American Chestnut Tree

The story of the American chestnut is well known to the people of Appalachia listening to stories told by their grandparents and great-grandparents. This tree, once called the redwood of the east coast, could grow eight feet in diameter and stretch 120 feet in height. Chestnuts trees were so productive, seeds from a single tree could sustain a pioneer family through a harsh winter.  Timber cut from the tree could last 100 years without rotting. From these reason and many more, the American chestnut was considered the most valuable tree in the eastern forest.

 

Beginning in the early 1900s, American chestnut trees in New York City began to die off in mass. The culprit was a pathogenic fungus (Cryphonectria parasitica) from eastern Asia that entered the trees through small cracks in the bark. Once infected, cankers would form around the trunk of the trees starving the tree of nutrients. They would then die off over a period of several months, surviving only within the root systems. Suckers would grow from the tree roots only to succumb again to the fungus before the trees reached 20 feet in height. Considering that American Chestnuts will only produce nuts when the trees reaches 20 feet in height and around 8 years in age, chestnuts seeds were completely eliminated from our forests in under a generation.

 

Fortunately, large chestnut trees planted outside of their original range can still be found. I’ve found multiple articles describing large chestnut trees from Oregon to British Columbia. Whether this is due to the fungus never being introduced to this part of the country or to the mild weather and soil conditions, their survival in the Northwest is not fully understood.

Outside of a few dozen trees surviving through a combination of luck, blight resistance, and hypovirus (a blight that kills the chestnut blight) the American chestnut tree was lost from the east coast. These remaining trees are now being used by researchers to develop blight resistant trees that could one day be introduced back into our forests.

Strategies for the Future:

Scientists are proceeding with three separate strategies to bring back the American chestnut tree. The first is to cross-breed the surviving pure American trees to produce future generations of trees with greater resistance to the chestnut blight. The second effort started by crossing an American chestnut tree with a naturally disease resistant Asian Chestnut tree to produce offspring that look like an American Chestnut tree while offering protection from the fungus. The third and most recent effort uses cutting-edge science to insert a wheat gene into chestnut trees to produce a tree that can fully resist the chestnut fungus.

I’m going to offer my opinion on each strategy from a non-scientific position (I’m an engineer) to discuss what the future may hold for each team of researchers.

The obvious strategy is to find surviving American chestnut trees that then can be cross-bred to develop trees that offer more resistance than their parents. Breeding these resistant trees with each other over several generations will hopefully produce an American chestnut with nearly the same blight resistance as an Asian chestnut. While this may appear to be a no-brainer, it actually requires the most amount of effort to complete.  The American Chestnut Cooperators Foundation is leading this effort through a nonprofit scientific and educational foundation formed by researchers from Virginia Tech’s Department of Plant Pathology, Physiology & Weed Research, as well as, scientists from Concord College in West Virginia.

Beginning in the 1970s, the foundation has worked with the public to identify large, surviving trees. A male catkin pollen is collected from an American chestnut to then pollenate a female flower from a second surviving American chestnut. The seeds are then collected in the fall to be distributed to volunteers that grow and monitor the offspring. The best trees are then cross-bred to other disease resistant trees for several generations (I believe that they’re up to the 6^th generation of trees).

Their website is http://www.accf-online.org/ but it’s infrequently updated. In addition, they don’t go into great detail as to the survivability of the 6^th generation trees, only stating that they may express some disease resistance. What this means is difficult to understand. My guess is that the even their best trees may only live for 30-50 years before succumbing to the fungus, instead of living to 200-300 years as the trees did prior to the 1900s. Either way, they’re making a great effort that should eventually pay off.

The American Chestnut Foundation is a nonprofit team founded in 1983 by a group of scientist in Virginia. This team initially cross-bred an American Chestnut with a blight resistant Asian chestnut to produce a descendant that could fight off the chestnut fungus while exhibiting some American chestnut features.  Trees showing good resistance were then back-crossed with pure American Chestnuts to produce trees that were 75% American Chestnuts and 25% Asian Chestnut.

