July 21 (Bloomberg) -- U.S. stock futures tumbled after the close of U.S. exchanges, dragged down by lower-than-estimated earnings at American Express Co. and disappointing forecasts at Apple Inc. Treasury yields and the dollar also dropped.

American Express, the biggest U.S. credit card company by purchases, fell 11 percent from its 4 p.m. close after second- quarter profit trailed analysts' estimates by 32 percent. American Express Co. said profit dropped because more consumers defaulted on loans, raising concern the U.S. economic slowdown will deepen.

Apple, maker of the iPod music player, lost 6.5 percent after saying sales and earnings will fall short of projections. SandDisk Corp. tumbled 12 percent after reporting a loss.

``This whole earnings season will be somewhat choppy,'' said Eric Marshall, who helps oversee $1.4 billion at Hodges Capital Management Inc. in Dallas. ``The technology companies that are more tied to making components or semiconductors or cell phones, those are more likely to be impacted by weakness in consumer spending.''