From a hillock in the San Joaquin Valley, Berne Evans III recently surveyed a citrus grove that stretches as far as the eye can see. "It's the largest clementine planting in the world," he said, smiling.

The groves make Mr. Evans the king of the Cuties, a brand of seedless, sweet and easy-to-peel mandarin that is storming the nation's fruit aisles and changing eating habits that span generations. The navel orange, after reigning supreme for decades, has a challenger.

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