Source: Nanci Hellmich and Melanie Eversley / USA Today A collection of retail heavyweights will join first lady Michelle Obama's campaign to bring fresh fruits, vegetables and other nutritious foods to the USA's "food deserts" — impoverished areas with little access to healthy foods. National chains, including Wal-Mart, Walgreens and SuperValu, and some regional retailers have agreed to open or expand more than 1,500 stores to bring more nutritious and fresh food to underserved communities. The companies' executives are joining [...]