Iconic country music singer-songwriter Merle Haggard has died, according to his manager. The Associated Press confirmed that Haggard died of pneumonia on Wednesday – his 79th birthday.

Haggard canceled tour dates in December after he checked into a hospital and learned he had double pneumonia. In an interview later with Willie’s Roadhouse on Sirius XM, he said he was nearly dead” during the two-week hospitalization. He later canceled shows in late January after his double pneumonia returned. At the end of last month, Haggard announced that he was canceling all of his scheduled shows for April on doctor’s orders.

Born in 1937 just outside of Bakersfield, Calif., Haggard endured a troubled childhood and was in and out of reform schools before being sent to prison at age 20. While at San Quentin, he began taking music seriously after seeing Johnny Cash's legendary performance at the facility, as heard on 1969's At San Quentin. Upon his release from prison, Haggard made a name for himself as a musician with intensely personal songs like "The Bottle Let Me Down" and "Mama Tried." He was widely successful throughout the '70s and '80s, perhaps best known for his 1969 hit, "Okie From Muskogee."

A huge influence on modern country music and alt-country bands like Son Volt and Wilco, Haggard continued to perform and release new music through his 2015 collaborative album with Willie Nelson, Djano & Jimmie. The album debuted at No. 1 on Billboard‘s Country Albums chart.

Haggard was inducted into the Country Music Hall of Fame in 1994.

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