The loss of the American chestnut tree (Castanea dentata) is often cited as one of the worst ecological disasters in modern times. An estimated 4 billion chestnut trees, one in four trees in our eastern forests, were killed as a result of a fungus, the chestnut blight (Cryphonectria parasitica). This fungus was native to Asia but was accidentally brought to New York in 1904. It quickly spread and within a few decades most American chestnuts lay dead on the ground. The species and communities that depended upon them for food and shelter were diminished in turn.
When the fungus girdles and kills the trunk of the tree, the root often survives and sprouts in the forest understory. The tree’s ability to root sprout is the only reason wild chestnuts persist in our forests today. The sprouts grow into trees but rarely mature to the point of flowering and producing seeds. Thus, the blight prevents the American chestnut from reproducing and evolving as a species. Without our intervention the chestnut is unlikely to reclaim its former territory and its outsized role in forest ecology and human culture.
The mission of The American Chestnut Foundation (TACF) and its 16 chapters is to return the iconic American chestnut to its native range. In our attempts to restore this species, we are blazing a trail to guide future restoration efforts for other trees in trouble.
The goal of the TACF breeding program has been to move the gene for resistance, which occurs in the Chinese chestnut, into the American chestnut’s genome. This goal has been approached in two ways: a standard back-cross breeding program and genetic engineering.
The Maine Chapter of TACF is one of many chapters engaged in a long-term back-cross breeding program with support from TACF’s staff and research farm in Meadowview, VA. Employing Maine wild chestnuts as mother trees, we aim to produce a blight resistant tree that is well adapted to Maine growing conditions.
We have long expected to see blight resistance work its way into the American Chestnut genome but this goal has remained elusive. Modern gene technology recently revealed that the combination of genes that confer resistance is more complicated and harder to pass along through standard back-cross breeding. After growing and tending hundreds of thousands of young chestnut trees over two decades, we have yet to produce a reliably blight tolerant tree that retains its distinctive American features. We have, however, learned a lot about chestnut trees!
A transgenic approach to blight tolerance has been underway almost as long as the back-cross breeding program. Managed by the State University of New York at Syracuse (SUNY Syracuse) this approach recently found success in a transgenic tree called Darling 58 or D58. Professor Tom Klak, from the University of New England (UNE) in Biddeford Maine has devoted years to advancing this approach.
Working with TACF and SUNY staff, Tom has developed ways to grow Darling 58 seedlings in the greenhouse and produce transgenic pollen for future breeding with native chestnuts. One big benefit of the transgenic approach is that the American chestnut genome does not become contaminated by the many extraneous Chinese chestnut genes that come with traditional breeding.
With help from many TACF volunteers and UNE college students, Tom Klak established a transgenic chestnut orchard in Maine in 2021-22. This federally regulated research site operates under a special permit from the USDA. Regulators are in the process of reviewing the potential use of Darling 58 for chestnut forest restoration.