Wild mushroom harvested in a town in Fukushima tested highest ever radioactive cesium so far in food after the Fukushima I Nuke Plant accident that I'm aware of: 28,000 becquerels per kilogram.
According to the data from Fukushima Prefecture, 13,000 becquerels/kg of cesium-134, and 15,000 becquerels/kg of cesium-137 were detected from the mushroom.
The town, Tanakura-machi, is located at about 73 kilometers southwest of Fukushima I Nuclear Power Plant.
From Yomiuri Shinbun (9/3/2011):
福島県は３日、同県棚倉町の山林で採取した野生のチチタケから、国の暫定規制値（１キロ・グラム当たり５００ベクレル）を大幅に上回る２万８０００ベクレルの放射性セシウムが検出されたと発表した。
Fukushima Prefecture announced on September 3 that 28,000 becquerels/kg of radioactive cesium was detected from wild mushroom, Lactarius volemus, harvested in the mountains in Tanakura-machi. The level of radioactive cesium vastly exceeds the national provisional safety limit of 500 becquerels/kg.
チチタケはシーズン初めのため、市場に流通しているものはないが、県は同日、同町や販売業者に、チチタケを含む野生キノコ（菌根菌類）の摂取と出荷を自粛するよう通知した。
It is still early in the season for this particular mushroom and it is not sold in the market. The prefectural government notified the town and the distributors to voluntarily halt harvesting and shipping of wild mushrooms including Lactarius volemus.
県によると、このチチタケは１日に採取された。県は今後、付近の野生キノコの放射性物質検査を行うとともに、摂取自粛を呼びかける看板の設置などを行う。
According to the prefectural government, the mushroom was harvested on September 1. The government is going to test the mushrooms nearby for radioactive materials, and put up signs calling for voluntary halt on harvesting.
同県では、これまで野生のキノコで暫定規制値を超えたのは、同県古殿町のチチタケの３２００ベクレルが最高値で、県は「ここだけ突然高い値が出たので困惑している。調査を続けて原因を調べたい」としている。
In Fukushima Prefecture, 3,200 becquerels/kg of radioactive cesium has been detected from Lactarius volemus harvested in Furudono-machi. The prefectural government says, "We are surprised at the extremely high number. We will continue to investigate and identify the cause".