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Investors drove down the price of Monsanto shares by 4 percent on Friday as South Korea joined Japan in suspending imports of U.S. wheat after an unapproved strain of genetically modified wheat was discovered in a field in eastern Oregon.

The strain of wheat, designed to resist harmful effects from Monsanto’s Roundup weedkiller, was never commercially developed by the St. Louis-based agriculture giant in large part because wheat growers did not want to risk retaliation from their biggest export markets.

Fields used to test new crop varieties are burned and checked for surviving crops. So the mysterious appearance of the Monsanto wheat has raised questions about how the strain traveled there and whether it is lurking in the commercial wheat crop.

As a precaution, South Korea, which last year bought about half of its wheat imports from the United States, said it would halt purchases while it runs tests this weekend on wheat and flour that it has already imported. The European Union is also testing supplies.

“This is an embarrassment for Monsanto, not as much with the public as it is with food companies, ” said Gene Grabowski, executive vice president of Levick, a communications and public relations firm. Grabowski, a former senior executive at the Grocery Manufacturers Association, said cereal and other food product firms selling in Japan or Europe haven’t wanted to go to the expense of making sure their wheat sources were free of genetic engineering.

“I was in board meetings where I remember food company CEOs who were very concerned about the idea that Monsanto was pushing for approval for biotech wheat,” he said. “They didn’t want it because they already had their hands full dealing with repercussions of biotech corn and soy.”