A survey of winter weeds on California’s Central Coast early this year again showed impatiens necrotic spot virus (INSV), a relatively new pest of lettuce, in malva, suggesting the ubiquitous weed may be a reservoir for the disease between lettuce seasons.
Steve Koike, Monterey County farm advisor, told of his survey and other findings during the annual meeting of the California Leafy Greens Research Board (CLGB) near Coalinga.
He added that malva, or cheeseweed, named a host in previous seasons’ weed surveys also, shows no symptoms of INSV when infected, precluding use of spot treatments of the weed as a way to manage the disease.
One of the tospovirus group, INSV was long known on the coast as a disease of flowering ornamental crops in greenhouses before it was confirmed on coastal lettuce fields for the first time in 2006. It goes to all types of lettuce.
Koike said its degree of incidence and severity has increased since, with the exception of moderate to low levels found in 2009. It has not been identified on lettuce in inland valleys.
Symptoms of INSV include leaves having tan- to brown-colored spots and dead areas, and it was mistaken for herbicide damage when it was first detected in lettuce fields. If plants are infected early, overall growth can be stunted.
Those symptoms are identical to tomato spotted wilt virus, which occurs in both coastal and inland lettuce and other vegetables. Precise identification requires laboratory testing or use of quick test immunostrips in the field.
“I don’t want to overstate the situation,” Koike said, “We’ve had from 1 percent to 10 percent losses from INSV routinely in the Salinas Valley, but in some fields we’ve seen up to 75 percent loss.
“It was different in 2009 in that disease incidence was much lower and less devastating, and it came in pretty late in the season, or after May when it typically appeared in past years.”
Koike said molecular analyses have demonstrated the Salinas Valley infections are not a new strain of the virus, since they are the same as samples from greenhouses and landscape ornamentals. During the past two summers he has done surveys on the virus’ principal vector, western flower thrips, and determined that species of thrips are the only known carrier.
He also observed that thrips populations do not seem to be different between infected and healthy plants. However, he reported lower thrips counts during 2009 and said they may be “a barometer” for infection.
His work during the past three years included “focused” surveys of weed hosts concentrated on areas where infections were found. A number of species, including hairy nightshade, shepherds purse, London rocket, fillaree, pineapple weed, annual sowthistle and malva, occasionally tested positive for INSV until the summer of 2009, when significant numbers of infected malva turned up. That was true again during the past winter’s survey, and Koike said malva may be a reservoir for the virus.
“And as far as we can tell, malva is symptomless, so you can’t tell which malva has the virus and which does not,” Koike said.
The surveys associated some vineyard areas with outbreaks of INSV. He said the grapes themselves are not a host of the virus, but the malva growing in and around vineyards is.
According to Koike, INSV is occurring in crops around the world where it was not previously seen, and he is sharing his research with plant pathologists elsewhere. Other crops recently found to attract the virus are fava bean, radicchio, basil, pepper, celery, and spinach.
Sweet alyssum, frequently used as an insectary species to attract beneficials in lettuce fields, is not considered a host of the virus, but is a host of the thrips vectors, he added.
Bacterial leaf spot (BLS) is a longtime disease of lettuce on the coast, and after its initial discovery there in the mid-1960s it developed into an economic pest in the 1990s. Sporadic in intensity and most prevalent in years of cool and wet conditions, it caused significant damage in 2009.
Carolee Bull, a USDA-ARS plant pathologist at Salinas, is leader of a team of scientists working on a CLGRB-funded project screening resistant cultivars in the field and in the greenhouse as a defense against BLS.
The current emphasis on BLS research has responded to the sporadic nature of the disease and its importance to the industry from year to year.
The team’s findings are used by USDA plant breeders for germplasm that can be passed on to commercial seed companies for development of resistant varieties. Among their studies in 2009 were 36 baby leaf lettuce types that are most susceptible and therefore would benefit most from resistance.
“We have found strains that will be very useful, and we have just started to look at how resistance functions, whether it affects bacterial populations or there are other mechanisms involved,” Bull said. The investigation points to BLS resistance as appropriate for the highly susceptible red leaf and red romaine.
“The remaining types, lolla verde, red oak, green oak, green leaf, and lolla rossa, were more variable and included resistant and susceptible individuals. Cultivar selections within these types will be more crucial when growing baby leaf lettuce in bacterial leaf spot prime environments,” she said.
Steve Fennimore, University of California, Davis weed specialist stationed at Salinas, led a group in a board project evaluating the herbicide Lorox (linuron) for crop tolerance and weed control in spinach.
“As people involved with spinach know, the herbicide situation is dire, if not downright frightening,” said Fennimore. He did trials in 2009 with Lorox, Ro-Neet, and other products to evaluate reports from the University of Maryland that Lorox is fairly safe on spinach.
“We found out that Lorox is not safe enough, however,” he said, explaining that the material at greater than the 0.2 pound rate was injurious to the crop, but 0.4 pound was needed for adequate weed control.
“So now we are looking for natural tolerance like spinach has for Ro-Neet or lettuce has for Kerb.”
The second objective of his research is to breed spinach for herbicide tolerance, just like breeding is done for protection from diseases or pests. They screened 390 germplasm lines of spinach for resistance to Lorox, applied at the 1-pound rate, in a field trial.
Surviving plants were collected, and the team found that three lines had more than 10 survivors, 12 lines had more than seven, and 22 lines had more than five. Those were removed from the field for growing out for seed in the greenhouse.
That seed will be harvested and planted for additional screening with applications of Lorox and other herbicides having a similar mode of action.