Lower salinity in Salinas irrigation water is the main cause of the difference, said Mark Ibekwe, a microbiologist with the U.S. Department of Agriculture’s Agricultural Research Service in Riverside, Calif.
Ibekwe and three colleagues published their findings, “Persistence of Escherichia coli O157:H7 in Major Leafy Green Producing Soils,” in the journal Environmental Science & Technology in October.
The results were based on laboratory-tested soil samples. Field studies of E. coli are typically limited to nonpathogenic varieties.
Increasing salinity in Salinas water would not be realistic or beneficial for leafy greens growers there, Ibekwe said. Nevertheless, the research underscores the importance of keeping new pathogens from entering the fields.
“You don’t want to introduce another variable into the farming environment that will ultimately cause adverse effects on the crops and result in lower yield,” Ibekwe said. “Because of how salinity will react with other factors there, we are not suggesting that.
“What we’re saying is that because we know there’s a longer survival in the Salinas area, we should be very, very careful in introducing pathogens from manure, poorly composted materials or any source at all into the farming environment,” he said.
Imperial and Yuma irrigation water has higher salinity because it’s drawn from rivers, Ibekwe said, whereas Salinas depends on groundwater.
To a lesser extent, the research also links longer survival to higher levels of organic carbon in Salinas soils. Total nitrogen is another contributor, the scientists said, but that shed little light in this case because the three regions have similar levels.
These factors add to a list of others previously identified such as temperature, pH level and moisture content.
The research was funded by the USDA’s National Institute of Food and Agriculture and the Agricultural Research Service.
It’s not the only E. coli research tied to the Salinas Valley.
In 2011, Steven Koike, plant pathology farm advisor at University of California Cooperative Extension, Monterey County, began a two-year study of survival on lettuce residue plowed back into soil. That project was funded for $118,000 by the Center for Produce Safety.