Apple cut its revenue guidance for its fiscal first quarter, sending its stock sharply lower in after-hours trading as concerns continue to mount over the company's future iPhone and China growth.

In a letter to investors, Apple CEO Tim Cook said the company now expects revenue of $84 billion in the quarter ending Dec. 29.

Apple had anticipated revenue of between $89 billion and $93 billion for the quarter. The number is also lower compared with the $91.5 billion in revenue analysts previously anticipated, according to earnings-tracker Refinitiv.

"Based on these estimates, our revenue will be lower than our original guidance for the quarter, with other items remaining broadly in line with our guidance," Cook wrote.

After being halted temporarily, Apple shares resumed trading at 4:50 p.m. ET, quickly falling over 8 percent to $145.12. The plunging shares wiped out more than $50 billion in the company's market value, according to Bloomberg data. Apple, which was trading around $146 in after-hours trading is now down more than 37 percent from its Oct. 3 high and has fallen mightily since becoming the first U.S. company to reach a $1 trillion market cap in August.

The bad news sent the Dow Jones Industrial Average down more than 300 points in after-hours futures trading.

Apple’s stock is widely owned by workers ranging from teachers to electricians to lawyers. With a roughly 3.5 percent weighting in the S&P 500 at the end of 2018, Apple is the second-biggest company behind Microsoft in the large-company stock index, an investment most people own in their 401(k)s.

Apple has faced a number of questions on its future growth for iPhones in the U.S. and abroad and its overall growth in China.

"While we anticipated some challenges in key emerging markets, we did not foresee the magnitude of the economic deceleration, particularly in Greater China," Cook said in his note. "In fact, most of our revenue shortfall to our guidance, and over 100 percent of our year-over-year worldwide revenue decline, occurred in Greater China across iPhone, Mac and iPad."

The company recently brought out a trade-in offer to try to get consumers to upgrade their older devices to newer iPhone XR, XS and XS Max phones. These phones aren't cheap, starting at $749 for the XR, for example.

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Apple saw "fewer iPhone upgrades than we had anticipated," according to Cook, noting that, among other factors, people were taking advantage of the company's cheaper battery replacement program, which it offered for the duration of 2018. By upgrading their iPhone's battery, people would see performance for their phones improve as software limitations Apple put in place to help weaker batteries last were lifted.

Apple raised the price of replacement iPhone batteries on Jan. 1.

The iPhone has long been a driver of Apple earnings. iPhone sales made up roughly 59 percent of its nearly $63 billion in revenue during the company's fourth quarter.

The trade war between the U.S. and China may have also played a role according to Cook. "We believe the economic environment in China has been further impacted by rising trade tensions with the United States. As the climate of mounting uncertainty weighed on financial markets, the effects appeared to reach consumers as well, with traffic to our retail stores and our channel partners in China declining as the quarter progressed."

Cook says the company also faced constraints with its other products.

"Sales of Apple Watch Series 4 and iPad Pro were constrained much or all of the quarter," the CEO wrote, adding that AirPods and the MacBook Air also dealt with constraints.

Not all will be bad for Apple in the quarter, however, according to Cook. The company continued to see growth in its other categories including Mac, iPad, wearables and services.

"Revenue outside of our iPhone business grew by almost 19 percent year-over-year, including all-time record revenue from Services, Wearables and Mac," said Cook. The services division – which includes Apple Music, the App Store and iCloud subscriptions – generated over $10.8 billion in revenue in the quarter.

Follow Eli Blumenthal on Twitter @eliblumenthal. Follow Adam Shell on Twitter @adamshell