|
In 1904, Hermann Merkel, chief forester of the Bronx zoo noticed something odd happening to the chestnut trees. The trees were developing strange lesions surrounded by spotty, orange-yellow patches. First the mighty upper limbs died, then the trunk. He called in mycologist William Murrill to examine the fungus. By the time Murrill published his findings just over a year later, the disease had spread to New Jersey, Maryland, Virginia, and the District of Columbia. Murrill warned his incredulous colleagues that the blight would kill every chestnut tree. The spores, carried on the wind, could spread an estimated 50 miles a year. Once the fungus Cryphonectria parasitica colonized a wound on an American chestnut, it became unstoppable.
Introduced on nursery imported Chinese chestnut trees from Asia, the blight spread with devasting speed. By the 1950’s virtually all of the canopy trees had died, 3-4 billion trees, wiped out in less than 50 years. In a range stretching from southern Maine to the Florida panhandle and west to the Mississippi River, the American chestnut (Castanea dentata) dominated the landscape. The American chestnut was a perfect tree. Massive, fast-growing, and rot-resistant, it was easy to mill into cabin logs, furniture, fence posts and railroad ties. Sweet, acorn sized nuts fed turkeys, deer, raccoons, and bears. And in the fall, when the chestnuts piled up in carpets half-a-foot thick, settler families collected and sold them by the bushel.
Almost as soon as chestnut blight began sweeping through the forests of Appalachia, scientists began trying to breed blight- resistant American chestnuts. Botanists first tried crossing American chestnuts with Asian chestnuts. Their efforts were unsuccessful. Federal funding dried up by the 1960s. Decades went by. Finally, by the 1980s, scientists tried backcrossing. They wound up with a tree that is fifteen-sixteenths American chestnut, with all the physical characteristics of an American tree and the blight resistance of an Asian tree.
In the 1990s work began to insert a blight resistance gene directly into the American chestnut’s genome. They called this genetically modified tree Darling 58. In 2023 the US federal regulators denied approval of Darling 58 release to the public due to concerns about its inconsistent blight resistance, poor growth, and a lab error that resulted in mislabeling the tree as Darling 58 when it was actually a different variety (Darling 54), rendering the data on its effectiveness unreliable.
All trees and seeds that are currently available are either products of cross-breeding efforts, referred to as “improved trees with intermediate blight resistance,” or they are wild-type American chestnut trees with no blight resistance. At this time there is no 100% blight-resistant American chestnut seed or seedling and there is no guarantee that any seed will grow free from blight.
What does the future hold? Today, there are a few American chestnut trees, largely in isolated areas outside of the tree’s historical range. There are even small populations in Minnesota and Wisconsin. The American chestnut is now designated as ‘functionally extinct,’ which means that although the species still technically survives, it cannot reproduce. The shoots rarely grow large enough to produce nuts and, therefore, no future generations. For now, efforts continue in hybrid breeding, genetic engineering and field trials. One hundred and twenty years after the blight was first discovered, scientists are getting close to a blight resistant American chestnut. This is turning out to be a tree-length task rather than a human-length one. Bringing the American chestnut back from extinction is kind of like bringing back the Woolley mammoth. Even if people can genetically engineer one, the ecosystem it was part of will be gone, rearranged in its absence.
| 
