Garden Variety: Tree discovery offers hope against chestnut blight

A mature healthy American chestnut tree was discovered a few weeks ago by a Canadian conservationist in far southern Ontario, about halfway between Detroit, Michigan and Niagara Falls. The tree is an anomaly because experts believe it may have survived exposure to a devastating disease known as chestnut blight. If the theory proves correct, the tree is the first of its kind and offers promise for reintroduction of this great species into North American forests.

Prior to 1900, American chestnut trees dominated the forests of the eastern U.S. and far southern Ontario. They were prized for high quality hardwood logs and lumber and for the chestnuts that were an important food source for deer, turkeys and other wildlife. American chestnut trees grew to 50 to 75 feet tall and were considered superior to oaks because of a faster growth rate.

Chestnut blight likely entered the U.S. on imported Asian chestnuts in the late 1800s and was initially identified in New York in 1904. An estimated 3 to 4 billion American chestnut trees died in the U.S. between that time and 1950.

To date, there is no known resistance to chestnut blight in American chestnut trees and no treatment for the disease once a tree is infected.

If the newly found tree is indeed resistant to chestnut blight, it can help plant breeders and scientists in a couple of ways. First, breeders can take cuttings from the tree and induce the cuttings to grow their own roots, thus creating clones. This method is also known as vegetative propagation. All named varieties of fruit, nut, and ornamental trees on the market are produced this way to ensure presence of the specific attributes for which the plant was selected.

Breeders may also be able to cross this American chestnut tree with others to breed resistance into additional trees, or they may cross it with Chinese chestnuts or existing hybrids to produce more resistant and desirable Chinese hybrids.

Finally, scientists should be able to analyze and compare the genetics of this tree to susceptible American chestnuts to pinpoint genes associated with resistance. Much of this work has already been done in the production of hybrids of Chinese chestnuts, but multiple genes are believed to be involved in resistance/susceptibility.

There are a few healthy American chestnut trees around the U.S. that have escaped exposure to the disease because they are outside of the native range of the tree. Many of these are protected but are still at risk from chestnut blight if the disease were to get to them. Planting of American chestnut trees is not currently recommended because of the risk of spreading the disease to new areas. Commercially-produced chestnuts are from Chinese chestnuts or from Chinese hybrids.

American chestnut trees infected with chestnut blight exhibit flagging and develop cankers. Trees typically die back to the ground within 4 years of infection. Trees’ root systems survive a little longer and send up suckers that then become infected. Suckers die back and regrow in a continual process that allows the disease to perpetuate within the native range of the tree.

Some oaks and other species are also susceptible to chestnut blight, but only exhibit severe dieback over a long period of time. This also allows the disease to perpetuate. In addition, chestnut blight may be moved short distances by wind or rain and longer distances by birds, insects and other animals.

Given the nature of tree growth, conclusive research and reintroduction of the American chestnut could take decades.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.