually succumbed to the blight as well (Anagnostakis, 1987, 2001). The fungus attacks the cambium and eventually girdles the branch or trunk, resulting in tree death. It is parasitic and co-evolved with the Asian species of chestnut trees. While the Asian species (the Chinese and Japanese chestnuts) have a natural immunity and rarely succumb to the blight, it wreaks havoc on the American chestnut, which has little to no immunity. The issue with the disease is that it usually kills the host before the tree is mature enough to flower and spread its genes. To make matters worse for this majestic tree, officials went on a campaign in the early to mid 1900s to save the lumber before the trees were affected. Many of the trees that could have potentially offered immunity to the blight were hacked down, reducing the population of adult trees even further. Chestnut trees are still somewhat common in our eastern forests, but are functionally extinct because of the lack of mature flowering adults.
Another infectious agent that affects the American chestnut tree is the oomycete Phytophthora cinnamomi (P. cinnamomi), which causes ink disease, or Phytophthora root rot, in chestnut trees and several other tree species (Robin, 2012). Oomycetes are a fungus-like eukaryotic microorganism. The organism essentially feeds on the roots of the host plant, weakening and killing the living root tissue. The black necrotic tissue caused gives it the condition the name “ink disease.” P. cinnamomi was first described in 1922 in Sumatra, but likely spread across the world long before (Robin, 2012). It has the broadest host range of any Phytophthora species, but is particularly destructive in the Southeastern United States (Robin, 2012).
The effort to restore the American chestnut species has been going on for over 90 years with relatively little progress made over much of that time (TACF). However, in 1983 The American Chestnut Foundation was founded and has been developing American chestnut populations that are resistant to the diseases caused by C. parasitica and P. cinnamomi. Resistance will allow natural selection to resume for this valuable tree species that is now functionally extinct which in turn, will increase biodiversity in our forests, increase food for wildlife, allow cultivation for nut production, and allow the harvest of high quality timber. The American Chestnut Foundation has been using backcross breeding to introduce genes for blight resistance from the Asian chestnut species into the American chestnut in order to grow trees that are phenotypically identical to American chestnuts, but carry the resistance found in the Chinese and Japanese species. The Chinese chestnut is the most commonly used species for backcross breeding programs. The current generation of hybrids are roughly 96% American.
The backcross breeding process is a long one. It begins with an American and a Chinese chestnut which are bred together. The offspring are grown out for several years in nurseries, orchards, or woodland settings. After a period of time to allow for establishment, the trees are intentionally injured and inoculated with the deadly fungus (C. parasitica). The trees get a canker because of the injury, which can be used to evaluate the trees resistance. Most trees succumb to blight, but some are immune and survive. The idea with backcross breeding is to only select the very best specimens, so generally over 90% of the population will be removed. The best of the surviving trees, which would be theoretically 50% Chinese, are then hand pollinated with pollen from an American chestnut tree. The offspring of the hybrid trees are again grown out and evaluated for resistance. The surviving trees are 75% American and 25% Chinese. The hybrids are again bred with full American chestnuts. This process continues until the tree is 15/16 (or 96%) American, which is the point where the program is today. The hope is to breed a tree that has the natural resistance of the Chinese tree but the form and function of the American chestnut.
In addition to backcross breeding, the New York chapter of the American Chestnut Foundation has been working on genetically modified American chestnut trees that are blight resistant. This process essentially just involves adding two additional genes to the American chestnut genepool to infer blight resistance. The most effective gene tested to date encodes for an enzyme called oxalate oxidase, which breaks down oxalic acid, one of the main weapons the blight fungus uses to kill the chestnut trees (TACF). This gene is actually rather common in the plant world, including in many of our crops, so it is proving safe for release into the environment. These trees are even more American ( >99.999%) than their backcrossed cousins.
Here at Reflection Riding, we are involved in the backcross breeding program. We have two small test orchards that are side by side on the property. This was implemented in the early 2000’s by a former board chair, Bill Brooks, who was a member of The American Chestnut Foundation. The first orchard (the southern set of trees) is the backcross orchard. These trees are American Chestnut Foundation hybrids crossed with American chestnuts from Tennessee. These hybrid trees have pedigrees going back all the way to the 1930’s in Connecticut and Maryland.The other orchard, the northern set of trees, are new hybrid trees being used to breed new sources of resistance into the backcross breeding program. These trees are F1 hybrids, and are 50% american and 50% chinese or japanese chestnut. Every year, chestnuts are collected from our trees and given to the The American Chestnut Foundation to create the next generation. Seedlings from these trees have been planted and are currently being grown out in experimental orchards across Tennessee.
The American Chestnut Foundation (TACF) website: https://www.acf.org/
Anagnostakis SL. 1987. Chestnut blight: the classical problem of an introduced pathogen. Mycologia 79: 23–37.
Anagnostakis SL. 2001. The effect of multiple importations of pests and pathogens on a native tree. Biological Invasions 3: 245–254.
Deason, Trent. 2018. Conservation and collection of Castanea dentata germplasm in the South. Honors Thesis. UTC.
Gentner, Kevin. 2018. Evaluation of blight resistance in chestnut F2 half-sibling and full-sibling families via small stem assay. Honors Thesis. UTC.
Hein, Kirsten. 2018. Implementing early screening methods to detect resistance to Phytophthora cinnamomi in backcross Chinese-American Chestnut hybrids. Honors Thesis. UTC.
Robin, C., Smith, I., Hansen, E.M. 2012. Phytophthora cinnamomi. Forest Phytophthoras 2(1). doi: 10.5399/osu/fp.2.1.3041.
Robinson, Anna C. 2016. Measuring Phytophthora resistance phenotypes in segregating testcross families of hybrid American chestnut trees. Honors Thesis. UTC.
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