California Tomato Growers Learning Ways To Overcome Parasitic Weeds

California’s processing tomato industry, for the first time this past harvest season, agreed to voluntary equipment cleaning and notification guidelines to prevent the spread of branched broomrape, a parasitic weed that attaches to roots and sucks out key nutrients.

The weed’s tiny seeds can be smaller than finely ground spices, survive dormant in soils for decades and be carried by wind, footwear and other methods. Its resurgence in 2017 in Yolo County threatens the productivity of this industry that brought in $1.6 billion in 2024.

The University of California, Davis, in conjunction with industry, federal authorities and state regulators, is playing a key research role in the battle against broomrape by testing and developing in-field sanitation guidelines for tomato harvesters and other field equipment. Researchers across campus are also evaluating herbicide treatments, weeding methods, ways to detect the weed and disrupt its ability to affect crops.

Under former state quarantine rules, any broomrape detection would require a field be destroyed before harvest. With the new guidelines, growers may harvest if they adhere to certain management practices, including equipment cleaning standards developed by the California Broomrape Board, formed in 2024 to advise the California Department of Food and Agriculture (CDFA).

“There are 1,000 acres that are actually reported but we know from observation that it’s probably much greater than that,” says Cassandra Swett, a UC Davis plant pathologist who is leading efforts to sanitize field equipment. “There are two main goals: reduce the economic impacts of broomrape on growers in the affected region by allowing them to harvest, and on the other side, keep broomrape out of the regions that do not currently have it.”

In 2025, nearly all growers and processors in the state signed on to compliance agreements regarding cleaning and notification, and this year the CDFA is expected to require them, according to Zach Bagley, Managing Director of the California Tomato Research Institute, which has been working on broomrape control issues since 2018.

“It’s overall seen as a positive in the industry,” Bagley says. “The driver for sanitation is not just the biology of this weed and the reality in the field, but it also has regulatory components.”

For more, continue reading at caes.ucdavis.edu.