Farm Progress

Some lettuce plants are fighters, prepared to deliver a knockout punch to disease challenges. Others are more susceptible.

Dennis Pollock 1

April 19, 2013

6 Min Read

Some lettuce plants are fighters, fully prepared to deliver a knockout punch to challenges such as bacterial leaf spot.

Others are more susceptible to those blows.

Sorting out which varieties fall into either of those categories was among the research findings presented to the California Leafy Greens Research Board in Coalinga.

The annual research conference was close to a seven-hour non-stop stream of findings from more than 15 researchers whose reports were richly technical, but also contained some practical advice for growers.

Here’s some of what they had to say:

• Carolee Bull, research plant pathologist with the U.S. Department of Agriculture, Agricultural Research Service in Salinas, looked at variability in resistance to bacterial leaf spot among different cultivars.

She found that the variety Little Gem was most resistant and Vista Verde highly susceptible. Another resistant variety is La Brillante, and other susceptible varieties include Salinas and Salinas 88.

Those plants that are able to resist, she said, have a mechanism she termed “hypersensitive reaction” that “turns off” the disease and causes rapid death of some cells, forestalling damage to more of the plant.

Bull likened it to “chopping off your hand because it is diseased so it doesn’t affect your entire body.”

Bull also looked at strains of the disease that differed in virulence.

• Eric Natwick, University of California farm advisor in Imperial County, talked of damage caused by thrips that can affect marketability of lettuce.

He said it is most likely to occur when market value is lowest “when we can least afford it.”

Thrips can also spread diseases like spotted wilt virus.


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Natwick explained it is the larvae that cause most of the damage, though adults can disperse the damage. He said there are several effective pesticides to address thrips, including Lannate, Entrust, Radiant, Mustang and Warrior.

Of those, he said, Radiant is most effective, and there is hope of getting a California label for Torac, which he said is also “quite good.”

But Natwick cautioned that Radiant is best used in rotation with other classes of chemicals. “If you keep using the same material over and over, it is not going to work,” he said, urging growers to practice “insecticide resistance management.”

• Tom Turini, UC vegetable crops advisor in Fresno County, said research showed variable responses by lettuce varieties to fusarium wilt and tomato spotted wilt virus.

He said most romaine plaints had lower incidence of fusarium wilt, but there were also a few iceberg varieties with relatively low levels of the disease.

He said Sawa Up had no fusarium wilt in the absence of tomato spotted wilt virus.  

Lettuce dieback, weeds, wilt

• Bill Wintermantel, with USDA/ARS in Salinas, looked at lettuce dieback caused by tombusviruses.

He said contributing factors for the disease include flooding or poorly drained soil, elevated salinity, plant stress and high temperatures. Wintermantel said the movement and stability of the virus in irrigation water, along with its soil-borne nature, causes this disease to persist in soils for many years.

• Steve Fennimore, with UC Davis, studied use of metam sodium for control of lettuce drop and weeds in lettuce.

Vapam suppressed weed in one crop and lettuce drop incidence in a second crop, Fennimore said.

Fennimore also talked of thinning trials in which a machine much like a highway striper was used. The machine applied various materials – including Scythe, sulfuric acid and AN 20 to thin rows.

Sulfuric acid proved most effective, he said. He said it is not clear that the automated thinning is economically viable, remarking that improvements are needed to increase machine speed and precision.

• Krishna Subbarao, plant pathologist with UC Davis, talked of his research on verticillium wilt.

He said lettuce crops that followed three consecutive crops of spinach were more vulnerable to the disease and spinach seeds tend to be more infested with the agent that triggers the disease.

• Lindsey du Toit, vegetable seed pathologist at Washington State University, also addressed the link between verticillium wilt and spinach.

She said research showed the highest presence of verticillum wilt occurred when non-treated seed was used, whether the soil was fumigated or not, adding that fumigation can exacerbate the problem. She said seed treatments did reduce transmission.

Du Toit added that contamination of spinach seed by phomopsis has put a strain on availability of seed because “$20 million of spinach seed from more than five countries is held up at U.S. ports of entry.”

• Steven Koike, plant pathology advisor for Monterey County, talked of an evaluation of winter cover crops and their susceptibility to verticillium wilt.

He said various legume cover crops are capable of becoming infected, but cover crops in general do not seem to be particularly susceptible. He noted that infected cover crops do not show symptoms and the amount of inoculum in the soil is a likely factor in the degree of infestation.

Downy mildew, nitrogen

• Steve Klosterman, with USDA/ARS in Salinas, discussed downy mildew, which he termed the most widespread and destructive spinach disease in California.

His research has included developing a DNA-based detection program and use of a trapping system to collect airborne spores. He said it appears hot dry winds over the Salinas Valley appear to move “a blanket of spores over the region.”

Spore traps were set up in sites that included Gonzales, Soledad and King City.

• Jim Farrar, director of the Western IPM Center, talked of the center’s role in research funding.

California leads the Western region in grant dollars spent between 2004 and 2012: $702,630, followed by Arizona at $461,490. Thirty four grants were provided in each state in that period.

• Richard Smith, UC farm advisor in Monterey, San Benito and Santa Cruz Counties, looked at treatment of tile drain and surface water to remove nitrates with a denitrification bed reactor, using plastic-lined pits filled with wood chips.

While the beds did lower nitrate levels, questions remain on whether use of them is economically feasible.

To develop nitrogen credits from prior crops, Smith also studied harvest residue from spinach, head and romaine lettuce and cauliflower harvesting to measure the release of mineral nitrogen, finding that most of the nitrogen from spinach residue was available in two weeks after incorporation into the soil. Lettuce and cauliflower residues mineralized more slowly, taking six to eight weeks.

Because mineralization can occur so quickly, Smith said, care must be taken with irrigation of the succeeding crop to keep nutrients in the root zone.

Looking at drip uniformity and the impact on water and fertilizer distribution, he found that leaks, plugged emitters and low pressure were especially problematical.

“Addressing water quality regulations will require improving irrigation management,” Smith said.

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