Jennifer Schultz Nelson
Extension Educator, Horticulture
We've all heard the familiar holiday tune that begins "Chestnuts roasting on an open fire....", but here in central Illinois chestnuts are not a familiar site among native stands of trees.
The American Chestnut, Castanea dentata, was at one point one of the most important forest trees in the eastern U.S. and southeastern Canada. It was also one of the largest, growing up to 150 feet tall with a trunk up to ten feet in diameter.
The species name 'dentata' is Latin for 'toothed' and is given to the American Chestnut in reference to its leaves. The leaves are up to eight inches long and four inches wide with a distinctly notched edge, much like a saw's teeth.
The wood from American Chestnut trees was at one time a prized commodity. It has a straight grain, is easy to split, and is very strong wood. The wood is highly resistant to decay, making it a good choice to use in outdoor projects.
As the song tells us, the nuts were also valued as food, and especially good roasted. Some cooks also use them raw, or ground into flour. As American Chestnuts have become harder to find, some stores more often sell the commonly available Sweet Chestnut.
The nuts were also an important source of food for both wildlife and livestock on farms. The stately trees were anchors in the landscape and a source of valuable shelter for birds and other wildlife as well.
But in the early 1900's the beautiful American Chestnut began to disappear from forests in the U.S. and Canada. The culprit is a familiar theme heard in other tragic demises of plant species: foreign disease.
Asian Chestnut trees (Castanea crenata or C. mollissima) were imported into the U.S. in the late 1800's as specialty trees. Unfortunately there was a hitchhiker on these trees, the fungus that causes chestnut blight, Cryphonectria parasitica.
The fungus infects a tree through wounds such as cracks in the bark, and the blight disease develops which eventually girdles and kills the tree. Scientists hypothesize there may be an insect or some other carrier that helps the fungus spread from tree to tree, or it may just spread through airborne spores or be washed from tree to tree by rain.
The Asian chestnut species have co-evolved some resistance or tolerance to this fungus, but the American chestnut had no exposure to the fungus prior to its arrival in the U.S. in the early 1900's, so it was completely vulnerable. The first trees to show symptoms were noted in the Bronx zoo in 1904.
Experts estimate that 3.5 to 4 billion trees were lost to chestnut blight across the eastern U.S. and Canada in less than 50 years. By some unknown mechanism, rare stands of American chestnut that had been planted outside of their native range were spared the death sentence of chestnut blight.
Today, researchers are very interested in these rare stands of trees. They are the only surviving relics that can provide clues on the American chestnut's role in forest ecosystems, and perhaps provide insight as to why these stands did not succumb to chestnut blight.
Other research efforts involve breeding the American chestnut to be resistant to chestnut blight. The method scientists use is called backcross breeding. Asian cousins of the American chestnut have resistance to chestnut blight, so they are the source of genes conferring resistance to chestnut blight in the breeding program.
An American chestnut is crossed with its blight resistant Asian cousin, and the offspring are genetically 50% American chestnut, 50% Asian chestnut. Among those offspring, individuals that most closely resemble the American chestnut and have blight resistance are crossed back to the American chestnut.
These offspring are 75% American chestnut, and 25% Asian chestnut. The process continues as before, selecting offspring that resemble the American chestnut but also have blight resistance. Breeders have carried this project out to the point of having blight resistant trees with 94% American chestnut genetics.
Though breeders appear to have had promising results, there are more challenges remaining. There are laws in many regions governing public lands where American chestnuts used to flourish that prohibit interference by man. Planting trees in itself violates these laws. Besides what the law says, many consider the new blight-resistant trees to be hybrids artificially created by man, so they should not be planted on public land because they are not 'natural'.
While public sentiment may prevent the blight resistant trees from populating the forests, there may be opportunities in the future for landowners to acquire these blight resistant trees for the landscape. Asian chestnut species are still available, and may be a good choice for landscapes with room for these large trees.