Q is right, of course.

Bluegrass started out as the particular sound of Bill Monroe's band. They had a number of imitators, which first got Mr. Monroe very angry. Later on, he was very pleased to be called the "Father of Bluegrass Music. But it took a while.

The genre came into existence in 1946, but it wasn't commonly recognized as a musical form in its own right, and given a name, until about the early '60s.

Peter.