There were still small groves and individual Elms around the country being managed through the fifties and sixties with targeted DDT spraying, and they were doing quite well. But when DDT was banned almost all of them got infected and died.
But you are correct that the large tracts of them not managed were wiped out. I was only speaking about ones they wanted to preserve, and there were still quite a few.
I’m only addressing the story of the American Chestnut and most of those were gone before DDT was invented in 1939.