i cannot find mention of his passing or an obit online
but i did find this article:

JOSH GRAVES was born Burkett H. "Buck" Graves, 27 Sept. 1928 in Tellico Plains, Monroe Co., Tennessee. His Graves ancestry is not presently known. He is one of the main people responsible for the acceptance and popularity of the Dobro® guitar in bluegrass and country music. In 1942, Graves joined the Pierce Brothers and began playing in Gatlinburg. Later he played with Esco Hankins and Mac Wiseman before becoming a part of the Wheeling Jamboree with Wilma Lee and Stoney Cooper. During a performance at the Grand Ole Opry, Graves made a big impression upon Lester Flatt and Earl Scruggs who invited him to join their Foggy Mountain Boys. He played with other groups and solo after that.