TE PUKE, New Zealand—Striding among orchards laden with fruit for export here, scientists in rubber boots and shorts scratch their heads and wonder if they have bitten off more than they can chew.

The problem isn't a lack of kiwis—the exotic green fruit that appears in everything from breakfast bowls to juice blends. A sunny summer has ripened them perfectly. Instead, the vexing issue is the kiwifruits' skin—it is furry and brown as it has been for decades.

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