Closer scrutiny during 2008 of the populations of Verticillium wilt show it mixed in lettuce and spinach, shedding light on the outbreak of the disease in lettuce in the Salinas Valley.
Krishna Subbarao, University of California, Davis plant pathologist stationed at Salinas, gave an update on his work with the disease during the annual meeting of the California Leafy Greens Research Board held near Coalinga.
Although the wilt pathogen, Verticillium dahliae, has been known in the Salinas Valley since 1995, it was found in a record 13 additional fields during 2008, bringing the total to 71 infected fields, comprising more than 1,500 acres, Subbarao said.
It is distributed from Watsonville to King City with concentrations around Salinas and Watsonville. In the Salinas area at least two lettuce fields infected with it were abandoned in 2008. Incidence of wilt in the other fields ranged from 20 percent to 60 percent.
A member Subbarao’s team of investigators, Zahi K. Atallah, a post-doctoral researcher at UC Davis, said they confirmed last year after DNA testing that Verticillium populations infecting lettuce and spinach are “one happy family,” virtually identical, mixed together, and appearing at the same sites.
The big question among growers and others about Verticillium wilt on lettuce in the Salinas Valley has been what caused it to appear there for the first time in 1995.
“It is not by chance that the Verticillium emerged in lettuce in 1995,” Atallah said, considering the expanded acreage of spinach since about that time.
It is likely, he added, that successive crops of lettuce, spinach, or other hosts contributed disease inoculum which, once it accumulated to high enough levels in the soil, ignited the infections in lettuce.
“It’s time,” Atallah said, “that we as growers, researchers, and seed companies started working together for a solution rather than blaming one another.”
Tests during 2008 also concluded that the Verticillium occurring in tomato in the San Joaquin Valley is separate and not a source of the infections in lettuce and spinach in the Salinas Valley.
Subbarao said additional studies are planned to look into connections between lettuce and the other hosts of the wilt.
In 2007, his trials revealed that the wilt on spinach goes to lettuce and wilt on lettuce goes to spinach.
The disease also cross-infects those two crops with several species of marigold, including French marigold grown for seed on the coast.
The wilt has two races — with Race 1 being the most common in coastal lettuce. Race 1 is pathogenic to the variety Salinas, while Race 2 is pathogenic to both Salinas and La Brillante.
Subbarao’s group has also developed a laboratory test to identify Race 1 from a variety of hosts.
The pathogen is internally and externally seed borne on lettuce. Infected seed are able to germinate normally and produce a plant, with the fungus growing along with the plant to infect the resulting seed.
During 2008, Subbarao’s group concentrated on tests of seed lots of currently available commercial lettuce cultivars for V. dahliae infestation. Nine companies submitted 171 seed lots for the evaluations.
The seed lots came mainly from the U.S., China, Chile, Australia, and the Netherlands. None of the lots from Australia and the Netherlands were infested, but the majority of those produced in the U.S. were infested.
They also examined samples of soil from locations where seed is produced. In the six soil samples they have tested they found no more than 20 microsclerotia per gram of soil. That raised the question about an airborne spread of the pathogen, and that issue is being researched in greenhouse trials this year.
Subbarao said for the disease’s symptoms to be visible in a commercial lettuce field there must be 150 microsclerotia per gram of soil. “Therein is the danger. Until that threshold is reached, the grower assumes there is no problem.” Microsclerotia levels in infested fields in the Salinas and Watsonville areas in 2008 reached as high as 2,200 per gram of soil.
In addition to V. dahliae, other similar-appearing fungal species were observed occasionally on seed.
Subbarao said it is unknown at this time what these have to do with V. dahliae, although they do not lessen its impact. On the resistance approach, Ryan Hayes, USDA breeder at Salinas, has developed materials having Race 1 resistance from five sources and these have been released to commercial seed companies. He is also screening plants to confirm their resistance to Race 2 in greenhouse trials.
A field at the Salinas USDA station has been dedicated to the ongoing series of resistance trials, which are scheduled to begin this spring.
Hayes is using a two-stage process to achieve high-level resistance to Race 2. Randomly selected lines are screened in the greenhouse, and those found to be candidates are advanced to replicated trials.
Thus far, most of the materials tested are capable of being symptom-free, although they are nevertheless colonized by the wilt.
Subbarao continues research on control of another serious disease in the Salinas Valley, lettuce drop caused by Sclerotinia minor.
Contans, a biological fungicide, he said, has proven to be more effective against field samples of S. minor that produce more sclerotia than those that produce fewer. An application of Contans is estimated to cost about $60 per acre — or about the same as that for Endura, a standard lettuce drop fungicide that is consumed by soil microbes and becomes ineffective after repeated use. Efforts are being made to optimize Contans costs.
Another dimension of the lettuce drop campaign is breeding to harness the “slow-dying” trait of some lines in developing resistance to the disease.
Susceptible lettuce may die from it in as few as two days, while other lines persist for 10 to 15 days, although they too eventually die. Hayes has identified a few families of crosses having the trait.
He is working with breeding lines of iceberg and romaine having reduced disease incidence of lettuce drop.
Two iceberg lines derived from Salinas 88, Great Lakes 54, and Holborn Standard have been selected and backcrossed for further breeding.
The Latin types Little Gem, Pavane, and Eruption have been consistently resistant, Hayes said, to infested field experiments and foliar inoculations with S. sclerotiorum. He hopes to transfer their lettuce drop resistance into full-size romaine types.