NEW YORK — The S&P 500 tumbled to a more than six-month low on Thursday, closing in negative territory for the year, on concern a decelerating Chinese economy will translate into slower global growth.Consumer stocks led the decline on Wall Street with Disney down 6 percent after a brokerage downgrade, while Apple fell 2 percent after a report that overall smartphone sales in China fell in the second quarter.Concern about the Chinese economy was underscored by a near 8 percent slide in the Shanghai stock index so far this week and after the Commerce Ministry said Wednesday exports could continue falling in coming months.“The largest issue is certainly the fact that we don’t know how much the Chinese economy is slowing,” said Art Hogan, chief market strategist at Wunderlich Securities in New York.“That’s manifesting itself in lower oil prices,” he said, pointing to the correlation between stocks and crude futures.U.S.