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Lettuce growers hope weeding, research can counter devastating plant virus
Population explosion of insect vector contributed to $100 million in losses in 2020
While most Californians are wholeheartedly embracing the wet start to winter, one group is welcoming the rain more warily (and wearily) – lettuce growers in the Salinas Valley.
“It's a blessing, yes, we need the water,” said Tony Alameda, managing partner of Topflavor Farms, which grows a variety of produce in Monterey and San Benito counties. “But, oh gosh: with that water, here come the weeds, here comes the habitat, here comes all the other problems that go along with it.”
Weeds are overwintering havens for a tiny insect called the Western flower thrips, which in turn carries the impatiens necrotic spot virus (INSV) – a plant virus that caused $100 million in lost gross revenue for Salinas Valley growers in 2020.
The agricultural community called it “the biggest problem we've seen in a long, long time,” said Mary Zischke, facilitator of a task force convened by the Grower-Shipper Association to address INSV and a related affliction, Pythium wilt.
Widespread crop failure in 2020
Since INSV was first observed in the state in 2006, the virus – which poses no threat to people – triggered significant crop losses in 2019, leading up to a catastrophic 2020. As Alameda's lettuces began to show the telltale “bronzing” of the leaves, efforts to bag up or remove the infected plants had no effect on the virus' implacable spread.
“Nothing seemed to work,” he recalled, “and you just watch those fields collapse, week after week, until you're just like, ‘Ugh, there's nothing here to even harvest.'”
After “100% crop failure” that year in his prime fields at the heart of the Salinas Valley, Alameda tried to dodge the virus in 2021 – shifting lettuce plantings to San Benito County and instead using his most valuable land for unaffected crops such as cilantro, leeks and radishes. By decamping to San Benito, Alameda was able to harvest 70% of his usual lettuce yield.
Generally, growers enjoyed a reprieve from virus pressures in 2021. Even in this “good” year, however, about one-third of all lettuce plantings in the Salinas Valley had at least a low level of infection, according to Zischke.
“Since we were attributing a lot of our so-called good fortune – on having less damage this year – to the cooler weather, we know we can't count on that to get us out of this problem,” Zischke said. “All the models point to the fact that we're in a warming climate, so we were fortunate this year.”
More research needed on thrips
Heat waves were a major driver of the INSV disaster of 2020. Although researchers have established a link between warmer temperatures and population increases of thrips, science still has a lot to learn about those disease vectors.
“Thrips are something we're trying to understand as much as we can, but it's pretty tough because they're a little mysterious in the way they get around and where they overwinter,” said Richard Smith, a University of California Cooperative Extension vegetable crops and weed science farm advisor for the Central Coast region.
Smith – along with U.S. Department of Agriculture research entomologist Daniel Hasegawa and California State University-Monterey Bay plant pathologist JP Dundore-Arias – provided an INSV update during an Assembly agriculture committee hearing in December.
Recent studies have identified several weeds as key “reservoirs” of thrips, including malva, marestail, and hairy fleabane. The ubiquitous mustards, fortunately, appear to be poor hosts for thrips, although their pollen serve as potential food sources.
Controlling those weeds – which are beginning to spring up as the days lengthen – is a top priority during the winter months, according to Smith. Aggressive weed management in the preceding winter was an important factor in limiting the virus' spread in 2021.
And because weeds recognize no boundaries, experts are also urging managers of non-agricultural lands to keep their properties as clean as possible, including industrial sites, equipment yards and the edges of roadways – namely U.S. Route 101, which runs through the center of the valley. Some growers have been volunteering to weed their neighbors' vineyards.
“We're encouraging everybody – as best they can – to knock down known weed hosts; that's really critical,” Zischke said.
Search for long-term solutions
Within the grower community, there is “nervous optimism” for the coming year, said Alameda, as he continues to hope for an innovation that would aid in the fight against INSV – whether a more targeted pesticide application or a beneficial insect that could deter the thrips.
However, both Alameda and Zischke pointed to the breeding of more resistant lettuce varieties as the ultimate solution to INSV – albeit one that is years away.
“We have a lot of different types of lettuce that we grow, so to move resistance into all the different types of lettuce we grow throughout the season … that's going to take time,” Zischke explained.
Research funding from the state and USDA – as well as projects supported by the California Leafy Greens Research Program – can help expedite that process. But, for Alameda, the INSV crisis underscores the need for more resources and farm advisors such as Smith, who has spent more than three decades cultivating relationships and building trust within Salinas Valley communities.
