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A foreign fungus nearly wiped out North America's once vast chestnut forests. Genetic engineering can revive them

In 1876 Samuel B. Parsons received a shipment of chestnut seeds from Japan and decided to grow and sell the trees to orchards. Unbeknownst to him, his shipment likely harbored a stowaway that caused one of the greatest ecological disasters ever to befall eastern North America. The trees probably concealed spores of a pathogenic fungus, Cryphonectria parasitica, to which Asian chestnut treesbut not their American cousinshad evolved resistance. C. parasitica effectively strangles a susceptible tree to death by forming cankerssunken areas of dead plant tissuein its bark that encircle the trunk and cut off the flow of water and nutrients between the roots and leaves. Within 50 years this one fungus killed more than three billion American chestnut trees.

Before the early 1900s the American chestnut constituted about 25 percent of hardwood trees within its range in the eastern deciduous forests of the U.S. and a sliver of Canadadeciduous forests being those composed mostly of trees that shed their leaves in the autumn. Today only a handful of fully grown chestnuts remain, along with millions of root stumps. Now and then these living stumps manage to send up a few nubile shoots that may survive for 10 years or longer. But the trees rarely live long enough to produce seeds because the fungus almost always beats them back down again.

2014 Scientific American, a Division of Nature America, Inc.

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The American Chestnut's Genetic Rebirth