Breeding for Blight Resistance
We use traditional breeding to generate hybrids that combine the blight resistance of Chinese chestnut with the timber-type form of American chestnut. Our strategy is to backcross Chinese chestnut × American chestnut hybrids to American chestnut over one or more generations to dilute out genes from Chinese chestnut except those that confer blight resistance. After nearly 30 years and three generations of breeding, we have nearly completed selection of the most blight-resistant chestnut hybrids in two seed orchards from our flagship breeding program in Meadowview, Virginia. We have recently incorporated genomics to accelerate selection of the most blight-resistant trees (learn more here). Selected hybrids have inherited between 60% and 90% of their genome from American chestnut and exhibit blight resistance on spectrum that is intermediate between American chestnut and Chinese chestnut. We have planted selected hybrids in over 40 restoration trials in the eastern U.S. to determine if these hybrids have sufficient blight resistance and competitive ability for restoration in eastern forests. We continue to improve blight resistance in our breeding program by inter-breeding the most resistant trees within each generation and selecting the most blight-resistant progeny. To financially support the research and development of American chestnut hybrids with improved blight resistance you can join our Seed Level Membership program and receive seeds from our most blight-resistant selections.
(A) Comparison of the stem form of pure American chestnut; (B) A selected BC3-F2 hybrid (85% American chestnut ancestry); (C) An F1 hybrid of American chestnut and Chinese chestnut (50% American chestnut ancestry)
Regional Breeding and Germplasm Conservation
Figure 2: Map of the American Chestnut Foundation orchard locations across the native range of Castanea dentata
The American Chestnut Foundation (TACF) has 16 volunteer-run chapters ranging from Maine to Alabama. Our chapter volunteers have bred chestnut hybrids with local American chestnut trees to incorporate genetic diversity that is critical for the restoration population to adapt to the American chestnut’s native range. Chapters are currently selecting the most blight-resistant hybrids in orchards across the native range. These orchards will eventually provide regionally adapted blight-resistant seed for restoration. Chapters are also currently conserving additional wild American chestnuts from across the native range in orchards. We intend to breed this diverse collection of wild American chestnuts with hybrids and transgenic blight-tolerant American chestnuts (pending U.S. federal approval) to increase the restoration population’s capacity to adapt to climate change. If you have found a wild American chestnut tree, you can report it to TACF by going to Tree Identification or using TreeSnap.
If you would like to join, volunteer with, or donate to your local chapter, please Contact TACF.
Combining Resistance to Chestnut Blight and Phytophthora
American chestnut is highly susceptible to the soil borne pathogen, Phytophthora cinnamomi, which causes root rot that kills plants. The range of P. cinnamomi is limited by prolonged freezing temperatures. Historically, this pathogen has affected American chestnuts in the southeastern U.S. As winters warm, P. cinnamomi is spreading north and is expected to reach New England by 2080. The American Chestnut Foundation is collaborating with Clemson University and the U.S. Forest Service to screen a genetically diverse population of American chestnut hybrid seedlings from TACF’s chapter breeding programs for resistance to P. cinnamomi (learn more here). Our hybrids demonstrate a range of resistance to P. cinnamomi and we plant the survivors at field sites where P. cinnamomi is present in the soil. Once these trees with resistance to P. cinnamomi grow large enough to flower, they will be bred with blight-resistant hybrids or transgenic trees to combine resistance to the two diseases.
Figure 3: American chestnut backcross hybrids with varying resistance to the Phytophthora root rot pathogen