Wall Street open lower on Tuesday, weighed down by a 6 percent slump in Chinese shares and Wal-Mart's weaker-than-expected quarterly results. Walmart shares fell as much as 3.2 percent to a nearly 2-1/2 year low of $69.58 and were the biggest drag on the Dow Jones industrial average. Disney fell 1.1 percent to $107.86 after a Wells Fargo downgrade and weighed on the S&P 500 and the Dow. Apple, for whom China is a key market, fell 0.5 percent to 116.60 and was the biggest drag on the Nasdaq and the S&P 500. Chinese stocks plunged again as the yuan weakened against the dollar, ...