ONCE you become accustomed to gas at $4 a gallon, brace yourself for the next shocking retail threshold: bananas reaching $1 a pound. At that price, Americans may stop thinking of bananas as a cheap staple, and then a strategy that has served the big banana companies for more than a century — enabling them to turn an exotic, tropical fruit into an everyday favorite — will begin to unravel. The immediate reasons for the price increase are the rising cost of oil and reduced supply caused by floods in Ecuador, the world’s biggest banana exporter. But something larger is...