Detroit, the very symbol of American industrial might for most of the 20th century, is drawing up a radical renewal plan that calls for turning large swaths of this now-blighted, rusted-out city back into the fields and farmland that existed before the automobile. Operating on a scale never before attempted in this country, the city would demolish houses in some of the most desolate sections of Detroit and move residents into stronger neighborhoods. Roughly a quarter of the 139-square-mile city could go from urban to semi-rural. Near downtown, fruit trees and vegetable farms would replace neighborhoods that are an eerie landscape of empty buildings and vacant lots. Suburban commuters heading into the city center might pass through what looks like the countryside to get there. Surviving neighborhoods in the birthplace of the auto industry would become pockets in expanses of green. Detroit officials first raised the idea in the 1990s, when blight was spreading. Now, with the recession plunging the city deeper into ruin, a decision on how to move forward is approaching. Mayor Dave Bing, who took office last year, is expected to unveil some details in his state-of-the-city address this month. “Things that were unthinkable are now becoming thinkable,” said James W. Hughes, dean of the School of Planning and Public Policy at Rutgers University, who is among the urban experts watching the experiment with interest. “There is now a realization that past glories are never going to be recaptured. Some people probably don’t accept that, but that is the reality.” The meaning of what is afoot is now settling in across the city. “People are afraid,” said Deborah Younger, executive director of a group called Detroit Local Initiatives Support Corporation that is working to revitalize five areas of the city. “When you read that neighborhoods may no longer exist, that sends fear.”