A key measure of the nation's manufacturing activity fell sharply in September, a purchasing manager's group said Wednesday, nearing a benchmark that indicates a recession in the sector.

The Institute for Supply Management's (ISM) manufacturing index fell to 43.5 in September, down from the August reading of 49.9. Economists were expecting a reading of 49.5, according to a consensus estimate compiled by Briefing.com.

The tipping point for the index is 50, with a reading below that indicating contraction in factory activity. A reading below 41 marks a recession. The index has hovered around the 50 mark for the past 12 months, with an average of 49.6.