Johnny Bush, a Texas singer whose rowdy honky-tonk hit “Whiskey River” became a signature song for his friend Willie Nelson but whose own career was interrupted by a neurological condition that robbed him of his voice for years, died Oct. 16 at a hospital in San Antonio. He was 85.

The cause was complications from pneumonia, said Ronnie McHan, his drummer and touring manager. He added that Mr. Bush had recently tested negative for the coronavirus.

Mr. Bush was a drummer, guitarist, fiddler and singer-songwriter with a distinctive, vibrato-heavy tenor that earned him the nickname “the Caruso of country,” a nod to Italian opera star Enrico Caruso. In the late 1960s and ’70s he recorded a string of country hits, including versions of his mentor Ray Price’s “I’ll Be There,” Marty Robbins’s “You Gave Me a Mountain” and Willie Nelson’s “Undo the Right” and “What a Way to Live.”

But soon after signing with RCA Records, his new label tasked him with composing a song of his own. He responded by writing “Whiskey River” with Paul Stroud, whom Mr. Bush later described as “an old rodeo cowboy friend.” Mr. Bush said he began writing it while traveling from Nashville to Texas by bus, jotting down lyrics on a paper sack that someone had used to bring him a cheeseburger:

Whiskey River, take my mind

Don’t let her memory torture me

Whiskey River, don’t run dry

You’re all I got, take care of me

Released as a single in 1972, the song reached No. 14 on the country charts while receiving near-constant airplay on Texas country stations. Thick with fiddles, driven by a propulsive bass line, it was an infectious anthem about alcohol-induced forgetfulness and the solace offered by an “amber current,” when the memory of an old love has left you feeling cold.

The song was shaped in part by Nelson, whom Mr. Bush called for advice. “Whiskey River” only had one verse and one bridge, he said, and didn’t it need something more? “Well, you’ve already said what you need to say,” Nelson told him, according to a Texas Monthly report. “In a country song, you tell your life story in two and a half minutes.”

When Nelson recorded a rockified version of the song for “Shotgun Willie,” his influential 1973 outlaw-country album, “Whiskey River” became an even bigger hit; Nelson opened his concerts with it for decades.

Mr. Bush and Nelson had been friends since the 1950s, when both men were unknown musicians performing in San Antonio beer joints. For a time, it seemed that Mr. Bush had the more promising career: When they partnered together in a group called the Mission City Playboys, it was Mr. Bush who convinced Nelson to join as a guitarist, not a singer, according to Nelson’s 2015 autobiography, “It’s a Long Story.”

“He had a tremendous voice filled with feeling,” Nelson wrote. He recalled having to fight back tears when he heard Mr. Bush sing a country rendition of the pop standard “Stardust,” which “made me see that a great singer can sing any song in any genre. Johnny Bush was a great singer.”

Their group, he added, “was a band that went nowhere fast, but we had fun going there.”

Mr. Bush often cited Nelson as his chief inspiration as a songwriter, once declaring that he compared every song he ever wrote to Nelson’s “Funny How Time Slips Away.” Through Nelson, he also met Ray Price, his other musical lodestar, playing the drums in the singer’s Cherokee Cowboys group for several years in the 1960s.

Mr. Bush left when Price began moving in a more pop-oriented direction but took with him “the fiddle-’n’-steel sound and the two-step shuffle beat that remain, to this day, at the heart and soul of true Texas honky-tonk,” music critic Rick Mitchell wrote in the introduction to “Whiskey River (Take My Mind),” Mr. Bush’s 2007 memoir.

By 1972, Mr. Bush seemed on the verge of bringing his honky-tonk sound to a larger audience. “Whiskey River” was rising through the charts when his voice suddenly started to slip. “The high notes — which in the past had come as easily and naturally to me as breathing — became raspy and strangled,” he wrote in his memoir. “It was as if my throat was being choked off.”

Doctors told him there seemed to be nothing wrong with his vocal cords. But his second marriage was struggling, and one physician prescribed anti-anxiety medicine and suggested he take time off. Mr. Bush lost his contract with RCA and soon lost his voice, finding that he was largely unable to talk. He was diagnosed with spasmodic dysphonia, a rare neurological disorder, and came to believe that God was punishing him for what he described as his “sinful behavior,” including years of womanizing and heavy drinking.

Mr. Bush ultimately launched a comeback, releasing albums including “Talk to My Heart” (1998), “Honky Tonic” (2004) and “The Absolute Johnny Bush” (2017). He was aided by vocal exercises he learned under Gary Catona, a noted voice coach, and by Botox treatment that he started in the 1990s, receiving a shot near the vocal cords every eight to 10 weeks. As a result, he estimated that his voice was no less than 75 percent of what it was in his prime.

In a phone interview, Tracy Pitcox, president of the country label Heart of Texas Records, said Mr. Bush was a key influence on many Texas country and “red dirt” artists, including Pat Green and Robert Earl Keen. “These younger artists were considering him what I’d call an elder statesman of this music,” he added.

Mr. Bush said he was simply trying to protect Texas’s “musical birthright” by honing a traditional sound, in contrast to “the powers that be in Nashville, who have either boldly or subtly set out to kill the original roots of country music.” Because of artists such as Nelson, Merle Haggard and George Jones, “I feel confident that our music will survive and stay real,” he wrote in his memoir. “If I have been able to play a role in this process, then I am grateful.”

Mr. Bush was born John Bush Shinn III in Houston on Feb. 17, 1935. He grew up in a home without electricity or plumbing, wearing clothes that his mother made by hand. His father worked in the printing business and played guitar, and his parents divorced when he was in the seventh grade.

After dropping out of high school, he worked in the oil fields and then threw himself into country music, going by Johnny Bush after a television announcer mixed his name up when he was 17, according to a biography on his website.

Around that same time, he married for the first time, to a woman he identified in his memoir only as Jean. Mr. Bush had several other marriages and in 1988 married Lynda Kilian. She survives him, as does a daughter from his first marriage, Gaye Lynn Litton; a stepdaughter, Christine Hecker; a brother; four grandchildren; and three great-grandchildren. A stepson, Trey Kilian, died in 2007.

Mr. Bush was inducted into the Texas County Music Hall of Fame in 2003. He had continued performing in recent years, donning a Stetson for each show and regularly ending the night with two of his most popular drinking anthems, “Green Snakes on the Ceiling” and “Whiskey River.”

“He acted as though he was a new artist just getting started,” said Pitcox, whose label released some of Mr. Bush’s last records. Pitcox added that he had tried to convince Mr. Bush not to perform his last show — in New Braunfels, Tex., last month — because Mr. Bush was increasingly ill, receiving supplemental oxygen.

But Mr. Bush was adamant that the show go on.

“It’s sold out,” he said. “I’ve got to do it. I don’t want to let the fans down.”