C A L I F O R N I A
Vegetables A SPECIAL GROWERS’ REPORT OF AG ALERT ®
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Tomato spotted wilt virus, inset, causes blotches and chlorotic spots on ripe fruit. Resistance-breaking strains of the virus can present a significant threat to tomato fields. Since 2016, the virus has also been detected in celery, lettuce, peppers and sow thistle in California.
Photo/University of California Agriculture and Natural Resources
Varied strategies sought to combat spotted wilt virus ByBob Johnson Formore thanadecade, San JoaquinValley tomato growers controlled tomato spotted wilt virus with variety resistance, the silver bullet of crop virusmanagement. Variety resistance, in this case from a dominant tomato gene—SW5—is a simple, economical and dependable way to control diseases. and Contra Costa counties by the end of the 2017 season. “The resistance-breaking strain has been detected in celery, lettuce, peppers and sow thistle,”Turiniwrote. “Detectioninsowthistleand lettuce inwinter, aswell as the increased distribution of the virus during 2016-17 suggests that the SW5 resistance-breaking strain will remain a production challenge into the future.”
Turini discussed tomato spottedwilt challenges during an April 6 episode of the UC Ag Experts Talk online seminar series: Considerations in management of Vegetable Crops Viruses. “TheSW5gene isnolonger functional forus inthisarea,”Turini said. “Weareconsistently finding the resistance-breaking isolate of the virus.” Tomato spotted wilt virus causes bronzing of the leaves and disfigures the fruit with chlorotic blotches. With the silver bullet no longer functional, growers must manage this virus using a complex combination of strategies, none of themas effective as variety resistance. “Control strategies shouldmakeuseofmultiple tacticswithout total relianceonanyone management approach,” Turini advised. He said an integrated pest management program for the virus should include
Thiswas toogoodtolast.With100,000acresof tomatoesplantedintheCentralValleywith the SW5 gene every year, it was only amatter of time before a resistance-breaking strainof tomato spottedwilt virus was identified. The first stage of failure for the resistant varieties came in2016, when the first resistance-breaking strainwas noted inFresnoCounty. That spring, a freshmarket tomato field in theCantuaCreek areahadasmuchas a 50% rate of spotted wilt, said Thomas Turini, vegetable crops advisor with the University of California Cooperative Extension in FresnoCounty. “By fall 2016, the same strainwas confirmed in two other areas within FresnoCounty,” Turini wrote in a summary of the events published by UC Agriculture and Natural Resources. His report, based on data fromthe California TomatoResearch Institute, may be found at ucanr.edu/sites/Vegetable_Crops/files/281645.pdf. By February 2017, Turini reported, the strain was detected in sow thistle in Cantua Creek and in the Huron areas. The area affected by the strain increased within Fresno County inprocessingand fresh-market tomatoes. It alsoaffected tomatocrops inMerced
See VIRUSES, Page 8
May 4, 2022 Ag Alert 7
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