I don't want to start a tiff on whether or not the use of instruments other than guitar, banjo, fiddle, bass, mando, or Dobro can be considered bluegrass (so please don't)...
... but I searched through some old threads here and saw that Allen Hopkins made this reference "Some oddities creep in -- accordion (Wilene Forrester w/Bill Monroe),..."
Is Monroe the only bluegrass traditionalist that's used the accordion? #What recording(s) is the accordion used on?
Gene