The American chestnut (Castanea dentate) was once very common to eastern North America. Since the early 1900s, however, tree numbers declines. Much of this was due to chestnut blight (a fungus).
By the 1950s the chestnut was almost disappeared. However, efforts to eliminate the blight are finally showing signs of success, raising the possibility that the iconic tree could flourish again.
The American Chestnut Foundation (ACF), an alliance of amateur enthusiasts and plant pathologists, has created blight-resistant hybrid trees by crossing descendants of original chestnuts with the smaller Chinese variety.
For further details, see Nature.