As verticillium wilt continues to spread in lettuce fields of California's Salinas Valley, plant pathologists and plant breeders are continuing their search for resistant varieties.
According to Krishna Subbarao, University of California plant pathologist at Salinas, lettuce wilt, caused by the fungus Verticillium dahliae, was identified on another Salinas ranch this year.
At the recent meeting of the California Lettuce Research Board in Seaside, Subbarao said nearly all the newly infected fields of that ranch had been sequentially planted to lettuce. Incidence of the wilt was less than 30 percent in all the fields, suggesting that it was recently introduced.
The soilborne disease, confined mainly in clusters around Salinas and Watsonville during the past decade, turned up in eight new locations during 2006, including one near Salinas for the first time on the west side of Highway 101 and one near King City.
“Although the disease was limited to California and the island of Crete until last year, recent reports from Greece and Italy suggest it is becoming more common on lettuce in other parts of the world,” Subbarao said.
More evidence of the spread of the wilt on the Central Coast and a potential threat to lettuce in the Santa Maria area was the extensive damage during the summer of 2007 to French marigold, a relative of lettuce, grown for seed near Lompoc.
Spinach-infesting strains of wilt have been found with incidence of up to 40 percent around Salinas, but spinach is harvested before the disease is expressed on the crop.
The implications for Salinas Valley lettuce from planting wilt-infested spinach seed remain unknown, but, Subbarao said, if strains from other crops also infect lettuce, the wilt could spread to other areas in the future.
The disease has been managed by fumigation, but it returns after a typical rotation cycle of two crops of lettuce and one of strawberries, requiring fumigation every other year.
Subbarao is currently testing soil samples from seed production fields provided by seed companies for the disease and plans to report results later to the board.
By charting the distribution of the disease's fruiting bodies, he has learned that edges of an infected field have the highest concentrations of the wilt.
Following fumigation, the spread of disease is linked to its initial distribution in a field and potentially on the reintroduction of inoculum from contaminated farm equipment.
Meanwhile, Ryan Hayes, USDA plant breeder continues work in Salinas to find lettuce germplasm with resistance to both Race I and Race II of the disease.
Among recent greenhouse selections he discovered ten examples having significantly lower disease incidence than the varieties Salinas 88 and La Brillante, and additional testing is underway.
In addition to mapping of distribution of the two races, Subbarao is testing Actigard, a plant defense elicitor compound that may have potential for control of the wilt in lettuce. He expects to disclose results of his greenhouse trials early in 2008.
Reporting on his work on another disease of concern on the Central Coast, lettuce drop caused by Sclerotinia species, Subbarao had an answer to concerns of potential resistance of S. minor to the fungicide Endura.
“In all likelihood,” he said, “the recent control failures are a result of rapid degradation of Endura in soils of fields where the fungicide has been applied repeatedly.
The best course of action is to discontinue using Endura in fields where control failures have been observed.”
His team efforts with Hayes' breeding work are centered on improved resistance in romaine and iceberg using plants having “slow-dying” resistance to S. minor and S. sclerotiorum. Although the trait has been known for years, this the first use of it in a breeding program.
Not completely resistant, plants with the slow-drying trait succumb to the disease in 10 to 15 days, but susceptible plants may survive infection by as little as two days.
Expanding his ongoing observations of Contans, a fungal biological material, Subbarao said he wants to investigate the use of Endura during the growing season, followed by treatment of the crop residue with Contans before incorporation into the soil.
In a project to learn more about the use of syrphid flies as a biological control of lettuce aphids, Erik Nelson, a postdoctoral researcher at the University of California, Berkeley, is working with various plants as habitats for syrphids.
Nelson said many organic lettuce growers plant strips of nectar-producing plants to attract and feed adult syrphids, but his goal was to try another strategy to support larval stages of syrphids.
This would be done by providing the most attractive host plants for non-pest aphids early in the crop cycle to sustain the syrphid population, whose larvae would be more abundant to feed on pest aphids when they appear.
In his field plots at Watsonville during the summer of 2007, Nelson screened Shasta daisy, common vetch, cilantro, spring barley, winter barley, sweet alyssum, bell beans, evening primrose, and Persian clover.
“Far and away, we found the greatest number of aphids on bell beans, and counts on winter barley were also high,” Nelson said. He found Shasta daisy, evening primrose, and cilantro had little potential, and the remaining species gave intermediate performance.
He said he hopes to plant the most promising candidates in larger trials to evaluate their effects on syrphids in lettuce fields.
Carolee Bull, USDA plant pathologist at Salinas, is probing the reason for a severe outbreak of corky root on a resistant lettuce variety in Watsonville in the fall of 2006.
Initially, using soil autoclaved to kill soil microorganisms and inoculated with a strain of the Sphingomonas suberfaciens pathogen generally blamed for corky root, she found resistant varieties held up against the disease.
However, in repeated tests with soil taken from the Watsonville site, both resistant and susceptible commercial varieties became infected.
Earlier research for the CLRB by Ariena Van Bruggen, UC, Davis plant pathologist, identified some 70 strains, among four Sphingomonas species, that cause corky root.
“This is a very unusual situation, where we may be looking at four or five groups among different species that all cause corky root,” Bull said.
Using 44 strains isolated from diseased roots of resistant cultivars stricken at Watsonville, Bull intends to design new methods to learn which strains attack each of those cultivars. Once that is accomplished, steps can be taken toward developing management strategies.