The persistent drought is taking a toll on producers of ethanol, with corn becoming so scarce that nearly two dozen ethanol plants have been forced to halt production. The Renewable Fuels Association, an ethanol industry trade group, provided data to The Associated Press showing that 20 of the nation’s 211 ethanol plants have ceased production over the past year, including five in January. Most remain open, with workers spending time performing maintenance-type tasks. But ethanol production won’t likely resume until after 2013 corn is harvested in late August or September. Industry experts don’t expect a shortage — millions of barrels...