When severely yellowed and stunted lettuce plants showed up in Salinas Valley fields in September of 2006, many growers mistook the damage for spray burn, but laboratory testing showed the cause was impatiens necrotic spot virus (INSV) and opened a new mystery for plant pathologists.
In the two seasons since, INSV has continued to spread in the Valley at an alarming rate, according to Steve Koike, Monterey County farm advisor, who is investigating it.
Well-known to coastal greenhouse producers of flowers and ornamentals, INSV, one of the tospoviruses, was not previously a problem in lettuce. Symptoms on lettuce include yellow, tan, and brown spotting on leaves, with stunting and necrosis.
Reporting at the recent meeting of the California Leafy Greens Research Board at Seaside, Koike said, “In the coastal area, we have INSV infecting all types of lettuce. There are still a few cases of tomato spotted wilt virus, which shows identical symptoms on lettuce, but they are insignificant.”
Tomato spotted wilt virus occurs in lettuce on the West Side of the San Joaquin Valley, but monitoring there thus far has not revealed INSV.
In his project for the board, Koike has been looking into the nature of the virus, its source, how it moves around, and its hosts other than lettuce.
After molecular testing of samples at the University of California, Davis, Koike said, “we now know the INSV from lettuce is not genetically different from what been found in commercial chrysanthemum or landscaping crops, and it's not a novel strain.
“We feel the INSV outbreaks may be driven by high populations of thrips vectors. They have been the cause of other recent tospovirus outbreaks elsewhere in the world.”
Screening infected plant samples from fields for thrips, he found that more than 95 percent were Western flower thrips, the common vector of viruses in vegetables. He noted that adult thrips do not carry the virus, but the juvenile phase does. Massive counts suggest that the disease has ample potential for being spread by these vectors.
While there may be a possibility of a new biotype in the thrips, Koike said there is no literature to indicate such.
A broad survey of weed populations in the Salinas Valley during the past two seasons for a source of INSV is inconclusive.
“We did not find a lot of weeds with the virus,” Koike said. “A few species, hairy nightshade, shepherds purse, London rocket, and annual sowthistle, have occasionally tested positive, but most have been negative. So we don't have an obvious source, based on this survey.”
He did offer one consideration about alyssum, often planted to attract beneficials in organic lettuce. Although it is not a host for the virus, alyssum interplanted in romaine showed high counts of thrips, averaging two adults and two juveniles for each tiny flower cluster and making it a prime reservoir for virus-vectoring thrips.
INSV has been found in Salinas Valley fava bean and pepper, not previously known to be hosts, and this may be a subject for additional research.
Ice plant around lettuce fields was also found to have tremendous populations of thrips.
Koike said “quick test” strips are available for rapid field testing of certain levels of viruses when symptoms suggest an infection.
Koike also detailed progress in another project focused on learning how generic and attenuated strains of E. coli Ol57 survive in soil and water. These forms are not the primary pathogen and do not have the genes that produce highly dangerous toxins.
The strains, resistant to an antibiotic to enable easy identification and recovery of them, were collected from coastal soil, water, and plant tissue. Growing romaine in test plots was inoculated by spraying on the pathogens.
In a departure from the many other laboratory studies of E. coli, the project deals with field conditions, including differences in fertilizer and moisture levels.
Using different combinations of strains and amounts of fertilizer, Koike and a team of other scientists found that the strains could be recovered from small, sprinkler-irrigated test plots for only a short period of up to 14 days, with the samples from soil having the longest persistence. This confirmed the findings of the trial done in 2007.
From a larger plot of three beds 145 feet long and split between sprinklers and drip irrigation, data on water were being analyzed at the time of his report.
Koike said no antibiotic-resistant E. coli was recovered from lettuce plants, with roots, soil around roots, and leaves all testing negative. Resistant E. coli was recovered from runoff water from sprinkler plots up to 39 days after inoculation. Both generic and attenuated types declined to the same end point at 39 days.
“This indicates, in a preliminary way, that we can use these strains to get some idea of what O157 is doing in the field,” he said.
The research also has value in mapping and accumulation of field data related to contamination events for development of a rapid response program for E. coli and Salmonella. It will hopefully yield information on pathogen distribution, sources, and other pertinent factors, Koike said.
In a report to the board on his project, Steve Fennimore, UC Cooperative Extension weed specialist at Salinas, said his career goal is to get as close as possible to a hand-weeding-free system for leafy vegetables.
He described his unsuccessful attempt to find a way to use drip irrigation on seeded lettuce the entire season instead of the expensive use of sprinklers first to activate herbicides. Richard Smith, Monterey County farm advisor, collaborated in the study.
For example, Kerb is now applied to a bed top and sprinkler irrigated. The water moves the herbicide in a layer though the top few millimeters of the soil and activates it where the weed seeds are.
Trials this year included Kerb, Prowl H2O, and a tank-mix of both applied via surface and buried drip, as well as Kerb through sprinklers.
Sowthistle, a relative of lettuce, stands up to Kerb at lower rates, and Prowl, while it is quite active against the weed, is too injurious, with unacceptable yield loss, to young lettuce seedlings.
However, he said, a new label for use of Prowl on lettuce transplants is coming. “So we know now that we will only be able to it on transplants.”
As for Kerb, it will still have to be activated by sprinkler, or applied at higher rates. “Those are the best options, and you'll have to look at the economics and make your decision,” said Fennimore.
In an effort to retain existing herbicides rather than wait for unlikely new products, Fennimore has been conducting trials on herbicide-tolerant lettuce germplasm.
Working with UC and USDA geneticists, he screened 350 mutant lines for glyphosate (Roundup) resistance and found 12 surviving plants. Survivors of further crosses are in screening. Only classical breeding methods are being used throughout the process.
Fennimore said he wants to find herbicides for spinach, which cannot be cultivated and is dependent on fumigants for weed control.