Back-crossing for several more generations produced the current tree that is 15/16 American chestnut and 1/16 Asian chestnut. These trees exhibit the look and form of an American chestnut while still having the blight resistance of an Asian chestnut. Considering that an American chestnut contains around 100,000 genes, these 15/16 American chestnut trees include over 6,300 Asian chestnut genes, some of which may be counterproductive to the ACF’s efforts.

 

While it would make sense to a layman that they should continue to back-cross for several more generations to eliminate more of these unwanted genes, the American Chestnut Foundation appears to be focusing on introducing these 93.5% American chestnuts into our forests. It may be that the trees look just like a pure American chestnut, so their efforts would be better spent on reintroducing this tree back into our forests.

It’s important to note that the while the American chestnut and Asian chestnut trees are cousins, their adult forms are quite different. As stated earlier, an American chestnut tree can grow up to 120 feet in height and live for 200 to 300 years. On the other hand, an Asian chestnut tree is more similar to an apple tree in that they only grow 20 to 30 feet tall and live 50 to 75 years.

I would like to learn from the American Chestnut Foundation whether they believe these disease resistant trees are expected to mature like an all-natural American chestnut or produce a tree midway between the American and Asian chestnut. Maybe the goal for these trees is for them to reach 75 feet in height and live 100 years.

The American Chestnut Foundation has a really good website that is updated frequently. They can be found at www.acf.org.

The last and most recent effort is to insert non-chestnut genes into a pure American chestnut to limit the growth of the chestnut blight.

The American Chestnut Foundation is also working with the State University of New York, College of Envornmental Science and Forestry (SUNY-ESF) to insert individual genes into an American Chestnut tree to enhance pathogen resistance using the tools of genetic engineer.

The science behind this effort is over my head, but is still interesting. Typically, when a powerful medicine is found to treat illnesses (ie penicillin), over many generations, the disease begins to build resistance and eventually fully overcomes science’s best efforts to eliminate it. The ACF is using a different strategy to address this problem.

Native Range Map

 

Using biotechnology, scientists have inserted a wheat gene called Oxalate Oxidase to break down the oxalic acid found in the blight fungus. Instead of killing the chestnut blight, this gene will instead deactivate the fungus, preventing it from killing the chestnut. As this gene does not force the fungus to breed for resistance, scientist expects that future generations of chestnut trees with this gene will continue to prevail against the blight.

Considering that inserting a gene from wheat into a tree requires government approval, the release of this tree to the public could still be 10 to 20 years away. To address this problem, the ACF has is growing 10,000 transgenic trees with the hope that they can get a jumpstart once/if the USDA signs off on this process.

In my opinion, this plan offers the greatest opportunity to allow future generations to see a 300 year old American chestnut producing nuts for bears, wild turkeys and and maybe even pseudo-passenger pigeons (more on that on another blog) into our forests. On the downside, any mention of genetic engineering will create an outcry from the public. Scientists are experiencing this in Key West, Florida as I write. Their plan to release transgenic mosquitoes to fight off the deadly Dengue fever threatening public health. While spraying mosquitoes with deadly chemicals is okay with the public anything hinting at genetically alternated insects is feared.

Other Items:

For years, everything I’ve read about the loss of the American chestnut only concerned the chestnut blight killing trees found at higher elevations in the Appalachians. However, I recently learned about another chestnut disease that killed off chestnut trees living at lower elevations in the early 1700s.

The American Chestnut Foundation’s website discusses Phytophthora cinnamomi which is a virulent pathogen of the American chestnut tree that causes the disease Phytophthora (otherwise known as “root rot”).

Apparently, the American chestnut tree had a range extending beyond the Appalachian Mountains down to the Gulf of Mexico, east to the low county of South Carolina and north along the entire east coast. Because this disease quickly killed off low elevation chestnut trees hundreds of years ago, scientist only recently learned that there was a second disease capable of killing the chestnut tree.

From my understanding, Clemson University in partnership with North Carolina University and the US Forest Services, learned about this additional disease when disease-resistant American chestnut trees quickly died off when planted in coastal South Carolina.

As the Asian chestnut tree also has resistance to this second disease, it is expected that protection from Phytophthora cinnamomi can be quickly established in the American chestnut.