The "patriarch of Appalachian music who with his brother Carter helped expand and popularize the genre that became known as bluegrass, died Thursday from difficulties with skin cancer," Webster wrote.
Stanley was already famous in bluegrass and roots music circles when the 2000 hit movie "O Brother, Where Art Thou?" thrust him into the mainstream. He provided a haunting a cappella version of the dirge "O Death" and ended up winning a Grammy.
"'O Brother' has done wonders for me and anyone who plays this style," Stanley told CNN in 2001.
Stanley was born in Big Spraddle Creek, Virginia, in 1927 and grew up playing and listening to traditional mountain music, according to his biography on the International Bluegrass Music Museum website.
After World War II, he and his brother Carter formed the Stanley Brothers and their Clinch Mountain Boys and gained regional success playing for crowds mostly in the Appalachian Mountains.
Ralph played banjo and sang. After Carter's death in 1966, he took on duties as front man for the groups, the IBMM said.
In the 1960s, the Stanleys were exposed to new audiences on college campuses, in Europe and at music festivals, thanks to the folk music boom. His group was the first bluegrass band to play at the Newport Folk Festival, in 1959, the IBMM said.
Ricky Skaggs and Keith Whitley were just two of the musicians who played in his band and later became well-known in their own right.
Despite health problems, Webster said, Ralph Stanley "continued to record and tour into his 80s, often performing with his son Ralph Stanley II on guitar and his grandson Nathan on mandolin."
Stanley was a member of the Bluegrass Hall of Fame and the Grand Ole Opry and was presented a "Living Legend" award by the Library of Congress in 2000.