Delta Air Lines' shares plunged the most in more than three years, pulling rivals lower, after disappointing ticket pricing forced the carrier to cut its revenue forecasts for the second time in two months.

Fourth-quarter revenue from each seat flown a mile, or unit revenue, will rise 3 percent from a year earlier, the Atlanta-based airline said in a regulatory filing Thursday. Delta previously forecast a 3.5 percent gain.

The news added to investor concerns that weaker pricing and the possibility for slowing global demand from trade wars could weigh on profits this year. The timing — coming only three weeks after a Delta update on Dec. 13 and following the busy holiday travel season — may have magnified investors' alarm, said Savanthi Syth, an analyst at Raymond James Financial Inc.

"We do not believe that the sharp sell-off in the U.S. airline shares beyond general market weakness is warranted, albeit understandable given the optics on a day with ample supply of negative headlines," she said in a note. Delta's reduced revenue outlook doesn't indicate general industry weakness, Syth wrote.

Buckingham Research Group analyst Dan McKenzie called Delta's adjustment "a blip, not a major revenue stumbling block."

The unit-revenue change would reduce fourth-quarter profit by 4 cents from the $1.30 a share previously expected, according to Syth's estimates. Delta was the first major carrier to comment on December trends and will report financial results later this month.

Delta shares closed down 8.9 percent to $45.61 after tumbling as much as 10 percent. Shares in other airlines fell sharply as well, bringing a Standard & Poor's index of airlines down 5.9 percent, the worst performing group on the S&P 500. The rout came amid a broad slump after Apple cut its sales outlook and weak U.S. manufacturing data was released.

"The pace of improvement in late December was more modest than anticipated," Delta said of average fares per mile. Demand from business and leisure travelers remained "healthy" in the quarter. Total revenue will rise about 7 percent, the carrier said, instead of the 7.5 percent that had been anticipated.

Given the timing of Christmas in 2018, investors may have expected that business travel would stay strong through Dec. 21, said Helane Becker, a Cowen Inc. analyst. "That appears not to have happened for Delta," she said in a note to investors.

The airline narrowed its outlook for fourth-quarter profit to $1.25 to $1.30 a share, the high end of its earlier expectation.