Back-Breeding Could Restore Chestnut Trees Ravaged by Blight
for National Geographic News
|December 29, 2005|
The American chestnut tree once dominated forests in Appalachia, until
an Asian blight virtually killed off the species a century ago.
Now conservationists say they can restore the region's chestnut forests if scientists succeed in their efforts to breed new, blight-resistant strains of the tree.
"Entire communities in Appalachia depended on the chestnut for everything," said Marshal Case, president of the American Chestnut Foundation of Bennington, Vermont. The nonprofit has been leading the effort to reestablish the trees.
Chestnut trees were integral to everyday life in Appalachia, Case said, where they were called "cradle to grave" trees: Craftsmen made baby cradles and coffins from the rot-resistant hardwood. The trees were also used to build houses, telephone poles, and railroad ties.
Wildlife thrived on the trees, which each year produced bumper crops of nuts.
Then Chinese chestnut trees imported into the New York Botanical Gardens and Bronx Zoo carried the blight to the United States. The blight was first identified in 1904.
Within 50 years the blight fungus had infected and killed about 99.9 percent of the American chestnuts from Georgia to Maine and west to the Ohio Valley. "It moved fast, about 50 miles a year," Case added.
At least 50 strains of the fungus have been identified, and Chinese chestnut trees are resistant to all of them. The goal of the Chestnut Foundation is to breed that resistance into American trees.
To Case, the American tree is more majestic than the Chinese variety.
"American trees can grow to more than 120 feet [36.5 meters] tall," said Case. "The Chinese trees are much shorter and bushier."
The Chestnut Foundation is working with its 13 state chapters, the U.S. Forestry Service, and about 30 universities to reach its goal. So far the effort has produced a couple dozen trees on an experimental farm that are highly resistant to the blight fungus.
But the project still has several years to go before it has parent trees with the desired genetic composition and hardiness to withstand the blight when planted out in the forest.
American chestnut trees still sprout in Appalachia, because the blight does not attack their roots.
But as soon as the sprouts grow one to two inches (two and a half to five centimeters) in diameter, most are infected quickly by the blight, says Rex Mann. Mann is a Forest Service staff officer for Kentucky's Daniel Boone National Forest.
Foresters have tried fungicides and various breeding methods to try to resurrect the American chestnut.
Recently scientists have found promise in a method called backcross breeding. The process involves selectively breeding trees that are part Chinese chestnut and part American chestnut with a parent tree that is wholly American chestnut.
The goal is to produce an American tree with genes from the Chinese tree that render it blight-resistant.
"This is the first time the backcross breeding process has been used on trees. It typically is used for crops," said Mann.
The breeding process takes place on a research farm in Meadowview, Virginia, run by the Chestnut Foundation, including a staff pathologist, research scientist, two farmers, and volunteers.
Researchers hand-pick male flowers from one tree and use them to pollinate female flowers on another tree.
It takes several generations to get the proper mix. The first breeding produces a tree that is half Chinese and half American. That tree and its descendents are then back-bred with American parents until the fourth breeding produces a tree that is only one-sixteenth Chinese chestnut.
This tree is then bred with a similar tree to produce an American chestnut with resistance equal to that of the original Chinese parent. Those descendents will then be bred with each other to produce what is hoped will be the new line of blight-resistant American trees.
"Both parents need to be resistant before we have a resistant tree," Mann explained.
He added that states are conducting their own breeding programs as well, because it is not known if a resistant tree bred in Virginia, for example, will grow well in other states with different temperatures, soils, moisture, and other conditions. This is why the Chestnut Foundation's breeding program uses American parents from a number of states.
On the Farm
About 22,000 trees on the farm in Meadowview are in varying stages of this cross-breeding program, according to Case.
At each breeding stage, the trees are infected deliberately with the blight. Only the survivors with high resistance are kept in the program.
Fred Hebard, a staff pathologist with the Chestnut Foundation in Meadowview, says that last year 26 backcrossed trees showed a fairly high level of resistance to blight. But the cankers caused by the blight were larger than he would have preferred.
"The level of resistance is there, but the canker size is bigger than I expected," he said. "But we are getting rid of most of the bushiness of the Chinese trees."
He said the next backcross should increase resistance. It will take another decade or two before chestnuts from the program are planted in the woods with a fair amount of confidence.
"I hope the next backcross will be able to be part of the resurgence of the American chestnut tree, but time will tell," Hebard said. "It won't be until the trees are 100 feet [30 meters] tall in groves in the woods until we'll know, and that will be 50 to 100 years from now."
That's a sight Mann would like to see. He developed a healthy respect for the important cultural and environmental role of the American chestnut from living in North Carolina and Kentucky.
"When my dad was a young man, he used to sell roasted chestnuts in Washington, D.C.," Mann recalled.
"No other tree in the woods could outgrow the chestnut, and almost every animal ate chestnuts," he said. "Of all the trees that grew in the forest, the chestnut was the most intertwined with the Appalachian culture."
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