https://www.waldwissen.net/en/forestry/forest-protection/fungi-and-nematodes/chestnut-blight

Chestnut blight is caused by the fungus Cryphonectria parasitica (formerly known as Endothia parasitica). The pathogen originates from East Asia, where it occurs as a weak parasite on the resistant Japanese (Castanea crenata) and Chinese chestnut (Castanea mollissima). In the early 20th century, the fungus was accidentally introduced into the USA with infected plant material, triggering a dramatic epidemic that almost completely eradicated the extensive forests of American chestnut (Castanea dentata) in the eastern USA.

This fungus was accidentally brought to America with the Japanese chestnut (which has some resistance) and almost completely ate the American chestnut (which has none).