US supplies of diesel have fallen to the lowest seasonal level ever, threatening to push prices even higher for the global economy's workhorse fuel just as heating and agricultural seasons kick into gear. Stocks of distillate fuel oil — a category of fuels that most prominently includes diesel — stood at 103.4 million barrels in the week ending August 21, according to data released Wednesday by the Energy Information Administration. The declining inventories at oil storage tanks, including the rail-fed diesel base in Wink, Texas, are raising concerns about supply tightness heading into peak demand periods.