The last thing the California lettuce industry needs is another disease, but a new one, impatiens necrotic spot virus (INSV), turned up in Salinas Valley fields in 2007.
Steve Koike, Monterey County farm advisor, described the disease during the annual meeting of the California Lettuce Research Board near Coalinga.
INSV, he said, has symptoms identical to tomato spotted wilt virus (TSWV). These include multiple, brown necrotic spots on leaves and petioles, resembling pesticide or fertilizer burn. Margins of infected leaves become wilted and yellowed, and the discoloration is often seen on new leaves near the center of the plant's growing point.
The disease, which makes heads of infected plants unmarketable, has been confirmed on iceberg, romaine, and greenleaf lettuces. It has been found in fields around Castroville, Chualar, Gonzales, Greenfield, Salinas, and Soledad.
Of the 20 or so fields confirmed with INSV last year, most had infection of less than 5 percent, although several showed 30 percent to 50 percent infection and significant crop loss.
“INSV,” Koike explained, “is a tospovirus and, like the more commonly known TSWV, it is vectored by thrips. Typically, INSV is known to be more of a greenhouse disease concern on flowering ornamental plants. The infection of lettuce, therefore, is an apparent departure from this pattern. The exact reasons behind the INSV outbreak on lettuce are not known.”
Of note is the previous impression of plant pathologists that INSV, a strain of TSWV, did not go to lettuce.
Koike said there has also been an increase in the incidence of TSWV on lettuce in the Salinas Valley in the last two to three years. It was identified there earlier, but was not considered a significant problem. It was also found last fall in several lettuce fields near Huron by Tom Turini, Fresno County farm advisor.
“INSV is very difficult to manage, and if it gets out of hand, you notice it right away,” Koike said.
He asked growers and PCAs to advise him of possible infected fields and bring in samples of any diseased plants having the symptoms to him for diagnosis at the Monterey County Cooperative Extension Office in Salinas.
Investigations of both strains this year are proposed. Working with Turini and Bob Gilbertson, University of California, Davis plant pathologist, Koike hopes to learn the source of INSV and devise management strategies for it, as well as to monitor developments of TSWV on lettuce in various parts of the state.
At the moment, UC recommendations for management are to avoid planting next to infected fields, control of thrips vectors, and crop rotation with nonsusceptible crops such as small grains.
Koike also reported on findings by his team of researchers probing how the food-borne pathogen E. coli survives in a simulated contamination situation under different irrigation systems.
The results showed it has a persistence of only a few days. Although water and environmental conditions likely influence bacterial survival, the trials did not find any difference between sprinkled plots at Hartnell College in Salinas and a drip system on a commercial field in the district.
“There are thousands of research projects on this organism, but we are interested in conducting practical field studies in a real situation with agriculture,” he said.
The project is developing information on how nonpathogenic, or generic, E. coli, which is not harmful to humans, persists under coastal California farming conditions in comparison with the deadly form, E. coli O157:H7, in laboratory studies.
The safe form used in the study can be identified under ultraviolet light.
Environmental data developed with the generic form can be the basis, he said, for gaining grower, handler, agency, and public acceptance of future controlled-application field trials with the reduced-risk isolates of E. coli O157:H7.
Applied research in the field is also needed so that industry and regulators can make informed decisions on growing practices and regulatory food safety policies for the field, he added.
In another E. coli team project, Carol D'lima, a postdoctoral researcher at UC, Davis, told the board of observations of how microorganisms travel through the air from livestock or composting operations to crops.
She noted that requirements are that lettuce and leafy green growers keep a 30 to 400 foot distance between fields and any livestock, compost facility, or other source of fecal organic matter likely to be contaminated.
“The foundation assumptions and appropriateness of these distances have not been adequately tested in environments pertinent to production of these commodities in California,” states the team's report.
The study is being done with bioaerosol assessments during mixing and turning at compost piles, and manure handling and movement of vehicles and animals at livestock facilities.
Three different types of air sampling devices are being used to collect airborne particles from locations downwind at various distances from 30 to 400 feet from the composting or livestock facility.
The board also heard a report from Jorge M. Fonseca, a postharvest specialist at the University of Arizona, Yuma. Fonseca leads a board-funded study there charting the level of risk to lettuce crops of animal fecal contamination (cow, dog, and pigeon) that could be carried by splashing from sprinkler irrigation.
“Our results as of February 2008 indicate that lettuce maturity and wind speed and direction are the key factors affecting the potential area of contamination from animal feces,” he said.
The maximum distance traveled by microbial indicators or fecal particles was up to three times longer when the field was irrigated on a day when wind speed was greater than 19 miles per hour than when the wind speed was less than 13 miles per hour.
“During early stages of the plant growth, fecal particles and bacteria traveled farther than when the lettuce had reached full maturity, regardless of low or high wind speeds,” he added.
The study also indicated that the survival of harmful E. coli depends on relative humidity and temperature and type of excrement. Its persistence in dog feces was considerably lower than in cow and pigeon feces. Salmonella and E. coli populations remained constant in cow feces for over 40 and 80 days, respectively.
“Current field trials,” Fonseca said, “aim to collect more data using other animal feces, and lab research aims to determine the survival of Salmonella in dog and pigeon feces, along with analyzing the effects of fluctuations in temperature and relative humidity conditions.”