Restoring Maine’s Chestnut Trees
The loss of the American chestnut (Castanea dentata) tree is often cited as one of the worst ecological disasters in modern times. An estimated 4 billion trees, one in four trees in our eastern forests, were killed as a result of an accidentally introduced pathogen, the chestnut blight (Cryphonectria parasitica). Although chestnut trees are still common in our forests today, they primarily survive as understory sprouts. The sprouts are able to grow quickly with canopy removal, but rarely have the opportunity to sexually reproduce in significant numbers before being killed by the chestnut blight. The American chestnut has ceased to evolve as a species.
The American chestnut generally has little, if any, resistance to chestnut blight. In Asia, however, where the pathogen originated, most native chestnut species and particularly Chinese chestnut are well defended against the blight. Over the course of their millennia of coexistence with the fungus, Chinese chestnut trees acquired the genetic material that confers resistance. Blighted North American chestnut species die, while blighted Chinese chestnuts suffer only cosmetic damage.
In the late 1970s, The American Chestnut Foundation’s founder Dr. Charles Burnham proposed a methodology of breeding to incorporate blight resistance into the American chestnut tree. By using a well-established plant breeding technique known as “backcrossing,” Dr. Burnham hypothesized that one could marry the best characteristics of both the American and Asiatic species.
The basic process goes something like this (see diagram below for a visual depiction of the backcross breeding program):
- An American chestnut is crossed with a Chinese chestnut to yield F-1 hybrid nuts that are moderately blight-resistant, and only moderately “American”.
- These F-1 trees are “back-crossed” with other pure Americans to dilute the un-wanted Chinese characteristics. The progeny of these crosses are purposefully inoculated with lab-grown blight fungus to evaluate their blight-resistance. The best trees are then back-crossed in 2nd and 3rd generations to further dilute the Chinese genes, while also selecting at each generation for moderate blight-resistance.
- The selected 3rd back-cross generation trees are allowed to cross pollinate each other. A small percentage of the progeny of these crosses can have all of the genes for blight resistance from their great-great-great-grandparent.
It’s actually a bit more complicated than this, and takes about six controlled pollination crossings. Each generation resulting from each crossing may take up to six years to evaluate for blight resistance. The TACF research farms in Meadowview, VA have been producing the first generation of potentially highly-blight-resistant American-type chestnuts, dubbed Restoration Chestnuts 1.0, since 2007. Here in Maine, the state chapter has been undertaking its own breeding program utilizing remnant Maine native trees in order to promote regional biodiversity and local adaptation. It is currently one generation behind the national restoration program, but making steady progress.