Ask Lemmy

Because the disease has become endemic to American forests.

The American Chestnut was the dominant tree in the ecosystem of the forests of Eastern North America. Per Wikipedia, "it was said that a squirrel could walk from New England to Georgia solely on the branches of American chestnuts." In the late 19th century, Japanese chestnut trees were imported, and they brought with them Asian Bark Fungus. American Chestnuts are quite susceptible to this fungus, and it largely wiped out the population.

The fungus infects the above ground portion of the tree, killing it. New shoots will emerge from the stump as the below ground portion of the tree isn't affected by the fungus, but the new growth doesn't get very far before the fungus kills it off again. We have no hope of eliminating the fungus from the forests.

So we've got these zombie tree stumps that will grow enough of a plant to keep the fungus alive and running (it also survives on other species of tree), but not enough to grow large and reproduce. There are some remaining adult trees here and there but the species is considered functionally extinct in the wild as it really isn't able to thrive because this fungus is among us. So unless we can hybridize or otherwise breed fungus resistant chestnut trees, we ain't got no American Chestnuts.

American chestnuts are also susceptible to ink disease and the Chinese Gall Wasp.

A lot of problems were caused by importing plants to North America; tumbleweeds aren't indigenous, they're Russian, and a massive fucking problem.