Then, last spring, some of Mr. Lahti’s fruit tested positive for genetically modified seeds. »’I was really surprised,’« Mr. Lahti said. »’I didn’t really know what was happening.”

He cut down all 170 of his trees and is now replanting, without any guarantee that the same problem - pollen from modified trees on other farms drifting on the wind to pollinate his trees - won’t happen again. From papayas in Hawaii, to corn in Mexico and canola in Canada, the spread of pollen or seeds from genetically engineered plants is evolving from an abstract scientific worry into a significant practical problem.