U.S. airlines marked the second-sharpest decline on the Standard & Poor’s 500 Index, following a weak revenue report from Delta Air Lines Inc. Passenger revenue for each seat flown a mile fell 7 percent in July from a year earlier, the country’s second-largest carrier by traffic reported Tuesday. The Atlanta-based airline blamed bad bets on currency exchange, excess seat supply on trans-Atlantic flights and reduced prices for tickets booked shortly before travel. The S&P Airlines Index fell 4.7 percent at 11:58 a.m in New York. Delta tumbled 6.3 percent to $36.98, the second-worst performance among companies on the S&P 500. “The group is off on the fact that you’re probably going to have negative headlines on PRASM numbers for July, which should not have been a surprise,” said Stephens Inc. analyst Jack Atkins. Delta warned investors two weeks ago that revenue for each available seat mile could be weak. “Maybe there was some expectation that things strengthened in the back half of July. I was not assuming that.” Airlines’ so-called unit revenue has fallen for more than a year, in part because of heavy competition in certain U.S. markets and an oversupply of seats in some regions overseas. Meeting Forecast Delta’s weak performance last month means it will need a sharp recovery in September to meet its outlook for this quarter, UBS Securities analyst Darryl Genovesi said in a note. The company had forecast that unit revenue would fall 4 percent to 6 percent in the period and said it would trim capacity next month. While advance ticket sales are improving, “we think it’s too early to assume a steep September recovery as a base case, given the close-in bookings weakness that we’ve been seeing over the past year,” he wrote. The airlines index was down 18 percent this year through Monday, while the S&P 500 gained 6.2 percent.