Two scourges that sometimes appear together but have not been definitively linked to each other continue to threaten lettuce: Pythium wilt and Impatiens Necrotic Spot Virus (INSV). The state budget allocated $1 million in new research funding aimed at developing more strategies to effectively protect the major crop. In addition, the California Leafy Greens Research Program received three grant proposals in December and will be reviewing them and voting on funding on Jan. 25, according to the program’s Jennifer Clarke.

In 2020, California’s lettuce crop was worth more than $2 billion and growers planted 80,000 acres.

Though all lettuce crops are at risk, the most significant economic impacts from the two pests have been seen in the Salinas Valley, with the 2021 crop somewhat less damaged than the devastated 2020 crop. In a hearing before the assembly ag committee last month, researchers and industry leaders posited that the drier conditions in the summer of 2021 prevented more damage, especially from Pythium wilt, which attacks the roots of lettuce, destroying it from below before the damage can be seen on the leaves.

Pythium wilt was first seen in 2011 in the Salinas Valley, but Jose Pablo Dundore-Arias, a plant pathologist at California State University Monterey Bay, said it didn’t present major challenges until 2018. “Now [it’s] considered the main root rot problem of lettuce.”

INSV is transmitted by the western flower thrips, tiny insects with huge impact because they’re highly reproductive, can survive in temperatures from 45-104 degrees Fahrenheit, feed on about 100 different plants, are not easily eliminated with pesticides and are so light the wind moves them across long distances, said Daniel Hasegawa, a research entomologist at USDA in Monterey Bay.

Mary Zischke of the Grower-Shipper Association said in a 2020 survey of 1350 fields, the average crop loss was 33% and some fields had 100% loss. She estimates the economic impact of the damage at over $100 million. In 2021, “we still had plenty of disease” but losses were lower, averaging about 10-15%, though she said organic farms were hit harder. That year, “we had below normal temperatures,” she said. “We can't count on that every year.”

Zischke said a task force formed in September 2020 has been meeting weekly to explore all options for protecting lettuce. Efforts to get approval for new chemical options have so far not been successful, she said, and currently only two products offer some limited success at controlling thrips.  

“The consensus, I believe, is that we feel like this is a true disaster,” said Mark Mason, a certified crop advisor and pest control advisor at Nature’s Reward, a large grower-shipper. He said growers rotated lettuce into different areas to try to avoid the pathogens “but it seems like we cannot outrun this thing.” That’s in part because Pythium wilt stays in the soil and can over-winter, infecting a crop that comes long after the previous infected one.

“Now growers and shippers are looking to other areas—that would be either outside of the state or even outside the country, down to Mexico, to try to grow lettuce to avoid these diseases,” Mason said. The impacts of that would reverberate throughout the community, he added. “This is a job-loss situation that we've got for our industry if we start moving our lettuce production outside of the area.”

Researchers hope to explore myriad different strategies, not focusing on pesticides as the top priority, Hasegawa said. Teasing out the relationship between INSV and Pythium is an important step.

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“If we can understand if one pathogen is making the plant more susceptible to infection by the other,” he said, “then that might guide us in terms of managing one pathogen versus the other.” Hasegawa said other lines of research include: the use of predatory insects to combat thrips; reducing the amount of pesticide used through more precise applications (potentially including drones or other new technology); increasing weed abatement to limit areas where pathogens can spawn, this can extend beyond crop fields to nearby publicly-controlled lands such as highway and railroad rights-of-way; breeding for stronger, healthier plants that can resist or survive the pathogens; improving detection; and expanding inquiries into RNA interference and other genetic tools.

“The toolbox for managing not only INSV, but Pythium as well, is going to have to be very diverse and very robust as we move forward,” Hasegawa said.

Funding from the Leafy Greens Research Program, the state and USDA will support ongoing research, with renewed urgency.

“It is critical to our state's economy as well as our overall national security” to safeguard California’s crops, said committee chair Asm. Robert Rivas, who represents the Salinas Valley.

“We need to take a very proactive approach, to partner with the work that has been done already, to see how we can overcome many of these challenges,” he said.

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