(Updates prices, sectors) July 19 (Reuters) - Canada’s main stock index was on course for its worst session in nearly five months on Monday, hit by a plunge in energy stocks as crude prices retreated on an OPEC+ decision to boost output. * The energy sector dropped 2.8% as U.S. crude prices were down 3.7% a barrel, while Brent crude lost 3.4%. * At 9:37 a.m. ET (13:37 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 246.07 points, or 1.23%, at 19,739.47. * The commodity-heavy index was also weighed down by a 1% fall in the materials sector, which includes precious and base metals miners and fertilizer companies, as gold futures ticked 0.1% lower to $1,812.5 an ounce. * The financials sector slipped 1.5%, while the industrials sector fell 1.1%. * Lumber provider Interfor Corp fell 6.2%, the most on the TSX, and the second-biggest decliner was miner First Quantum Minerals Ltd, down 6%. * On the TSX, 29 issues were higher, while 199 issues declined for a 6.86-to-1 ratio to the downside, with 21.54 million shares traded. * The largest percentage gainers on the TSX were OceanaGold Corp, which jumped 1.7%, and Tricon Residential Inc , which rose 2.0% after the rental housing company announced a joint venture to acquire more than 18,000 single-family rental homes for about $5 billion. * The most heavily traded shares by volume were Royal Bank of Canada, Baytex Energy Corp, and Bombardier Inc. * The TSX posted no new 52-week highs and no new lows. * Across all Canadian issues there were 13 new 52-week highs and 22 new lows, with total volume of 49.78 million shares. (Reporting by Amal S in Bengaluru; Editing by Aditya Soni) Our Standards: The Thomson Reuters Trust Principles.