The E. coli outbreak that has caused at least 7 illnesses in Michigan (1), Minnesota (3), and Wisconsin (3) invovles hazelnuts likely grown and harvested in Oregon.  Jeremy Olseon at the Star Tribune spoke with Minnesota Department of Agriculture official Ben Miller about the outbreak: 

The three Minnesota infections involved men over age 50 from Hennepin, Stearns and Redwood counties. Two were hospitalized; all three have recovered.

Hazelnuts grow on trees, but they are harvested after they fall to the ground, where the contamination likely occurred, according to Ben Miller of the state Department of Agriculture’s dairy and food inspection division.

“The fact that they do spend some time on the ground increases the risk of environmental contamination,” he said. The investigation is now focusing on farms in Oregon that produce the majority of U.S. hazelnuts.

Hazelnuts mature during the summer months (turning in color from green to hazel) and are harvested in late summer and early autumn.  It is this time of year when the nuts fall to the ground. 

According to Wikipedia, in the United States, hazelnut production is concentrated in two states, Oregon and Washington, while they are also grown extensively just to the north, in the Fraser Valley of British Columbia, Canada. In 1996, the in-shell production in Oregon was about 19,900 tons (18,000 tonnes) compared to 100 tons (91 tonnes) in Washington