The S&P 500 suffered its biggest decline since February 2009. Losses exceeded 10% for 13 of the companies. Just three stocks rose on the benchmark index, which is down 11% since July 22. The Dow erased its 2011 gain. ‘It’s unbelievable . . . It’s left everybody with this mindset that things are not good,’ said David Joy, an Ameriprise strategist in Boston.