Malva, or cheeseweed, may play a role as a reservoir host in the spread of impatiens necrotic spot virus (INSV) in Salinas Valley lettuce fields, but how to deal with the disease remains a mystery.
That word came from Steve Koike, Monterey County farm advisor, in his year-end report on the virus before the California Leafy Greens Research Board meeting in Seaside.
INSV first was discovered in Valley fields in 2006, initially in romaine, where a few fields showed as much as 70 percent infection, which Koike termed “impressive” for any disease of the crop. The virus has long been known in greenhouse flowering plants along the coast.
Vectored by Western flower thrips, it is related to tomato spotted wilt virus (TSWV), which occurs in inland and coastal lettuce. Koike said, however, thus far INSV has not been detected in lettuce in inland counties.
Symptoms of the viruses are identical and can only be distinguished with laboratory or quick test immunostrip products. Infected leaves show tan to brown spots with dead areas. These signs were at first mistaken for herbicide damage. Yellowing and brown spotting on younger leaves and possible stunting of the plant are also symptoms.
INSV goes to all types of lettuce, and although it increased in 2007 and 2008, its incidence in 2009 was moderate to low.
“In the 2009 season,” Koike said, “it started late, and we started seeing a buildup in August and September instead of in late April and May in the two previous years.”
During 2009 he learned that the outbreak is not due to any new strain of the virus. It is the same that occurs on chrysanthemum, dahlia, and other ornamentals. He also confirmed that the sole vector on lettuce is Western flower thrips, the predominant thrips species drawn to lettuce.
The 2009 season was the second year of his investigation of weed hosts. Although a survey of 156 leaf samples from sites where INSV was heavy in 2008 revealed only one example of INSV-infected shepherdspurse, the 2009 survey was quite different.
“On one ranch with five or six hotspots of INSV, we took over 100 samples at three different times in late summer, and we found that 35 percent to 40 percent of the malva had INSV and about 8 percent to 10 percent of the shepherdspurse tested positive for it,” he said.
That being said, there’s another problem, he added. “There are no symptoms of INSV in infected malva. Even though you don’t want the weed anyway, we can’t pinpoint where to spray for control measures. The same thing is basically true about shepherdspurse.”
Although his project for the board is completed, Koike said he plans to continue to monitor INSV because he suspects it and TSWV will be a long-term concern for the lettuce industry. He and Richard Smith, Monterey County farm advisor, will be surveying INSV in the weed hosts this winter.
“Not only are there reports of INSV damaging crops worldwide, but it is showing up on new hosts, including lettuce, radicchio, basil, fava beans, celery, and spinach,” Koike said.
He asked growers and PCAs to be watchful of fields having INSV symptoms. With various host crops being grown in proximity, he warned that thrips will move from one infected host to another.
Collaborating in the INSV research were Robert Gilbertson, University of California plant pathologist, and Tom Turini, Fresno County farm advisor.
In another project for the board, Jim Correll, plant pathologist at the University of Arkansas, and Koike have been probing downy mildew in spinach, which has been prolific with new strains in the last 10 to 15 years.
The two have had a research partnership during the past 17 years, investigating races of downy mildew in spinach occurring in the U.S. as well as Europe.
When Correll began his research on the disease in 1989, races 1 and 3 were primarily in California, Arkansas, and Texas. Race 4 appeared in the early 1990s, and the spinach industry started voluntarily-funded research on it. By the late 1990s another two races had appeared, and the California spinach industry was funding research on downy mildew. Additional races continued to emerge.
By the time the California Leafy Greens Research Board was established in 2008, pathologists and breeders were working on what is thought to be a 12th race. Correll and Koike exchange germplasm with European counterparts and screen varieties for resistance to the pathogens.
Correll said the newest strain, designated UA2209, was recovered multiple times from several different cultivars from the Salinas area. An international group will meet in January of 2010 to decide if it will be officially termed race 12.
Strain UA2209 gained notice because it was able to overcome the resistance of newly-released cultivars bred to stave off races 1-11. Fortunately, Correll said, they were testing new germplasm having resistance to race 11 when the new strain appeared. “We immediately stopped our evaluations to examine what was happening with this new strain,” he said, adding that one of the traits of an older spinach variety, Califlay, seems to be holding up quite well to the new strain.
They are also working with breeding companies to learn more about separating components of resistance.
Correll speculated that the increase of races that have appeared in the last decade or so is due to dramatic changes in spinach production, including plant density, 12-month growing seasons, and the number of varieties putting selective pressure on the pathogen.
A contributing factor in the spread of the downy mildew, which goes only to spinach, could be the “green bridging” of older fields providing disease inoculum for new fields.
Correll said they are using laboratory techniques developed at UC Davis for research on downy mildew of lettuce to monitor the disease.
“We’ve also initiated some ‘fingerprinting’ tests to at least track isolates of the disease a little more clearly when they show up in different parts of the world,” he said.
These could possibly reveal whether the races originated from minor mutations or wholesale recombinations of genes in the pathogen.