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, shown in file photo above, has begun to cause new problems in San Joaquin Valley fields. A University of California Cooperative Extension farm advisor says a new strain of the disease appears to have overcome resistance bred into certain tomato varieties.
Integrated tactics may limit new tomato virus strain ByBob Johnson
during the last decade. “The resistance-breaking strain has not been reported north of Merced,” Turini said. “It has not been found in Yolo County or further north.” Resistant varieties have given tomato growers a 10-year respite fromthis challenging virus, which can live in awide range of common crops andweeds, fromwhich it is easily carried to tomatoes by western flower thrips. Turini indicated it may have only been a matter of time before the virus evolved to overcome the protection provided by resistant varieties that are widely planted. “Inmost of California, we’ve been using single-gene resistance to TSWV and in 2016, a resistance-breaking strainwas detected,” Turini said. “Within the areas we’re testing, we see this resistance-breaking strain. We have been selecting for it by using plants with the SW5 gene.” The first sign of trouble came in the spring of 2016, when a fresh-market tomato grower in the Cantua Creek area of Fresno County suffered a 50% infection rate in a field planted with a TSWV-resistant variety. The resistance-breaking strainhas since spread to areas of Fresno,Merced, Kings and Kern counties, and has been detected in a range of weeds and crops, including lettuce.
A new strain of tomato spotted wilt virus is challenging Central Valley farmers, who find their crops are suffering significant damagewith varieties that until a fewyears ago were resistant to the disease. University of California researchers advise tomato growers in the San JoaquinValley to adopt integrated pest management programs to control tomato spotted wilt virus. Known by the shorthand termTSWV, the virus causes fruit distortions and bronzed, spotted or yellow leaves. “Tomanage TSWV, in some areas you need to try several tactics, including sanitation andmaybe thripsmanagement,” saidTomTurini, UCCooperative Extension vegetable crops farm advisor in Fresno County. Turini has been monitoring the advance of the new strain of TSWV from Fresno to nearby counties, in cooperation with specialists on the UC Davis campus. He made his remarks on the challenge this new development creates as UC and UC Cooperative Extension researchers presented their latest work during the 2021 California Processing Tomato Production virtual workshop. Fortunately, he said, Sacramento Valley tomato growers can, at least for now, still control the disease using resistant varieties that have become the standard
See VIRUS, Page 8
May 5, 2021 Ag Alert 7
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