A Tough Nut to Crack

Emily Dobry ‘20 and Bryan Hed

Emily Dobry ‘20 and Bryan Hed

Credit: Penn State Behrend

Scientists at Penn State research facility contribute to effort to revive the American chestnut

If you were to design the perfect tree, it would be the American chestnut. It was known to grow tall and straight and be rot-resistant, making it prized at one time for lumber, furniture, and musical instruments. Despite its tremendous size, once felled, it could be easily split. It was also prolific, sending up new shoots that grew fast.

The average mature American chestnut tree could be up to 100 feet tall and 4 to 7 feet thick at the trunk. A healthy specimen might be expected to live for 400 to 600 years and produce several bushels of nuts each year, literally raining down food for humans and animals.

In the early 1900s, this majestic species made up a substantial portion of the eastern hardwood forests. There were almost

4 billion American chestnut trees in the United States until a fungal pathogen transported on chestnut trees imported from Japan and China wiped the species out in less than forty years.

It is considered to be the greatest ecological disaster to ever strike the world’s forests.

Smallpox for trees

“The pathogen is native to Chinese and Japanese chestnuts, so the two co-evolved, but the American chestnut had never been exposed to it before and had little natural resistance to it,” said Emily Dobry ’20, a student in Penn State’s Plant Sciences Horticulture master’s program who is doing research work at the University’s Lake Erie Regional Grape Research and Extension Center (LERGREC) in North East, Pa. “Think smallpox for trees.”

Today, there are fewer than 1,000 American chestnut trees, largely in isolated areas outside of the tree’s historical range in the eastern half of the United States, along the Appalachian mountain ridge and through New England.

A few of them can be found at LERGREC, where researchers have been conducting a trial since 2013 with fifteen chestnut trees—five each of the American, Chinese, and American- Chinese hybrid species developed by scientists, all planted in one long row.

“The idea was to plant American and Chinese chestnuts side by side with some of the hybrids that have been developed and allow them to be challenged with chestnut blight over the years,” said LERGREC’s Bryan Hed, a plant pathologist.

“Over the years, most of the trees have suffered dieback from disease, insects, or weather, and have had to be cut back and renewed from sucker growth from the original rootstock,” Hed said. “Interestingly, the Chinese trees have fared the worst. The hybrid trees are notable exceptions; three of them are currently 17 to 21 feet in height.”

‘Functionally extinct’

“The American chestnut is now designated as ‘functionally extinct,’ which means that although the species still technically survives, it cannot reproduce,” Dobry said. “The shoots rarely grow large enough to produce nuts and, therefore, future generations.”

Originally, scientists hoped to engineer a tree that was as close to an American chestnut as possible with just enough genetic material from the Chinese chestnut to resist the blight.

“It sounds simple to breed that resistance into the American chestnut,” Dobry said, “but it has turned out to be much more complicated.”

The most promising hope for the American chestnut now is probably transgenic—that is, the development of a genetically modified tree.

“Researchers have developed partially blight-resistant trans­genic American chestnuts that are capable of surviving infection from the pathogen that causes blight,” Dobry said. “It doesn’t kill the pathogen; it’s still present, but it doesn’t destroy the tree.”

A place in the forest

Reintroduction may not be simple for the American chestnut, as the oak tree has risen to fill its place.

“It will be interesting to see what happens,” Dobry said. “It will be a challenge for the American chestnut to establish itself into the forest ecosystem again.”

And the American chestnut may bring its own problems to the forest. In 2018, while Dobry was a Behrend student and undergraduate researcher at LERGREC, she discovered on one of the trees a fungus that was atypical for the American chestnut species.

“Initially, we assumed this was an unusual presentation of chestnut blight infection, but after taking samples and doing research, we did not find blight, but a pathogen commonly known as chestnut brown rot,” she said. “At the time, there had been no published report of this fungus in our hemisphere.”

As a graduate student, Dobry continues to study the domestically isolated strain, examining whether it may be harmful to species of trees closely related to the chestnut, like oak trees.

“If this fungus proves pathogenic to oak, it could be another blow to forest dynamics, which is why the chestnut project and others like it are so important to protect the diversity and future of our forests,” she said.