Answer:
The chestnut blight.
Step-by-step explanation:
The chestnut blight is a fungal disease, caused by the Cryphonectria parasitica, a parasitic fungus of the chestnut trees. It was introduced in America in the early 1900s by imported Asian chestnut trees.
There have been estimations that there were about 3 - 4 million chestnut trees in the eastern North America, and about 25% of them were the American chestnut. After the blight, there were roughly 100 trees left.