USDA releases new lettuce lines resistant to threats
By Bob Johnson U.S. Department of Agriculture re- searchers have released red leaf, green leaf and romaine lettuce breeding lines that combine resistance to crop threats including leafminer, corky root and downy mildew. Plant breeders may request limited amounts of seed to cross the new resistance with commercially acceptable varieties.
someone comes out with a variety with new resistance, the guys will switch to it. With lettuce, it is more complicated.” Recently released USDA breeding ma- terial has more complex sources of resis- tance that could help growers keep up with the fast-changing disease. In a HortScience article announcing the release, USDA research geneticist Beiquan Mou said more than 30 major
genes for downy mildew resistance have been identified in lettuce. “The resistance based on single dom- inant major genes, however, has not been durable because new isolates of the pathogen have continued to evolve and render this race-specific resistance ineffective,” Mou wrote. He concluded in the article that “quan- titative resistance” to the pathogen has been found in several lettuce cultivars that typically become infected. University of California pest manage- ment guidelines say growing resistant cultivars is the most effective means of controlling downy mildew. But the UC guidelines say resistant cultivars are not available for all areas and seasons. “Because the pathogen is highly vari- able and dynamic, resistant cultivars do not remain resistant indefinitely and can be overcome by new virulent isolates,” UC said. Widespread adoption of drip irrigation has made downy mildew more manage- able in lettuce than spinach, but the dis- ease remains a constant concern. Bianchi said the best strategy is to ap- ply a fungicide while the lettuce plants are still young. “We’re putting preventive materials out there to make sure the plants don’t get infected,” he said. “The materials are effective, but they are expensive.” Another crop concern is leafminer flies, which damage lettuce by punctur- ing the leaves to suck out food and by de- positing eggs that hatch and mine their way through the leaves. “Damage caused by adult stings and larval mining reduces photosynthetic ca- pacity, renders lettuce leaves unmarket- able and provides entrances for disease organisms,” Mou wrote. He said chemical control of leafmin- er usually lasts only a short time be- cause leafminer can develop a high degree of resistance to a broad range of insecticides. “Lettuce cultivars with a high level of resistance to leafminer are not currently available,” he wrote. Another threat found in the Central Coast region, corky root, makes plants develop lesions that eventually make it difficult to absorb water and nutrients. Mou said yield losses in severely infested fields can reach up to 100%. “Crop rotation, cover cropping, re- duced nitrogen fertilizers, drip irrigation and improved soil drainage may reduce corky root to a certain extent,” he wrote. The Salinas-based California Leafy Greens Research Board supported the work that led to the release of the new let- tuce breeding lines. So far, the new lines have been trialed only in Salinas and may not grow well or be effective in other ar- eas of the state. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
Downy mildew is a persistent chal- lenge in areas with a cool and moist cli- mate. Such climate conditions make the Salinas Valley the nation’s Salad Bowl. But they also favor the pathogen, which can leave lesions and spots that make the crop unmarketable. “Downy mildew is always an issue,” said Richard Bianchi, a San Benito County vegetable grower. “Even if you dry it up, the spots are still there. With spinach, if
December 1–6, 2023 • Grand Sierra Resort and Casino, Reno, NV Save the Date
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California Marketplace Come shop the bounty and artisan goods of our state and local vendors, enjoy food and beverage samples, and visit sponsor/exhibitor booths. Workshops Educational topics covering three paths: Marketing, Branding & Sales, Policy, and Professional Development. Sunday Activities Join us for fun tours, annual cornhole tournament, and more, as well as the many offerings of the Grand Sierra Resort and Casino. Research and Grant Updates Dr. Amrith Gunasekara, CAFB director of science and research, discusses exciting research and grants led by the California Bountiful Foundation.
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