BEIJING — Apple’s $1,000 iPhone is a tough sell to consumers in China unnerved by an economic slump and the trade war with the U.S.

CEO Tim Cook said in a letter to shareholders Wednesday that demand for iPhones is waning and revenue for the last quarter of 2018 will fall well below projections, a decrease he traced mainly to China.

The iPhone is Apple’s biggest product, accounting for about 60 percent of its revenue in the July-September quarter, the latest reported. Its stock fell 10 percent Thursday, the worst drop in five years, wiping out about $75 billion in market value.

Apple is the latest company grappling with increasing Chinese consumer anxiety. Other brand names such as Ford and jeweler Tiffany & Co. already have reported abrupt declines in sales to Chinese buyers.

China still is one of the fastest-growing economies, with 2018’s expansion forecast at about 6.5 percent. But China’s tariff fight with the U.S. and bad news about tumbling auto and real estate sales are undermining consumer confidence after two decades of almost unbroken rapid growth.

“It’s definitely worrying,” said China specialist David Dollar, senior fellow at the Brookings Institution.

The Apple news shows how vulnerable many big U.S. companies are to a slowdown in China, especially as the world’s two biggest economies are in a contentious trade dispute.

“It’s not going to be just Apple,” Kevin Hassett, chairman of the White Council of Economic Advisers, told CNN Thursday.

He said many companies have a lot of sales in China and will “be watching their earnings downgraded next year until we get a deal with China.”