(For a Reuters live blog on U.S., UK and European stock markets, click LIVE/ or type LIVE/ in a news window.) * S&P 500, Nasdaq on course for worst day in four months * Ford rises on $11.4 bln investment with SK Innovation * Indexes drop: Dow 1.62%, S&P 1.99%, Nasdaq 2.64% (Updates to afternoon, adds dateline, changes byline) NEW YORK, Sept 28 (Reuters) - Wall Street stocks tumbled in a broad sell-off on Tuesday as the upward trajectory of U.S. Treasury yields and deepening inflation concerns dampened risk appetite and sent investors fleeing the equities market. All three major U.S. stock indexes were deep in red territory, with market leading megacap stocks weighing heaviest. The S&P 500 index and the Nasdaq Composite index were on track for their biggest daily percentage drop since May, and their largest monthly declines since September 2020. “It’s all about rising yields,” said Peter Cardillo, chief market economist at Spartan Capital Securities in New York. “Part of it is this time of the year and part of it is inflation fears,” Cardillo added. “That’s why investors are hitting the brakes.” “September has lived up to its expectations.” The benchmark index was also setting a course for its weakest quarterly performance since the COVID pandemic brought the global economy to its knees. Weakness pervaded across most asset classes, including gold, suggesting widespread risk-off sentiment. Benchmark U.S. Treasury yields continued their ascent, reaching the highest level since June as inflation expectations heated up and fears grew that the U.S. Federal Reserve could shorten its timeline for tightening its monetary policy. Stocks extended their decline after Treasury Secretary Janet Yellen said she expected inflation to end 2021 near 4% and warned lawmakers their failure to avert a government shutdown as the nation moves closer to exhausting its borrowing capabilities could cause “serious harm” to the economy. Senate Republicans appeared set to strike down Democrats’ efforts to extend the government’s borrowing authority and avoid a potential U.S. credit default. A Conference Board report showed consumer confidence weakened unexpectedly in September to the lowest level since February. The Dow Jones Industrial Average fell 563.86 points, or 1.62%, to 34,305.51, the S&P 500 lost 88.42 points, or 1.99%, to 4,354.69 and the Nasdaq Composite dropped 395.85 points, or 2.64%, to 14,574.12. Nearly half of the S&P 500’s components were down 10% or more from their 52-week high. That included 64 stocks that had fallen 20% or more. Among the 11 major sectors of the S&P 500, all but energy were lower, with tech and communications services suffering the steepest percentage losses. Apple Inc, Microsoft Corp, Amazon.com Inc and Alphabet Inc were down between 2.3% and 3.8%. Among other stocks, Ford Motor Co gained 1.3% on news that it would partner with Korean battery partner SK Innovation to invest $11.4 billion to build an electric F-150 assembly plant and three U.S. battery plants. Declining issues outnumbered advancing ones on the NYSE by a 4.77-to-1 ratio; on Nasdaq, a 4.49-to-1 ratio favored decliners. The S&P 500 posted 17 new 52-week highs and five new lows; the Nasdaq Composite recorded 47 new highs and 106 new lows. (Reporting by Stephen Culp; Additional reporting by Noel Randewich in New York and Devik Jain in Bengaluru; Editing by Richard Chang) Our Standards: The Thomson Reuters Trust Principles.