Fresh produce that is consumed raw can be a source of pathogens that cause human illness. This is particularly true for produce that is irrigated with untreated surface water at times close to harvest. Michigan has many blueberry farms, and these farms often irrigate their crops using untreated water drawn from irrigation ponds. This research was conducted to quantify E. coli levels in irrigation water sources at six blueberry farms in Michigan, and associate these levels with factors such as, rainfall, pH and conductivity of the water, and susceptibility of the water sources to runoff. Samples of water and blueberries were collected during summer 2015. Water samples were obtained twice weekly and blueberry samples once weekly. The Colilert Quanti-Tray/2000 method was used to quantify E. coli. Among the water samples (n=111), 95% contained detectable E. coli, and 17% of samples had E. coli levels greater than 126 MPN/100 ml, - the maximum geometric mean (GM) of E. coli level allowed in agricultural water under the FDA Produce Safety Regulation. The maximum detected level of E. coli was 4,611 MPN. Among the blueberry samples assayed (n=68), only 4.4% tested positive for E. coli, with the maximum level detected being 12 MPN E. coli per 100 grams of blueberries. These results indicate a high likelihood of detecting E. coli in surface irrigation water sources used on selected Michigan blueberry farms, and will be useful in establishing initial water quality profiles for these farms as required under the new FDA Produce Safety Regulation.
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