Can an ambitious breeding effort save North America’s ash trees?

Since a devastating fungal blight popped up in the Bronx Zoo in 1904 and went on to kill at least 3 billion chestnut trees, North American forests have been swept by one plague after another, including a fungus that kills elms and an aphidlike insect that kills hemlocks. No tree has come back—but Koch hopes her approach can usher in an unprecedented era of tree revival.