Bluegrass musician Bobby Osborne performs with Del McCoury, not pictured, at the International Bluegrass Music Association Awards show on Sept. 27, 2012, in Nashville, Tenn. Mark Humphrey/Associated Press file

Bobby Osborne, the lead singer and mandolinist of the Osborne Brothers, a group that broadened the commercial appeal of bluegrass music and recorded two of the genre’s most enduring standards, “Rocky Top” and “Ruby, Are You Mad?” died June 27 at a hospital in Gallatin, Tenn. He was 91.

His son Bobby Osborne Jr. confirmed the death but did not give a cause.

Osborne was one of the last living musicians from the genre’s first generation, having begun his career in the late 1940s as a guitarist with the Lonesome Pine Fiddlers. He was also the Grand Ole Opry’s oldest regular performer at the time of his death; he joined the show’s cast with his brother Sonny in 1964 and last appeared on its stage on May 19.

“Rocky Top,” a brisk paean about a rural transplant in the city who longs for the simple comforts of his native hamlet in Tennessee, was written by Nashville tunesmiths Boudleaux and Felice Bryant, a husband-and-wife team who wrote many pop hits for the Everly Brothers and Roy Orbison.

Since the Osborne Brothers first recorded the song in 1964, it has become an unofficial fight song for the University of Tennessee football team and was adopted as a Tennessee state song in 1982.

To the consternation of bluegrass purists, the Osborne Brothers amplified their instruments and added drums, piano, and electric bass to their lineup. They also recorded with pedal steel guitar and, occasionally, string sections.

The willingness to experiment netted the group over a dozen entries on the Billboard charts during an era when country radio was largely ignoring the subgenre. Their other hits included “Once More” (1958), credited to the Osborne Brothers and Red Allen; “Making Plans” (1965); and “Tennessee Hound Dog” (1969).

Fans revered Osborne for the keening-like quality of his voice. In a typical bluegrass harmony, one voice supplies the melody while a tenor and baritone harmonize above and below it. However, beginning with the recording “Once More,” the Osbornes inverted their harmonies. Osborne’s falsetto tenor took the melody with the other two voices blending below his lead. This “high lead” style became the group’s signature.

“We knew then that we had caught onto something that … we had never heard before,” he told NPR in 2017. “So we got the guitar out of the trunk and found out what key we were in, and we sang that song all the way home so we would not forget that type of harmony because that’s what we wanted to do.”

Robert Van Osborne Jr. was born in Hyden, Ky., on Dec. 7, 1931, and moved with his schoolteacher parents to Dayton, Ohio, when he was 9.

He took up electric guitar and worked in country bands as a teenager, singing in the baritone style of honky-tonk singer Ernest Tubb.

“Music was the only thing that we cared anything about,” he later told the roots music magazine No Depression. “We didn’t want to be farmers, and we didn’t want to be anything. Just music was the only thing that we wanted to do, that’s it.”

His voice changed – unusually, it got higher; too high to imitate Tubb’s drawling baritone but high enough to emulate Bill Monroe, the father of bluegrass.

Moving to Bluefield, W.Va., in 1949, he and his younger brother Sonny, then a teenage prodigy on banjo, joined the Lonesome Pine Fiddlers. The two brothers first recorded with their sister Louise on lead vocals for a tiny label in 1951.

After Marine Corps service during the Korean War, Osborne – by then playing mandolin – started his group in Detroit with Sonny and singer-guitarist Jimmy Martin, but they quarreled with Martin over leadership.

Red Allen replaced Martin as singer-guitarist when the group was signed to MGM Records. In 1956, they had their first hit, “Ruby, Are You Mad?,” essentially a variant of the Appalachian folk melody “Reuben’s Train,” with both brothers playing the banjo.

They were one of the first bluegrass groups to play on college campuses after a 1960 performance at Antioch College in Ohio brought them to the attention of other northern schools. Other notable engagements included the Newport Folk Festival.

The Osborne Brothers played at the Nixon White House in 1973 – reportedly the first bluegrass group to perform at the executive mansion.

The group received an award for best vocal group from the Country Music Association in 1971. They were inducted into the International Bluegrass Music Association’s Hall of Fame in 1994.

Sonny Osborne retired from performing in 2005 and died in 2021. Osborne started a new group, Rocky Top X-Press, with his son Bobby Jr. His last album, “Original,” was released in 2017. His son said he had finished another record at the time of his death.

His first marriage, to Patsy Kline, ended in divorce. Survivors include his wife of 38 years, the former Karen Harr; three sons; a daughter; a sister; five grandchildren; and six great-grandchildren.

“People ask me now, ‘When you gonna quit?'” Osborne said to NPR in 2017. “Why, they ask the wrong guy. I don’t intend to quit as long as I can do what I’m doing now. Now, if I get to where I can’t sing or can’t play or can’t think good or whatever, then I might have to. But I ain’t gonna quit ’til then.”