Researchers seek to safeguard Salinas Valley lettuce A stormy winter could portend anoth- er devastating year for lettuce farmers in the Salinas Valley. They saw approxi- mately $150 million in lost gross revenue in 2022 due to the impatiens necrotic spot virus and associated diseases, according to University of California Cooperative Extension farm advisors and other researchers. for the tiny insect, the Western flower thrips, that carries impatiens necrotic spot virus, or INSV. Resources assigned Pearsons to the area last November and hired Yu-Chen Wang in October as UCCE plant pathology ad- visor for the three counties.
“We don’t know if thrips are just so per- sistent and so stable in that pupal stage that maybe they will emerge unaffected,” said Kirsten Pearsons, UCCE integrated pest management farm advisor for Santa Cruz, Monterey and San Benito counties. “There’s just so much about their biology and ecology in the Salinas Valley that we just don’t know.” The mystery of thrips, INSV and soilborne diseases—namely Pythium wilt—is why UC Agriculture and Natural
But the rains could benefit growers, as thrips in the soil—during their inter- mediate stage of development—might be drowned in the waterlogged fields. As with so many aspects of the INSV crisis, the ultimate effects of flooded fields on thrips populations remain unknown, researcher say.
“They’re stepping in at a critical mo- ment,” said Richard Smith, the region’s UCCE vegetable crop production and weed science advisor, who retired in January af- ter a 37-year career. “They’ve gotten grants funded already, and that’s just incredible. They’re hitting the ground running.” Experienced in disease diagnosis and collaboration with growers and industry partners, Wang said her pathology back- ground—paired with Pearsons’ entomolo- gy expertise—will be crucial in addressing INSV and other diseases. One priority is untangling the dynamics of INSV and Pythium wilt co-occurrence, which is the subject of ongoing research by J.P. Dundore-Arias, a plant pathologist at California State University, Monterey Bay. While the vegetables may tolerate one disease or the other, their one-two punch often deals a lethal blow. “We have so many problems now, whether it’s fusarium (wilt) or Verticillium (wilt) or Pythium or INSV,” said Mark Mason, pest control advisor for Nature’s Reward, which primarily grows lettuces on 5,000 acres across the Salinas Valley. Mason said co-infections on his crops— sometimes with three or four diagnosed diseases—make it difficult to assign mon- etary damages to a specific pathogen, but he noted he has seen fields with 100% loss. According to the Grower-Shipper Association of Central California, about 11,500 acres were deemed not harvestable in 2022, representing 12% of total state let- tuce acreage. Given the gravity and complexity of the disease, Pearsons said she has been field- ing calls from growers seeking new and better solutions, ways to improve existing tools, techniques borrowed from other crop systems and additional biological or chemical means of control. Although there are a couple of pesticides that manage the disease-carrying thrips reasonably well, growers and researchers are worried about their diminishing effi- cacy due to overuse. Plus, they only con- stitute a short-term fix. “Managing the thrips will only reduce the amount of INSV that can get trans- mitted,” Pearsons explained. “You can kill 99.9% of the thrips, but you get one thrips that has INSV that enters a field, and now you have an infected lettuce plant. All of the thrips are going to come, and they can spread it from there; pesticide slows things down, but it’s not going to eliminate it.” Finding disease-tolerant lettuce cultivars is a more sustainable approach. Trials con- ducted last year by Smith, Wang and others identified several varieties that appeared to hold up well to Pythium and INSV. While additional research could maximize their potential benefit, Wang said even the har- dier cultivars will lose their resistance over time, and a multi-layered INSV strategy with “integrated management tools” is crucial. “We realized, when this thing started See LETTUCE, Page 18
Recent rains might mean more weeds, which serve as overwintering “reservoirs”
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10 Ag Alert February 22, 2023
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