Feb 26 (Reuters) - Canada’s main stock opened lower on Friday, a day after posting its worst session in nearly a month, with energy and materials stocks tracking weakness in commodities, while data showed domestic producer prices rose 2% in January. * At 9:30 a.m. ET (14:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 28.75 points, or 0.16%, at 18,194.79.