Testing Blight Resistance in American Chestnuts

Results from eight years of field trials

stacy-clark-chestnut-seedling
Stacy Clark stands next to a 10-year-old hybrid chestnut seedling planted as part of a long-term research project on the Cherokee National Forest. Photo by Stacy Clark, USFS.

The American chestnut (Castanea dentata) was a keystone tree species in the eastern U.S., once found in the forest overstory from Maine to Georgia. The loss of the “mighty giant” to chestnut blight (Cryphonectria parasitica), a fungal disease accidentally imported from Asia in the early 1900s, reduced the once dominant chestnuts to remnant understory sprouts.

After eight years of field testing, USDA Forest Service research forester Stacy Clark and her colleagues evaluated blight resistance and survival of the backcross-generation American chestnut seedlings, known as BC3F3.

Their results were published in Forest Ecology and Management.

Researchers with The American Chestnut Foundation have spent decades developing a backcross breeding program to produce trees with the blight resistance of Chinese chestnut (C. mollissima) and the favorable growth characteristics of the American chestnut. Their hope was that the hybrid seedlings would be capable of restoring long-term ecosystem processes and functions.

In 2009, the Southern Research Station partnered with the Foundation, as well as the University of Tennessee’s Tree Improvement Program and the Southern Region of the National Forest System to test blight resistance in hybrid trees.

The team planted 1,100 chestnut seedlings in the Cherokee, Jefferson, and Nantahala National Forests in real-world forest conditions. The seedlings were planted in mesic, even-aged regeneration harvests. They are the first forest field tests of blight resistance of BC3F3, the third generation of the third backcross, and the most advanced breeding generation currently available.

“The primary objective of this study was to quantify blight resistance of genetic families of the BC3F3 breeding generation planted in the three test sites over eight growing seasons,” says Clark. “Our secondary objective was to examine natural pathogen infection, specifically to determine if the first infection could be predicted from previous tree growth or size.”

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Repeated browsing by white-tailed deer on American chestnut seedlings will stunt growth and may eventually lead to mortality. Photo by Stacy Clark, USFS.

The researchers also compared BC3F3 with less advanced breeding generations, such as BC1F3 and BC2F3, along with disease-resistant Chinese chestnut and disease-susceptible American chestnut.

After eight years, blight had infected between 17 and 54 percent of all trees planted: 40 percent of trees in VA, 17 percent of trees in TN, and 54 percent of trees in NC.

The BC3F3 generation exhibited resistance more similar to the Chinese chestnut than the American chestnut.

Blight was more likely to infect trees after they grew three to five inches in size at the base, as the study showed.

The researchers found that 31 percent of the planted seedlings died prior to blight detection. This non-pathogen-related mortality was probably due to factors typical of hardwood plantings, including repeated deer browsing and native and nonnative pest damage.

“To my knowledge, our research represents the oldest study examining blight resistance of the BC3F3 generation planted in forest reintroduction trials,” says Clark. “We found that blight resistance of the BC3F3 trees was not as good as the Chinese chestnut but was better than the American chestnut. Eight years of data is a remarkable research accomplishment, but it’s not sufficient to determine if the seedlings have enough blight resistance for restoration plantings. If restored, the American chestnut would return one of the most reliable and nutritious hard mast producers and its many ecological benefits back to its native range.”

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The Forest Service manages approximately 15 million acres (shaded red) within the former range of the American chestnut (shaded blue), offering substantial opportunities for research and restoration. USFS image.

The American chestnut was a forest species of great economic and ecologic value — providing ecosystem services such as mast production, rot-resistant lumber, carbon storage, insect diversity, and tannins for tanning leather. The leaves and nuts were a source of food for wildlife, birds, and many mammal species and a commodity product and food source for Appalachian subsistence farmers.

Read the full text of the article.

For more information, email Stacy Clark at stacyclark@fs.fed.us.

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