US STOCKS-Wall St begins last trading week of 2015 in the red
* Apple biggest drag on S&P, Nasdaq
* Exxon, Chevron fall as crude prices slide
* Valeant slumps as CEO goes on medical leave
* Disney up as latest Star Wars rakes in $1 bln
* Indexes down: Dow 0.46 pct, S&P 0.56 pct, Nasdaq 0.67 pct (Updates to early afternoon)
By Abhiram Nandakumar
Dec 28 (Reuters) - Wall Street began the last trading week of the year in negative turf, as the oversupply pressure on crude oil prices showed no signs of abating.
The S&P 500 energy sector was down 1.87 percent, easily the worst performer on the index. Exxon's shares were down 1 percent at $78.57, while Chevron was down 1.9 percent at $90.34.
The S&P 500 slipped back into negative territory for the year on Monday, down 0.5 percent, while the Dow Jones industrial average was down 2 percent. The Nasdaq Composite index was up 5.8 percent. Continuación...