ZZ Top, the classic 1970s and '80s boogie-rocking band that was to headline Musikfest's opening night on Sands Steel Stage in 10 days, has canceled.

Musikfest does not plan to schedule a replacement act, leaving the festival without an opening night act for the first time ever, said Musikfest spokesman Mark Demko.

The band's bassist, Dusty Hill, 65, is scheduled to undergo surgery to remove two kidney stones this week, so the band has canceled six dates — from the July 25 start of its tour through the Aug. 1 Musikfest date, publicist Bob Merlis said.

"The kidney stones were discovered in the course of a routine physical examination, where Hill's doctor advised him that they should immediately be removed, lest they cause more serious problems down the line," Merlis said in a statement.

He added: "It is expected that Hill will make a full recovery following two weeks of recuperation."

ZZ Top's tour is now set to launch on Aug. 6 in Sturgis, S.D., with the band joining forces with Jeff Beck for a string of tour dates immediately thereafter, Merlis said.

The cancellation is a blow to the 10-day Bethlehem festival. The band was among the biggest sellers this year.

The show had sold 5,227 tickets for the 6,560-seat venue, according to the festival's online ticket site. That made it the best-selling headline act for this year behind country artist Keith Urban, who sold out his Aug. 10 closing-night show.

It's not the latest-date cancellation by a headliner at Musikfest. Sheryl Crow canceled in 2001 — just two days before the festival started and five days before her show. The show had been a sellout.

Musikfest replaced Crow with a second show by modern rock band Fuel, which had sold out an earlier performance.

ZZ Top, known as the Little Ol' Band from Texas, has sold 30 million copies of its 24 studio and live compilation albums in its career, which has stretched more than 35 years. Its breakthrough album was 1973's "Tres Hombres," which sold gold and broke the Top 10.

"Fandango!" in 1975 gave the band its first Top 20 hit, "Tush."

But by far its most popular disc was 1983's "Eliminator," which sold more than 10 million copies, with hits such as the No. 2 "Gimme All Your Lovin'."

The follow-up disc, "Afterburner," sold more than 5 million copies and was the band's highest-charting album, hitting Top 5. A 1992 Greatest Hits album sold 3 million copies. A dozen of its discs have gone gold or platinum.

The band's most recent studio disc, "La Futura" in 2012, went to No. 6. A compilation album, "The Complete Studio Albums, 1970-1990," was released in June 2013.

In all, the band has had 25 Top 20 songs on various charts, with its last in 1996.

ZZ Top was inducted into the Rock and Roll Hall of Fame in 2004.

ZZ Top ticket-holders will be issued a refund and should contact the ArtsQuest Box Office at 610-332-3378 starting at 10 a.m. Tuesday. Individuals who purchased tickets via cash will need to present their tickets in person at the box office.

As a "thank you" to ZZ Top ticket buyers, ArtsQuest is waiving service fees on tickets they purchase for other Musikfest headliners through 5 p.m. Wednesday.

More info: http://www.musikfest.org.

John.moser@mcall.com

Twitter @johnjmoser

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