Ag Alert February 14, 2024

Virus

Continued from Page 8 UCCE plant pathology farm advisor based in Watsonville. “It is critical that growers continue mak- ing a special effort to reduce weed popula- tions in all the usual areas, as well as areas that may not have received as much atten- tion in the past,” Wang said. Wang is part of a team of UC research- ers looking for answers to INSV and has screened lettuce varieties for resistance to the disease. She is also partnering with Daniel Hasegawa, a USDA research ento- mologist in Salinas, in efforts to track and curtail the spread of the disease. In a December report, Hasegawa cited several factors as contributing to vastly lower INSV infections in the region’s let- tuce crops. “Our current theory is that the colder weather and precipitation suppressed thrips populations at the early part of 2023,” Hasegawa wrote in Salinas Valley Agriculture, an online publication of UC Agriculture and Natural Resources. “This coincided with an abundance of weeds and grasses that germinated in early 2023.” Hasegawa concluded “because the weeds were newly germinated, they were not infected with INSV and, thus, served as ‘clean’ hosts for thrips to feed on and re- produce on.” As a result, he said, the thrips that moved onto lettuce crops were gener- ally not carrying the disease. Atmospheric river storms in January and March 2023 caused an estimated $600 mil- lion in damages to Monterey County farms and ranches, including $54.4 million in let- tuce crop losses. Yet at the same time, the chilly and wet weather reduced the winter reproduction rate for thrips. Hasegawa said thrips pop- ulations recorded from February to April last year “were some of the lowest we have observed” and “a stark contrast to 2022.” Even as the thrips population rebound- ed from May to November, Hasegawa said, “the opportunities for thrips to acquire the virus from infected plants were much low- er than in previous years.” An abundance of new, uninfected grass- es and weeds growing as a result of the at- mospheric river storms served to reduce the threat, he said, leading to substantially lower virus loads in and near lettuce fields. In a continuing effort to prevent thrips populations and the INSV threat from building up early in the season, UC and USDA researchers set up mon- itoring networks throughout Monterey County to provide early warnings for lettuce growers. Every year in December, the region prohibits any lettuce from growing in the fields. The policy, which was ad- opted decades ago to control another lettuce disease—the mosiac virus—is also believed to be beneficial in reduc- ing INSV by controlling potential hosts for thrips. “Winter weather would impact the pop- ulation of overwinter thrips, which might

acquire and spread INSV for the next sea- son,” Wang said. Thrips populations are monitored throughout the Salinas Valley during the growing season, and the results are posted online to give growers an idea of the pres- sure in their area. Once thrips feed on a plant infected with INSV, they can carry the disease and spread it to new plants.

When the virus builds up early in the season, it is all but impossible to prevent spread of the disease in the summer months because there is no effective treat- ment once a plant is infected by INSV. Agricultural officials in Monterey County are on the lookout for thrips populations and INSV dangers that can spread from a list of 10 suspect weeds: common purslane, lambsquarter, field bindweed, shepherd’s purse, nettleleaf

goosefoot, hairy fleabane, annual sowthistle, malva and burning nettle. More information, including data on overwintering and year-round thrips pop- ulations in Salinas Valley, may be found at the UCCE Monterey County website at www.cemonterey.ucanr.edu. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)

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February 14, 2024 Ag Alert 9