Japan halted a major purchase of white wheat from the United States Thursday.
How the crop got on an 80-acre field in Oregon is a mystery, The Washington Post reported.
Monsanto tested a similar strain in Oregon between 1994 and 2005 but the product was never approved for commercial use.
The strain was identified in Oregon recently when a farmer tried clearing a field using Monsanto's herbicide and discovered that the wheat could not be killed, the Post reported.
Monsanto said in a statement that the strain is not harmful and that it will likely not be found elsewhere.
Food safety groups drew the opposite conclusion.
"This was not from a recent trial, which means it's been sitting there in the environment," said Andrew Kimbrell, executive director of the Center for Food Safety. "It's highly doubtful that it's just on one farm."