Researchers identify new threat to American chestnut trees

Emily Dobry is a graduate student who has been conducting research at Lake Erie Regional Grape Research and Extension Center. She discovered a new fungal threat for the American chestnut tree. Credit: Penn State Behrend
The American chestnut was an ideal treetall for lumber companies. It was straight, rot-resistant, and easy to split. It was also prolific and produced many new shoots, which grew rapidly.

The species was a significant part of eastern hardwood forests in the early 1900s. The United States had nearly four billion American chestnut trees. Each tree grew up to 100 feet tall and had trunks between four and seven feet thick. The average lifespan of healthy trees was 400-600 years. They produce several bushels each year.

It can be hard to find healthy American chestnuts today. The species was decimated by a fungal pathogen that infected trees from Japan and China. It took less than 40 years. This is the largest ecological catastrophe to hit the forests of the world.

"The pathogen is indigenous to Chinese and Japanese chestnuts so the two coevolved," stated Emily Dobry, a Penn State Behrend graduate currently in Penn State's plant sciences, horticulture master’s-degree program. She is currently conducting research at the University's Lake Erie Regional Grape Research and Extension Center, (LERGREC), in North East. The American chestnut was not exposed to it before and so had very little natural resistance. It can be described as "smallpox for trees."

Today, there are less than 1000 American chestnut trees. They can be found in isolated locations in New England and along the Appalachian mountain range.

One of these is LERGREC. Researchers have conducted a trial with 15 chestnut trees since 2013, which includes five each of American-Chinese hybrids species and one row of Chinese American-Chinese.

Bryan Hed, a LERGREC plant pathologist, stated that the idea was to plant American chestnuts alongside some hybrids, and allow them to be challenged by chestnut blight over time.

He said that most of the trees had suffered from diseases, insects, or weather and needed to be cut down and renewed. The hybrid trees are a notable exception: Three of them currently measure 17-21 feet high.

Sucker growth is used to revive trees from their original rootstock.

Dobry stated that the American chestnut has been declared "functionally extinct", which means that, although technically it still exists, it can no longer reproduce." "The shoots are rarely large enough to produce nuts and so, the future generations will not be able to reproduce."

Transgenic, the development of a genetically altered tree, is currently the most promising hope for American chestnut. Scientists are working to create a tree that is as American as possible. They will use just enough genetic material from the Chinese chestnut for resistance to the blight.

Dobry stated that researchers have created partially resistant transgenic American chestnuts to blight. These trees can withstand infection by the pathogen that causes it. It doesn't kill it; it still exists, but it doesn’t destroy the tree.

As scientists attempt to reintroduce American chestnuts, the current balance of species in American forests could be a problem. The oak tree has now taken its place.

Dobry stated that it would be difficult for the American chestnuts to reestablish itself in the forest ecosystem.

Other problems could be caused by the American chestnut. Dobry, an undergraduate researcher at LERGREC in 2018, discovered a fungus not normally found in the American chestnut species.

She said that initially, she thought it was a rare case of chestnut blight. But, after collecting samples and conducting research, we discovered that the pathogen known as chestnut brownrot, not blight, was what she had suspected. There had not been a published report on this fungus in our region at the time.

Dobry continues to research the domestically isolated strain and examine whether it might be harmful to closely related trees, such as oaks.

She said, "If this fungus proves to be pathogenic to oak it could be an additional blow to forest dynamics." "This is why the chestnut project, and other similar projects, are so important in protecting the diversity and future forests."

To understand the ancestry of trees and their immunity, explore further using DNA "fingerprinting".