Viruses Continued from Page 7
“sanitation, site selection, thrips con- trol and plant resistance.” Turini said, “Themost effective tomato spottedwilt virus control strategy is an in- tegratedmanagement program.” The first part of that strategy is to plant tomatoes inplacesandat timeswhere they are less likely to suffer extensive damage. “Avoid sites where therewere previous to- mato spottedwilt virus outbreaks,” Turini advised. A related strategy is to plant to- matoes earlier in the season rather than later, when thrips have had time to spread the virus throughout the growing region. Drenching transplants inthe insect con- trol product Verimark can reduce thrips feeding for a while, but it is not a stand- alone control for the virus. “Insecticidesarenotsomethingtorelyon for control of this virus,”Turini cautioned. Plant pathologists are trying to findnew sources of variety resistance, but it could takesome timebefore thisworkbears fruit. “Currently, there isnoalternative toSW5 in commercial varieties,” Turini reported. “There are other approaches to resistance being testedunder greenhouseconditions with plans to evaluate these lines under field conditions.” Beet curly top virus—another import- ant disease—stunts the plants, which turn yellow or bronze and then die. It
New strains of tomato spotted wilt virus have been able to infect a long-resistant tomato variety. Early-season planting is advised to reduce threats of virus outbreaks.
causes economic losses in several crops that include processing and fresh-market tomatoes, celery, lettuce and peppers. This disease caused so much damage inCentral Valley tomatoes one recent year that the California Department of Food and Agriculture created a Beet Curly Top Virus Control Board. “We lost entire fields, hundreds of mil- lions of dollars of processing tomatoes to beet curly top virus in 2013,” Turini said. The board monitors the vector, beet leafhoppers, and their rate of infection and tries to knock down the insect in high pressureyearsas theymigrateeveryspring from the foothills to the western areas of the San Joaquin Valley in Kern, Kings,
Fresno andMadera counties. In a 2015 small plot study, Verimark transplant drench increased yield in to- matoesunderbeet curly toppressure from 36 tons to 45.6 tons. Managing crop viruses by controlling the insect vector, however, can be ineffec- tive because the pest frequently dies after spreading the diseasewithin the field. “Aphids transmit watermelon mosaic and cucumber mosaic viruses on the sty- let,” Turini said. “The insect loses the virus after feeding a few times, so killing aphids is too late to control the disease.” Cucumbermosaic reduces leaf sizeand retards growth on a wide range of crops, including tomatoes, spinach, celery, saf-
flower, beans, blackeyes, peppers, beets, potatoes, melons andmany ornamentals. This disorder can reduce the quality of melons even if the crop is able to set fruit. In thewarmer southernareas of theSan JoaquinValley, silverleaf whiteflies canbe an issue for a range of crops, including let- tuce, alfalfa,melons, squash, broccoli and other cole crops. “They can damage the product without a virus but, in the 2020-21 season, we de- tected cucurbit yellow stunting disorder virus,” Turini said. (Bob Johnson is a reporter in Monterey Coun t y . He may b e c on t a c t ed a t bjohn11135@gmail.com.)
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8 Ag Alert May 4, 2022
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