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Lettuce trials show ways to counteract Pythium wilt

Issue Date: February 17, 2021
By Bob Johnson
A trial in a Salinas Valley field shows a variety of lettuce that tolerates damage from Pythium wilt, left, and a susceptible variety, right. A University of California Cooperative Extension farm advisor reports “devastating” outbreaks of the plant disease in 2020. He cautions farmers to avoid planting lettuce in fields with prior heavy infestations, as one of several techniques to avoid future losses from the root-rot disease.
Photo/Richard Smith, UC Cooperative Extension
In this lettuce field damaged by Pythium wilt, younger leaves remain upright while older leaves wilt downward.
Photo/Richard Smith, UC Cooperative Extension

The epidemic of wilted plants that struck lettuce fields from Castroville to King City without warning last summer has Salinas Valley growers and researchers anxiously wondering what 2021 will bring.

The cause was Pythium wilt. After being virtually unknown in Monterey County vegetable fields until less than a decade ago, Pythium wilt steadily became more widespread. It caused troubles in 2018 and then widespread damage in 2020, presumably leaving behind pathogens that will remain viable in the soil for years.

"The disease started slowly in 2020 and was just in scattered fields, but by midsummer the infestations grew into devastating outbreaks," said Richard Smith, University of California Cooperative Extension farm advisor based in Salinas.

"We don't know if it will hit again in 2021, or when in the year," Smith said. "It is not clear when it might be most destructive, whether it will be an issue early in the season or remain a late-season issue."

Pythium is a root rot that, at its most extreme, can make entire lettuce fields wilt and then collapse, much like other viral or fungal diseases that are more familiar in the Salinas Valley.

Although researchers are still scratching their heads over why it's hitting the Salinas Valley, and why now, Smith and California State University, Monterey Bay, assistant professor of plant pathology J.P. Dundore-Arias have already developed recommendations for farmers. They presented the recommendations during an online Pest Management Series.

"Avoid fields with prior heavy infections," Smith cautioned. "Although we saw it as far south as King City, the most severe infections were north of Gonzales."

Rotation out of lettuce should be practiced for years, the specialists said, because Pythium leaves behind spores that can remain viable in the soil for quite some time.

"The spores can survive several years in the soil without a host," Dundore-Arias said. "Like downy mildew, it can be spread by insects."

Many species of Pythium can survive in different environments and disperse by swimming or in the wind, he said: "The ability to move to new fields in the wind is important because of the wind in the Salinas Valley."

Irrigation management is another tool, he said, because the disease thrives in ground that is too wet.

"Overwatering is a problem and overfertilizing, especially with nitrogen, is a problem too," Dundore-Arias said. "If you overwater your field, you not only create perfect conditions for the pathogen, you also reduce root growth."

Pythium thrives in moist soil, the researchers said, and the symptoms appear worst when the plants wilt in hot weather—a difficult combination because it is a natural response to heavily irrigate wilting lettuce.

"We think it may have become worse during the heat spells related to the fires this summer, when many growers increased watering," Smith said.

While researchers track down leads on why the disease took off during the summer, and when it might return, they have already made significant progress on identifying which lettuce cultivars appear susceptible or tolerant, and which may provide genetic resistance that seed companies could use to cross into other varieties.

Researchers said they were able to screen cultivars for tolerance or susceptibility after Pythium broke out in one Salinas Valley field where a seed company was holding variety trials.

"There appears to be some reasonably good genetic material available for seed companies to work with to develop tolerance or resistance to this disease, hopefully relatively quickly," Smith said.

Some green leaf varieties were "highly susceptible," he said, whereas red leaf varieties appeared tolerant.

"There were differences among the head leaf varieties, but no romaine was particularly good," Smith said.

Pythium is technically not a fungus but a water mold, such as downy mildew and Phytophthora, but he said widely used fungicides may help control the disease.

Smith said he tried using Ridomil in six fields late in the season, and the material helped in two of them—although the difference was modest.

"We were hoping for more, but there is cause for optimism because it had some effect," he said. "Maybe earlier timing can make it better. We are also going to look at other materials."

Smith and Dundore-Arias have published results of their ongoing Pythium studies on the Cooperative Extension Salinas Valley Agriculture Blog, which includes the results of the variety screening, the Ridomil trial and other management suggestions, available at

Their studies revealed one more reason the 2020 Pythium outbreak took many growers and pest control advisors by surprise: Many of the ailing plants suffered from more than one disease.

"At the beginning, there was a lot of confusion," Smith said. "People were blaming collapsed fields on impatiens necrotic spot virus (INSV) when it was Pythium. The way to tell is that Pythium causes root rots."

Smith found in one field that only a handful of plants had INSV but nearly 7% were infected with Sclerotinia lettuce drop. In another field, INSV was widespread and nearly all the plants with Pythium also had that virus.

One ominous discovery is that while Pythium infections tend to be near the soil line or slightly below, some lettuce plants had infections in 2020 that were a lot deeper in the ground.

"Some plants had infected taproots deeper into the soil, which is concerning," Smith said.

(Bob Johnson is a reporter in Sacramento. He may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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