American chestnut trees, once the “redwoods of the East,” explained Tennessee Tech University’s Jed Young, an assistant professor of agriculture, were hit in the early 1900s with an Asian fungus to which they had little resistance.
“The trees used to grow more than 100 feet tall and more than 6 feet in diameter,” said Young. “Peoples lives and livelihoods revolved around the tree. They built homes, barns and fences from the wood, fed livestock and made a living in the lumber industry.”
Young said though the species, which now only produces shrubby root sprouts, is still being attacked by the fungus, efforts to restore it to its former glory have stepped up considerably in the past couple of decades.
An intensive breeding program is underway in a few orchards, including the 480-tree orchard cultivated on Tennessee Tech’s campus. Young and his colleagues use the backcross method to transfer the blight resistance of the Chinese chestnut to the American chestnut.
According to the American Chestnut Foundation, advances in genetics revealed where early researchers who tried backcrossing missed the mark. Now, scientists including Young are confident the current method will succeed. The backcrossing effort is about 10 years old; the Tennessee Tech project has been in operation about a year, with the orchard being planted only a few months ago. Both TTU's participation and the national effort will soon reach important milestones.
Backcrossing allows a single trait, in this case blight resistance, to be transferred to another plant. It starts with crossing a Chinese and an American chestnut to produce a hybrid — one-half Chinese, one-half American. Then the hybrid is backcrossed to another American chestnut, resulting in a tree that is three-fourths American and one-fourth Chinese.
“The goal is to continue the crossing until we have a tree that is 15/16 American and sorting so that the 1/16 Chinese trait holds the genes that code the plant for resistance to the fungus,” explained Young.
Once the American chestnuts are 2-3 years old, a survival of the fittest episode will take place when Young and his colleagues introduce the fungus to the orchard to kill all but the most resistant. They hope at least a few of the trees will have developed resistance to the pathogen, but they won't know which ones without killing the others.
“The inoculation process is how we cull the wheat from the chaff, so to speak. The chances are we will kill 95 to 99 percent of the trees,” explained Young. “But with the 1 to 5 percent left, we will begin breeding a resistant population, possibly with trees from other orchards around the country.
“Hopefully between four to eight years, we’ll have trees resistant enough to be put back into the environment,” said Young. “That means in as little as 20 years we could see bunches of trees surviving to maturity.”
There are seven American chestnut tree orchards in Tennessee, and fewer than a dozen universities conducting breeding programs nationwide, including Tennessee Tech and the University of Tennessee at Chattanooga.