As the spread of Verticillium wilt of lettuce accelerated this season in California’s Salinas Valley, plant pathologists and breeders stepped up their quest for ways to control it.
Research leader Krishna Subbarao, University of California, Davis plant pathologist stationed at Salinas, disclosed progress during a gathering of the California Leafy Greens Research Board held at Seaside.
The occasion was the first series of research reports delivered since the board evolved on April 1 of this year from the former California Lettuce Research Board to add funding support for projects on spinach and mixed greens.
The wilt, said Subbarao, “was very active this year and since it was first discovered 13 years ago, there was never a year in which so many new lettuce fields succumbed to it.”
By early October, he added, 13 new fields picked up the disease, bringing infected acreage in the Valley to more than 1,500 acres.
Typically, during each of the past several years only four to six new fields were found to be infected. The majority of incidence is around Salinas, although it spans from Watsonville to King City. At least two fields were abandoned, and others varied in loss from 20 percent to 60 percent.
Many fields have been fumigated and planted to strawberries for two seasons, only to have the wilt reappear when lettuce was planted.
One of Subbarao’s approaches has been to investigate seed lots for infestation of the causal fungus, Verticillium dahliae. He is continuing to test seed and soil samples provided by cooperating seed companies having commercial cultivars.
“For the disease to occur in a commercial lettuce field, it takes 150 microsclerotia per gram of soil. But of the lots we have assayed, we’ve found no more than 20 per gram of soil,” he said.
Since the soil infestation levels were too low to trigger extensive Verticillium wilt development, he added, “the level of seed infestation observed could possibly have come from the airborne spread of the pathogen.”
That led him to probe into whether the pathogen has an airborne phase, and he plans to report on his findings at the annual meeting next March.
The laboratory studies include surrounding a disease-free lettuce plant with concentric rings of infected plants and examining their seed for infection to learn if the fungus is transported through the air. Another part of the experiment is spraying the pathogen on flowers of clean plants and later testing the seeds.
Although the wilt has occurred thus far only in the northern part of the coastal lettuce-growing region, Subbarao said destruction of French marigold fields by wilt last year near Santa Maria aroused concerns for infection of lettuce there. He wants to know if the wilt that goes to marigolds and spinach also goes to lettuce.
Verticillium dahliae has two races. To distinguish them, samples are inoculated on La Brillante and Salinas 88 lettuce. If a sample is pathogenic on both, it is Race 2. If it is pathogenic only on Salinas, it is Race 1, which has been the most commonly occurring in Subbarao’s work.
Other research on isolates of the disease of spinach and other crops shows that those from spinach are thought to include both races. “Most of these may have already been introduced into California lettuce fields,” Subbarao said.
The wilt has great diversity in the Salinas Valley, and thus far sources of it are known to be Washington state, Denmark, and the Netherlands.
Meanwhile, Ryan Hayes, USDA breeder at Salinas, has identified sources of resistance to Race 1 and released them to commercial seed companies. He has also identified sources of resistance to Race 2 and is confirming them.
Hayes said plans are to evaluate Verticillium-resistant material at the USDA Station at Salinas in the spring of 2009. He is working closely with UC, Davis geneticists in identifying improved resistance.
Subbarao is also continuing his investigation of lettuce drop, caused by Sclerotinia minor. Recent seasons of trials with Contans, a biological fungicide, showed that it performed against S. sclerotiorum, but not S. minor. Subbarao did laboratory plating trials with Contans applied to isolates of S. minor on pieces of potato and observed results at five, 12, and 22 days after treatment. He concluded that isolates having abundant sclerotia benefit more from Contans than those producing fewer sclerotia.
He also did field trials with different rates and timing of applications of Contans and Endura in the 2008 season. Those results will also be disclosed to the board in March. Endura, a long-time fungicide for lettuce drop, was found to be ineffective against lettuce drop when applied repeatedly because it was consumed by soil microbes.
Another avenue Subbarao is pursuing is another look at the “slow dying” response some iceberg and romaine lines have to S. minor. These plants survive for 10 to 15 days, while other more susceptible plants collapse in as little as 24 hours after infection.
The trait has been known for some years, but in an approach not previously considered, he is supplying inoculum of the pathogen and Hayes is working to see if it can be used in finding resistant plants.
“The earlier evaluation of slow-dying resistance was done using a single isolate of S. minor,” Subbarao said. “In light of the variation in virulence that we have detected in populations of S. minor, it is important to re-screen these lines to make sure that slow-dying resistance is stable across the virulence spectrum of the pathogen population.”
They have evaluated the lines against five isolates of S. minor, and analysis of the results will be complete in time for the March meeting.
The pair is also continuing work to develop other sources of resistance to both species of Sclerotinia. At this point, Subbarao said, they have identified a link in the germplasm to early flowering, but transferring the resistance to horticulturally superior lettuce backgrounds is difficult. A few families from crosses between low-susceptible lines are being evaluated in the field.