Apple (NasdaqGS: AAPL) was down nearly 3% in Monday’s premarket and the stock looks ready for another test of the key $500-a-share level as the tech giant weighs down the popular PowerShares QQQ (NasdaqGM: QQQ), an ETF hitched to the Nasdaq-100 Index.

AAPL is the largest individual stock holding in QQQ at 15.5% of the portfolio.

The company has cut orders for LCD screens and other parts for the iPhone 5 this quarter due to weak demand, Reuters reports.

Apple shares have corrected more than 20% since the September high, which has hurt the tech-heavy QQQ. The Nasdaq-100 has been trailing the S&P 500 due in large part to the weakness in Apple.

For example, QQQ has slipped about 1% since the end of August while SPDR S&P 500 (NYSEArca: SPY) is up 4%. [Equal-Weighted Nasdaq ETF Dodges Apple Sell-Off]

The chart below shows the relative performance of QQQ versus SPY. When the chart is falling, the Nasdaq-100 is lagging the S&P 500.

Full disclosure: Tom Lydon’s clients own QQQ, AAPL and SPY.

The opinions and forecasts expressed herein are solely those of John Spence, and may not actually come to pass. Information on this site should not be used or construed as an offer to sell, a solicitation of an offer to buy, or a recommendation for any product.