University of California plant pathologists are trying some new methods for early detection of potential contamination of deadly E. coli O157:H7 in irrigation water used in lettuce production.
Trevor Suslow, a specialist in postharvest pathology at UC, Davis and leader of a team of investigators, told a recent meeting in Seaside of the California Lettuce Research Board (CLRB), that scientists are taking irrigation water samples from Salinas Valley fields.
He said the on-farm investigations of soil, lettuce plants, and irrigation water thus far have not found the harmful bacterium in samples collected from production blocks.
The samples are being used in evaluations of a novel, membrane-based method, which he said is superior to existing practices, to trap and identify pathogenic forms of E. coli.
Additional samples, including runoff water and surface water, will be gathered from other lettuce districts. Parallel research will seek ways to simultaneously determine fecal pollution and its source.
Recent investigations by federal and state agencies, Suslow reported, suggest that fecal contamination from domestic animals and wildlife, as well as human sources such as municipal waste or septic drain field effluent, “may contaminate shallow aquifers, surface water, irrigation reservoirs, or farmed land and crops due to seasonal flooding.”
Goals of the research are to re-examine the potential of irrigation water as a risk factor for E. coli O157:H7 contamination and find a practical way for the lettuce industry to monitor for that contamination.
Other researchers, Suslow said, have found low dosages of chlorine to be effective in reducing populations of the bacterium, depending on the water quality. Another mitigating technique is treatment of water with copper, but the treatment has been shown to lose effect over time.
Heat treatments of contaminated lettuce seed, at temperatures low enough to allow germination, have had little impact on the pathogen.
On other fronts, an experimental project is looking into how generic E. coli survives a seed coating process, while USDA-ARS scientists in Beltsville, Md. evaluate ways to detect E. coli on airborne particles.
In other approaches to management of E. coli, Steve Koike and Michael Cahn, Monterey County farm advisors, say their field trials with beds sprayed with nonpathogenic strains of E. coli showed no significant difference between drip and sprinkler irrigation with respect to persistence of the bacteria.
Before describing the trials, Koike pointed out that although the 2006 outbreak of E. coli O157:H7 was in spinach, the pathogen is also an issue for lettuce, so pathologists are looking at it in both crops.
The nonpathogenic strains, selected for natural resistance to rifampicin, an antibiotic used in the recovery and identification process for E. coli, were collected from central coast sources of soil, irrigation water, and lettuce.
“Our simulation of a one-time, high-level contamination event resulted in very short persistence” of the E. coli strains used in the trials, Koike said.
In one trial, which was not planted, eight days after inoculation the bacteria in the soil declined to or near the detectable level, and by 14 days none was detected. In the second trial, planted to romaine grown to harvestable size, none was detected six days after inoculation, either in the soil or plants.
Linda Harris, a food microbiologist at the Western Institute for Food Safety and Security at UC, Davis, said the thrust of investigations is focusing on how harmful E. coli bacteria behave in the environment.
The institute was established in 2002 as a joint program of the university, state and federal agencies, and the food industry. It includes the Produce Safety Center formed in the spring of 2007 to research E. coli.
“We have data on how the bacteria behave in the laboratory, but very few studies have been done in a natural setting,” she said.
At the present, she added, persistence of the pathogen remains an issue, “but there is nothing from a single experiment that shows anything that a grower should do differently.”
She said her work thus far confirms existing knowledge that although a rapid die-off of the pathogen occurs initially, it does persist.
Reporting on another concern in the 2007 growing season, Koike said impatiens necrotic spot virus (INSV) has shown up in Salinas Valley lettuce and the exact reasons for the outbreak are not known.
More familiar as a greenhouse disease of flowering ornamental plants, it was not known to go to lettuce until it was first identified in valley fields in September of 2006. It is vectored by thrips.
INSV infections have been found in fields in Castroville, Salinas, Chualar, Gonzales, Soledad, and Greenfield and confirmed on iceberg, romaine, and green leaf lettuce.
Of the some 20 fields confirmed with the disease this year, Koike said most had limited disease incidence of less than 5 percent.
“However,” he said, “in several fields disease incidence was 30 percent to 50 percent or higher, and significant crop loss was experienced. Because of this apparently new problem and the fact that affected plants are unmarketable, growers, PCAs, and field personnel should be aware of this disease.”
It is related to tomato spotted wilt virus (TSWV), a disease found in the Salinas Valley in previous years, but only a minor concern until now. Koike said the wide distribution of both this year “appears to be a novel development.”
Symptoms of INSV-infected plants are leaves with brown to dark-brown spots and dead areas, resembling burn damage by pesticides or fertilizer. Spotting and yellowing can be seen on newer leaves near the center of the plant’s growing point. Young plants infected with it may show overall stunted growth.
Koike, who hopes to investigate the disease with Robert Gilbertson, a UC, Davis plant pathologist, asked that anyone finding possible INSV cases submit samples to his diagnostic lab at the Cooperative Extension Office in Salinas.
W. K. Frankie Lam, entomology staff research associate for Monterey County, gave an account of his trials in managing aphids and thrips in romaine. In the aphid study in Salinas, he found the several insecticides used effectively managed the pests, which were mostly lettuce aphid, and no phytotoxicity to the plants was observed.
In his Greenfield trial on thrips including Assail and several experimental insecticides, he concluded that the products were effective against thrips for seven to 10 days after application.
However, he added, the plot was adjacent to an onion field and the insects moved continuously from the onions to the romaine, suggesting that to control high thrips populations in such a situation, weekly application of insecticide may be required.