There's always "the first"..
From Jiji Tsushin (10/7/2011):
Shizuoka Prefecture announced on October 7 that 1,033 becquerels/kg of radioactive cesium exceeding the provisional safety limit of 500 becquerels/kg was found in the dried shiitake mushrooms produced and processed in Izu City in Shizuoka. It was found by a voluntary testing by a retailer. The prefectural government plans to conduct its own testing of the remaining products kept at the producer on October 8.
According to the prefectural government, the shiitake mushrooms in question were harvested in Izu City in March and April, and processed in the city by the middle of April. The producer sold to 5 retailers in and out of Shizuoka. Some are considered to have been already consumed, but the retailers have voluntarily removed the mushrooms from their stores.
If the mushrooms were harvested in that time period, radioactive cesium must be from the fallout on the surface of the mushrooms. The Ministry of Education is yet to conduct the aerial survey of Shizuoka Prefecture, and the Hayakawa radiation contour map only shows the east side of the Izu Peninsula with some radiation. Izu City is located in the center of the peninsula.
(The map showing the location of Izu City is from this blog. Thank you.)