The chestnut blight was accidentally introduced to North America around 1900-1908, either in imported chestnut lumber or in imported chestnut trees, and by 1940, mature American chestnut trees had been made virtually extinct by the disease.
The blight appears to have been introduced from either China or Japan. Japanese and some Chinese chestnut trees are resistant to the fungus: they may be infected, but the blight does not usually kill them.
Despite the devastation the blight caused to the American Chestnut tree, the root collar and root system of the tree are fairly resistant to the blight, so a large number of small American Chestnut trees still exist as shoots from existing root bases. However, the shoots are seldom able to grow enough to reproduce before the blight attacks them.
The fungus is spread by an unknown agent, but it is local in range, so many American Chestnuts survive where there is no other tree within 10 km. Also, there are at least two pathogens which weaken the fungus (hypovirulence) and many trees survive that way.
Surviving chestnut trees are being bred for resistance to the blight, notably by The American Chestnut Foundation, which aims to reintroduce a blight-resistant American chestnut to its original forest range within the early decades of the 21st century.