Alarm bells went off in Minnesota when Prairie Home Companion talker Garrison Keillor mentioned to Minneapolis’s Star Tribune last week that he might give up host duties and become producer for a “successor show.” The newspaper is examining “the ripple effects” that it says “would be enormous” for state businesses if that happened — even at the state fair, where Keillor appearances routinely draw 7,000 to 11,000 fans. Keillor’s mild stroke on Sept. 7 and four-day hospital stay has him pondering his future.