Sentiment among U.S. small business owners plunged in September as pandemic-induced supply disruptions and a persistent labor shortage weighed on the economy’s recovery.

The National Federation of Independent Business said Tuesday that its optimism index fell in September by one point to 99.1, the lowest since March. Five of the 10 components declined, while just three improved.

“Small business owners are doing their best to meet the needs of customers, but are unable to hire workers or receive the needed supplies and inventories,” NFIB chief economist Bill Dunkelberg said in a statement. “The outlook for economic policy is not encouraging to owners, as lawmakers shift to talks about tax increases and additional regulations.”

Owners expecting better business conditions over the next six months decreased to the lowest level since December 2012.

Labor costs and quality were cited as the biggest threats to small business owners.

That’s in part because small businesses are struggling to find workers to fill open positions. Fifty-one percent of all owners reported job openings they were unable to fill, a record high for the third consecutive month.