Commodity ETFs are down this quarter. That could be long-term positive

It’s been a rocky summer to be a commodities trader, but that could give investors looking to diversify away from stocks and bonds some long-term upside. More than halfway through the third quarter, many of the biggest commodity ETFs in the United States are in sizable holes. That applies to index-style funds like Invesco’s Optimum Yield Diversified Commodity Strategy ETF (PDBC) as well as funds focused on single commodities, like the United States Oil Fund (USO) . The slump in commodities could be a warning sign about slower economic growth . Notably, one of the exceptions among commodities has been…