US SMALL/MIDCAPS-Stocks fall, led by energy, materials

NEW YORK, Sept 17 (Reuters) - Small- and mid-capitalization stocks dropped on Monday, led by a decline in materials and energy shares following a selloff in oil and other commodities.

Among energy companies with the biggest declines were Forest Oil Corp, down 4 percent at $8.81, and SM Energy Co , down 2.9 percent at $56.08.

Oil prices dropped in volatile trading, with Brent falling more than $5 a barrel before settling at $113.79, down $2.87. Traders said the selloff appeared to have stemmed from a problem with an automated computer trading program.

Materials shares in small and larger-cap markets also fell after JP Morgan Securities downgraded several stocks, including Cliffs Natural Resources Inc.

Among other downgrades, container board maker Rock-Tenn Co was cut by Longbow Research to "neutral" from "buy." Its shares dropped 6 percent to $63.67.

The S&P MidCap 400 index was down 1.1 percent, while the S&P SmallCap 600 index was down 0.7 percent. In comparison, the benchmark S&P 500 was down 0.3 percent.