The bacterium Escherichia coli is all around us, in both harmful and harmless forms, and no satisfactory way has been found to predict an outbreak of the deadly E. coli O157:H7 that contaminated spinach this past summer near San Juan Bautista, Calif.
That's the word from Trevor Suslow, Extension specialist in postharvest pathology at the University of California, Davis, who described efforts to manage E. coli O157:H7 during a recent plant disease seminar at Salinas.
Suslow, who has extensively researched E. coli and other pathogens in cantaloupes, has long urged good agricultural practices to avoid microbial or other contamination. One practice is keeping livestock and manure piles away from cropland and sources of water used in irrigation and packing.
His and other researchers' findings are being considered as the produce industry searches for ways to keep pathogens from spinach and other crops.
Suslow is preparing a water-quality action threshold plan to survey amounts of E. coli in surface water supplies. It is based on three years of research he did for the California Lettuce Research Board.
As part of the project, between 2003 and 2006 he analyzed more than 500 water samples from reservoirs in various locations in the Salinas Valley and elsewhere in the state.
Most generic E. coli bacteria are non-infectious and some may even be beneficial, but Suslow said those related to fecal contamination can be harmful to both animals and humans. “These may or may not be captured in the same tests used to identify non-pathogenic E. coli,” Suslow said.
Citing previous outbreaks of O157:H7 in Missouri, Wyoming, New York, and Ontario, Canada, Suslow said they were all associated with contaminated well water.
But using water-quality indicators has limitations, he cautioned. “There is no single indicator that meets all the criteria we need for predictability, practicality, or cost effectiveness.” Sophisticated tests are necessary to analyze samples properly, and commercial laboratories should be consulted in any program.
His draft, aimed at establishing baselines for future reference, sets an action threshold, based on bimonthly testing for at least one season, of either 1,000 fecal coliform or 126 generic E. coli per 100 milliliters in any single water sample. The number 126 is used because it is the value used in EPA standards for recreational water quality.
Steps to be taken if the threshold is exceeded include temporarily increasing the testing frequency to five times every 30 days, attempting to determine the source of the change, seeking professional advice, retaining all test results, and documenting all decisions. If the high counts continue, the water source can be disinfected or shut down.
Suslow's research in lettuce underscored the uncertainties in testing irrigation water for the pathogens and using the results to predict what might be taken up by the crop.
A particular reservoir was chosen for sampling because it had a history of higher counts of non-pathogenic E. coli. After a series of irrigations, he said, one might assume that abundant E. coli would be found in the crop.
“But that did not materialize in anything we've done,” he added. Of 40 plant samples from a furrow-irrigated field in 2005 only one plant had a comparatively high population of non-pathogenic E. coli, although E. coli was in the irrigation water and the soil.
Results like these, he added, demonstrate the challenges of how to sample, what to look for in the sampling, and how to interpret data.
The O157:H7 contaminating fresh spinach this past summer in California carried Subtype 0124, identified in 1998 as the source of a highly virulent toxin.
Cattle are the main reservoir for O157:H7, but it causes no visible clinical illness in them. In humans, however, it causes bloody diarrhea and potential life-threatening complications. Horses, goats and other domestic animals can also be reservoirs, and vectors can be humans, or even slugs, nematodes and Canada geese.
The pathogen can survive even after being frozen, and scientists say it may not be activated until it is in the digestive tract of a host.
E. coli that gets into groundwater supplies can show up in wells days, years, or centuries later, depending on the soil strata and location of aquifers.
Another speaker at the plant disease seminar, Steve Koike, Monterey County farm advisor, said Salinas Valley growers again coped in 2006 with spinach downy mildew, which first appeared in 2005 with increased acreage of the crop. The infections have been Race 10, which also occurs in desert fields, and no new race has been identified.
Lettuce downy mildew has broken out in ornamentals such as strawflower, and a new Fusarium disease appeared with considerable damage in some cilantro fields, he said.
Tomato spotted wilt virus, a thrips-vectored disease common to peppers, has shown up on lettuce, where it has been rarely seen in the past until reports of it increased in the past two to three years, and on radicchio.
The wilt infections are dark brown, dry patches that may resemble phytotoxicity damage on foliage. It is difficult to diagnose in the field and, like other viruses, often requires lab tests to identify.
Koike said the diagnostic lab at the Monterey County Extension office can identify diseases from samples brought in by growers and PCAs. The lab handles some 800 plant disease identifications in crops and landscaping each year from Monterey County and elsewhere in the state.
Koike is senior author with Albert O. Paulus of the University of California, Riverside and Peter Gladders, a plant pathologist at Cambridge, United Kingdom, of a new book entitled Vegetable Diseases, A Color Handbook.
The 448-page, hardbound publication, now available from Academic Press, covers major vegetable disorders, their development, and recommended management of them, and has more than 600 color photographs. It can be ordered online at http://books.elsevier.com.