As far as I've checked, the report has never been "proven", but never been "disproven" either.
Also, keep in mind the warning of Professor Toshiso Kosako, that there will be "a chaos" in the fall harvest season.
From the report that appeared on Japan Business Press on May 14 (the report was from their free subscription part of the site, written by Satoshi Kawashima, the former editor of Nikkei Business and the founder of Japan Business Press):
また、ある食品メーカーが独自に調査した結果では、福島第一原発から50キロ以上離れた水田の土から、政府が発表している数値よりケタ違いに高い放射線が検出されたという。
According to the private survey by a food company, an extremely high level of radiation, order of magnitude higher than what the government has been reporting, has been detected in the soil in the rice paddy more than 50 kilometers away from Fukushima I Nuclear Power Plant.
この食品メーカーによると、現時点でその結果を公表するのは影響が大きすぎるため発表は控えているとのことだが、その田んぼの土からは高い濃度のプルトニウムも検出されたそうだ。
The food company says it is withholding the information for now because it may cause panic, but it says the survey has also found a high level of plutonium in the same rice paddy soil.
There has been no follow-up to this report. Back in May, almost all tweets on the subject in Japan were from people who outright dismissed the report as "baseless rumor" and criticized Japan Business Press for fear-mongering. "There's no way plutonium can travel outside the Fukushima plant!" was one typical tweet. (Never mind it did.) "They are manufacturing the story" was another.
Ah. Trust your government.