The Castanea genus includes multiple species under the Chestnut and Chiquapin common names. These trees are all within the larger Beech (Fagaceae) Family. Castanea have a large distribution between North America, Europe and Asia.
Chestnut Blight (Cryphonectria parasitica), is a fungal infection that originated in Asia and coevolved with the Chinese (C. mollisima) and Japanese (C. crenata) chestnuts. Thus these species are resistant to infection.
American Chestnut (Castanea dentata) was once a prominent forest trees throughout Appalachia and the eastern US, ranging from Alabama to Maine and also dipping into southern Ontario and Michigan. The tree was highly valued for the rot resistant lumber as well as seasonal harvest of abundant and highly nutrient dense nuts.
At the beginning of the 20th century Chinese and Japanese chestnuts were being imported to the US, for commercial production of their larger nuts. Unfortunately they also carried the blight, which spread over the next 50 years through the native range, effectively exterminating the species.
The fungal spores enter the bark through injury or fissures, where then fungus multiplies and kills the tree by secreting oxalic acid. This is toxic to the bark and vascular tissue of the tree resulting in trunk and canopy death. The fungus cannot live in the soil due to competition with other microorganisms. Thus the roots continue to survive and will send up new shoots. These shoots may grow for some time, before the blight reinfects them and they once again succumb. Thus few forest trees survive long enough to reach reproductive age and two separate trees in close proximity are required for pollination. Thus species is now considered “functionally extinct” within the native range.
For more complete reading I suggest these links.
Castanea species readily hybridize and humans have been doing such for over a hundred years for a variety of reasons. Initially for commercial nut production factors including nut size, taste and ease of harvest. After introduction of blight to American work began on hybridizing for blight resistance by crossing American with Chinese trees. This work was performed by individual breeders and scientific scholars until ultimately the American Chestnut Foundation was founded in 1983 as the prominent breeding organization.
They have been focused on a backcross program where hybrid trees are backrossed to a pure AC, selecting for blight resistance with AC characteristics. With the timeline of trees reaching reproductive range, innoculation and assessment of resistance the program is now producing 15/16ths American hybrids.
It was originally hypothesized that resistance was related to a few specific genes. Thus with enough backrossing these few genes could be isolated from the Chinese genome and the remaining genome would be American. However as the program continued and especially once genetic sequencing technology and gene analysis was available, it has been determined the resistance genes are much more dispersed throughout the genome and thus transfer is much more complicated.
An alternative breeding strategy was developed involving genetic engineering in the 1990s. Since the blight kills the tree by secreting oxalic acid, it was theorized that insertion of a gene that comes for an enzyme that can degrade the toxin would confer resistance. The oxalate oxidase gene (OxO) is common in grass species, in particular wheat and was inserted into the AC genome. This work was primarily carried out through the State University of New York College of Environmental Science and Forestry (SUNY/ESF). These trees have been titled “Darling” trees (also OxO trees), after the original supporter of this project.
This project has not been without setbacks, in part due to labeling error, withdrawal of funding from TACF, needing regulatory approval of GMO trees, concerns if resistance can confidently be transferred to subsequent generations as well as concern over competitive growth rates. Research continues with different gene promoters and gene insertion point analysis.
History of AC Breeding
Identification
There are many identification resources more through and readily available than this page. Identification of pure AC trees vs high percentage hybrids can be very difficult, especially with the history of hybridization. Key factors include long canoe shaped (pointy at both ends) leaves with deep, hooked margins. The underside of the leaves are smooth, and the surface is thin and “matte,” with buds pointing away from the branch.
Potentially significant LSAs in Michigan are likely to be found on old farmsteads or isolated areas and have the potential to be quite old and large, even though blight is now endemic through our state.
There are some unique factors for identification in Michigan, given that a vast majority of the state is out of the Native Range. LSAs in Michigan are most likely to be found on old farmsteads/woods, where the nuts were brought from people moving from the native range to our state. These LSAs have been protected from blight attacks, through isolation as well as some hypovirulence towards the blight. The fungus can be attacked by a virus, which weakens it and allows the ACs to survive. Extensive research was performed on this topic by Dr. Dennis Fulbright with Michigan State University during his career. Dr. Fulbright passed away in 2019 but did leave vast contributions to the chestnut knowledge base.
American Chestnuts of Michigan 