The American Chestnut.
It was pretty much wiped out by a disease that infects its cousin, the Chinese Chestnut, which has defenses against it.
"The American chestnut tree reigned over 200 million acres of eastern woodlands from Maine to Florida, and from the Piedmont plateau in the Carolinas west to the Ohio Valley, until succumbing to a lethal fungus infestation, known as the chestnut blight, during the first half of the 20th century. An estimated 4 billion American chestnuts, up to 1/4 of the hardwood tree population, grew within this range."
http://www.acf.org/range_close.php
They're trying to reintroduce blight resistant strains by cross breading with Chinese trees.