Assessment of Generic E. coli in Surface Irrigation Water Sources and Fruit in Selected Michigan Blueberry Farms

Introduction: Fresh produce that is consumed raw can be a source of microbial pathogens that can cause human illness.  This is particularly true for produce that is irrigated with untreated surface water at times close to harvest.  Michigan has a large number of blueberry farms, and many of these farms irrigate their crops using untreated water drawn from irrigation ponds.  

Purpose: The purpose of this research was to quantify generic E. coli levels in repeated samples obtained from irrigation water sources at six blueberry farms in Michigan. 

Methods: The farms for this research were selected based on their use of surface water as irrigation water sources, and on their susceptibility to runoff from their surroundings.  Samples of water and blueberries were collected from late June until mid-September in 2015.  Water samples were obtained twice weekly and blueberry samples once weekly during the course of the experiment. The Colilert and Quanti-Tray/2000 method was used for quantifying generic E. coli levels.  

Results: Among the water samples (n=111), 94% contained detectable generic E. coli, and 19% of samples had generic E. coli levels greater than 126 MPN/100 ml. The maximum detected level of generic E. coli was 2,742 MPN.  Among the blueberry samples assayed (n=68), only 4.4% tested positive for generic E. coli, with the maximum level detected being 4 MPN generic E. coli per 100 grams of blueberries. 

Significance: These results indicate a relatively high likelihood of detecting generic 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.