Funding aims to help create new value-added products

By Bob Johnson 

The unusually cold and wet winter of 2022-23 appears to have brought Salinas Valley lettuce growers a reprieve from impatiens necrotic stunt virus, which has devastated the region’s crop in recent years.

In December, researchers with the U.S. Department of Agriculture and the University of California Cooperative Extension concluded that last year’s atmospheric river storms that caused flooding on Monterey County farm fields likely disrupted the spread of the deadly lettuce virus known as INSV.

Monterey County farmers in 2022 suffered an estimated $150 million in crop losses from the disease, which is spread by milimeter-long insects, or thrips. Two years earlier, lettuce crop losses were estimated at $100 million.

The county’s farmers typically harvest 100,000 acres of lettuce valued at $1.2 billion.

After the fallout of 2022, the county enacted a mandatory abatement program for select weeds that can host INSV and the thrips that carry the disease to lettuce.

The county called on residents who noticed any of the key weed hosts near lettuce production areas to immediately notify the agricultural commissioner’s office.

“There could be many other factors impacting the virus levels, including INSV host weeds,” said Yu-Chen Wang, UCCE plant pathology farm advisor based in Watsonville.

“It is critical that growers continue making a special effort to reduce weed populations in all the usual areas, as well as areas that may not have received as much attention in the past,” Wang said.

Wang is part of a team of UC researchers 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 entomologist 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 lettuce 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 reproduce on.” As a result, he said, the thrips that moved onto lettuce crops were generally not carrying the disease.

Atmospheric river storms in January and March 2023 caused an estimated $600 million in damages to Monterey County farms and ranches, including $54.4 million in lettuce crop losses.

Yet at the same time, the chilly and wet weather reduced the winter reproduction rate for thrips. Hasegawa said thrips populations 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 rebounded from May to November, Hasegawa said, “the opportunities for thrips to acquire the virus from infected plants were much lower than in previous years.”

An abundance of new, uninfected grasses and weeds growing as a result of the atmospheric 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 monitoring 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 adopted decades ago to control another lettuce disease—the mosiac virus—is also believed to be beneficial in reducing INSV by controlling potential hosts for thrips.

“Winter weather would impact the population of overwinter thrips, which might acquire and spread INSV for the next season,” 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 pressure 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 treatment 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 populations 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.)