Back-Breeding Could Restore Chestnut Trees Ravaged by Blight

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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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