Apple Inc. AAPL, -1.59% shares fell below $100 for the first time in 15 months, gripped by Thursday’s sharp market declines and signs of slowing growth in the two pillars of the company’s recent success: the iPhone and China.

The Cupertino, Calif., company’s 4.2% drop on the day was greater than the overall market’s slide and the 3% slump in the Nasdaq Composite, which closed down 10% from its May peak. Since early December, Apple has shed $100 billion in market value.