Fusarium Continued from Page 6
the new CR4 variant has made varieties previously resistant to Fusarium race 1 sus- ceptible to the disease. The trial of potential sources of resis- tance to the new variant was supported by D’Arrigo California, the California Department of Food and Agriculture and the Salinas-based California Leafy Greens Research Board. Once an area of a field is infested with Fusarium wilt, it will remain viable and ready to infect lettuce indefinitely. Farmers are instructed to take care to not spread the pathogen to other areas of the field on equipment or boots. Before replanting lettuce, they are ad- vised to wait for seed companies to offer varieties with resistance to the variant. The new CR4 variant, which overcomes resistance to race 1, is now a focus of con- cern in the Central Coast region. “CR4 is the new fusarium in Salinas and Santa Maria,” Putnam said. Farm advisors say there are resistant va- rieties available to plant in areas that do not have the new variant. “Growers around Watsonville should have enough resistant varieties to plant,” said Yu-Chen Wang, UCCE plant patholo- gy farm advisor based in Watsonville.
Infected lettuce plants show symptoms of Fusarium wilt. Research being conducted in a field infested with the soil-borne disease revealed varieties of romaine lettuce with resistance to Fusarium, while numerous iceberg varieties demonstrated high-mortality rates.
Generally, romaine lettuce holds up bet- ter than iceberg against all known variants of fusarium wilt, Wang said. In the Soledad trial, iceberg varieties Tombstone, Rhodenas and Tamarack all suffered 90% mortality, and most of the Yucaipa, Telluride, Oso Flaco and Primo plants also died, researchers reported. There were 22 iceberg varieties with mor- tality below 50% in this year’s trial. Varieties that had mortality below 5% included
Balboa, Fontina, San Andreas, Paraiso, Powerball, San Miguel and Meridian. Among the Romaine lettuces, there were 21 varieties with marketable yields of 90% or more. “There were more susceptible varieties last year,” Putnam said of the continuing research. “But I think the seed companies removed those this year.” Fusarium wilt is a warm-weather dis- ease that causes the plants to wilt and die
as the season grows hotter. It started in the warmer growing regions of Yuma and the Central Valley and later spread to the Central Coast region. Because the pathogen can survive in the soil indefinitely, the main management strategies are to move lettuce crops to clean ground or plant resistant varieties. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
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September 4, 2024 Ag Alert 7
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