* TSX down 13.18 points, or 0.12 percent, at 11,240.05
* Index's energy and materials groups drop
* Potash Corp down 1 percent after Citi downgrade (Adds details, quote)
TORONTO, Sept 14 (Reuters) - Toronto's main stock index drooped on Monday morning, pressured by weaker commodity prices and a drop on world stock markets due to concern about a U.S.-China trade spat.
The index's key energy and materials sectors dropped 0.4 percent and 0.5 percent, respectively. Heavyweight decliners included oil company EnCana Corp (ECA.TO), down 1 percent at C$62.88, and miner Teck Resources TCKb.TO, which fell 0.8 percent to C$28.13.
"The thing I'm looking at right now is the issue of trade friction," said John Johnston, chief strategist for Harbour Group at RBC Dominion Securities. "That's certainly not good for equity investors."
U.S. President Barack Obama announced safeguard duties on tire imports from China late last week and China responded quickly by saying on Monday that it would request World Trade Organization consultations with the United States over the duties. [ID:nSP459289] [ID:nLD514738]
"It throws pebbles into the gears the global economy," said Johnston.
At 10:04 a.m. (1404 GMT), the S&P/TSX composite index .GSPTSE was down 13.18 points, or 0.12 percent, at 11,240.05, with six of its 10 main groups lower.
Another source of pressure on the index was Potash Corp of Saskatchewan (POT.TO), which dropped 1 percent to C$95.86. Citigroup on Monday lowered its rating on fertilizer producers Potash and Mosaic Co (MOS.N) to "hold" from "buy", citing the likelihood of limited opportunity for fertilizer application this autumn among its concerns. [ID:nN14501206]
Heavyweight advancers were Thomson Reuters (TRI.TO), up 2.2 percent, and Research In Motion RIM.TO, up 0.4 percent. Thomson Reuters said its tax and accounting business will buy the Abacus Enterprise range of software and related business operations from Deloitte LLP.
$1=$1.09 Canadian Reporting by Jennifer Kwan; editing by Peter Galloway