Alameda would like to see a renewed focus on bringing “bright, young, passionate people who live and breathe this stuff” to the region, so growers are better equipped to handle the inevitable next calamity.
“Hopefully this is a wakeup call to all,” he said. “This is a valued industry – you have to take care of it; it cannot be taken for granted. The ‘salad bowl of the world' cannot rest on its laurels.”
Protecting California’s parsley crop
California parsley is produced typically in high volumes and with high quality. However in the past few years, growers began to observe unfamiliar disease issues in their parsley fields. Leaf spots, blighted foliage and yellowed plants contributed to loss of quality and reduced yields. Steven Koike and Oleg Daugovish, UC Cooperative Extension advisors in Monterey and Ventura counties respectively, stepped in to investigate the new parsley problems. They collaborated with farmers and pest control advisers to understand the extent of the problems and to obtain samples of the diseased crops.
The UC Cooperative Extension plant pathology diagnostic lab in Salinas was successful in isolating and identifying several pathogens that were responsible for causing the disease symptoms. Working with USDA, they found that three new diseases were present in California parsley crops: bacterial leaf spot, Stemphylium leaf spot, and Apium virus Y disease.
Two of these problems are seedborne, so future management will include the use of pathogen-free seeds. The Apium virus Y pathogen is found in weeds, so growers will need to remove poison hemlock, among others.
Previous to this research, some growers were spraying symptomatic fields because they believed that a disease called late blight was responsible for the disease symptoms. Growers have now ceased making these sprays, eliminating the use of unnecessary chemicals and saving costs.
Click here for more on this research.
Crop diseases a concern in strawberries and tomatoes this year
Record reporter Reed Fujii spoke to Brenna Aegerter, UC Cooperative Extension advisor in San Joaquin County. She said she's never seen such losses due to beet curly top virus.
"In my eight years here in this county, I had only seen curly top in two fields," she said. "The virus was present in every tomato field I have seen this season, though in most fields, the incidence was so low as to not be a concern."
Beet curly top is spread by beet leafhopper. The insect don't like tomatoes and peppers, but will briefly feed on the crop and infect them before moving on, Aegerter said. The high leafhopper population is most likely a cyclic peak.
"The hope is we'll go back next year to not seeing it," Aegerter said.
Amy Asman of the Santa Maria Sun used UC Cooperative Extension materials for her story on the serious pallidosis-related disease threat in local strawberries. For detailed information about the strawberry decline, see UCCE advisor Surendra Dara's story in the Strawberries and Vegetables blog.
Foodborne illnesses and the 100K Genome Project
Bart Weimer, professor in the UC Davis School of Veterinary Medicine, serves as director of the 100K Genome Project and co-director of the recently established BGI@UC Davis facility, where the sequencing will be done. Other collaborators include the U.S. Centers for Disease Control and Prevention and the U.S. Department of Agriculture.
The new five-year microbial pathogen project focuses on making the food supply safer for consumers. The group will build a free, public database including sequence information for each pathogen's genome — the complete collection of its hereditary information. The database will contain the genomes of important foodborne pathogens including Salmonella, Listeria, and E. coli, as well as the most common foodborne and waterborne viruses that sicken people and animals.
The project will provide a roadmap for developing tests to identify pathogens and help trace their origins more quickly. The new genome database also will enable scientists to make discoveries that can be used to develop new methods for controlling disease-causing bacteria in the food chain.
"This landmark project will revolutionize our basic understanding of these disease-causing microorganisms," said Harris Lewin, vice chancellor for research at UC Davis.
The sequencing project is critically important for tackling the continuing outbreaks of often-deadly foodborne diseases around the world. In the United States alone, foodborne diseases annually sicken 48 million people and kill 3,000, according to the CDC.
"The lack of information about food-related bacterial genomes is hindering the research community's ability to improve the safety and security of the world food supply," Weimer said. "The data provided by the 100K Genome Project will make diagnostic tests quicker, more reliable, more accurate and more cost-effective."
"We see this project as a way to improve quality of life for a great many people, while minimizing a major business risk for food producers and distributors," said Mike McMullen, president of Agilent’s Chemical Analysis Group.
A consumer-focused article about the project is available on the FDA website.
(This article was condensed from a UC Davis news release. Read the full press release and watch a video of Bart Weimer giving an overview of the project.)
