Mustard cover crops tested for lettuce disease control

Mustard cover crops appear to be a positive cultural practice and should be evaluated for their potential as a means of reducing lettuce head drop disease and weeds in coastal California counties.

That is the conclusion of Richard Smith, Monterey County farm advisor, after a series of trials with commercial varieties of white and Indian mustard species planted as covers between lettuce crops.

Smith, whose studies were funded by the California Lettuce Research Board, said lettuce drop, caused by the fungus, Sclerotinia minor, is the primary soilborne disease of lettuce in the Salinas Valley.

There has been a flurry of interest among growers in the valley in mustard crops as a cover crop to provide control of head drop and weeds. Many research projects around the world, Smith noted, have shown mustards develop glucosinolate compounds that are antagonistic to disease and weed pests.

In reporting on his project to the board’s recent meeting in Seaside, Smith said he recently planted short- and long-range term crop trials with BQMulch, from New Zealand. “It is a variety of Brassica napus that had a very dramatic effect in controlling a high population of S. minor in a trial in Tasmania, Australia.”

Glucosinolates reside mostly in the roots of this variety, and Smith said this may improve the efficiency of release of the compounds in the soil. In the mustard covers, the chemicals are in the plant shoots, and stands must be incorporated into the soil before the decomposing matter can release the chemicals.

Hopefully, he added, future cover crop varieties will be developed for improved glucosinolate content.

Sclerotinia impact

In detailing his final results from the Indian and white mustard trials, Smith said they have “a slight but significant impact on Sclerotinia.” He also saw some disease reduction with covers of Merced rye used as a comparison.

He also recorded a reduction in weeds in two of the short-term trials at thinning time. However, in the long-term trials, none of the cover crops reduced weeds at thinning and there was no significant reduction of weed seed in the soil seedbank after two cover crop cycles.

All the cover crop plots (including the two mustards and Merced rye) for 2004 and 2005 showed improved head weight yields over the fallow, untreated check plots.

Michael Cahn, Monterey County farm advisor, told the board his project on irrigation of lettuce demonstrated that germination with drip can be improved on a fine, sandy loam soil with water treated with polyacrylamide (PAM) and discharged at a high rate thorough the tubing.

PAM compounds are used in a variety of ways, from wastewater treatment to processing of fruits and vegetables and manufacture of cosmetics.

Sprinklers still dominate

Despite the expanding use of drip in lettuce crops in the Salinas Valley during the past 10 years, overhead sprinklers are still the primary method to germinate a crop.

Some growers have attempted to use drip, he said, but it is often unreliable because of difficulty in moving moisture from the tape to the seed line.

Use of drip for the entire growing cycle could save water, cut labor costs, reduce runoff, and perhaps avoid diseases carried by splashing water.

“Our project evaluated strategies to increase the lateral movement of moisture, including using polyacrylamide to increase the viscosity of the water and thereby decreasing infiltration.”

Using tape discharge rates of 0.22, 0.45, and 0.67 gal/min/100 ft., Kahn found that at the high rate lateral movement of water and germination was improved.

The addition of PAM did not improve lateral movement in commercial, surface and subsurface drip fields on Clear Lake clay and Salinas clay loam soils, which had bulk density from 0.86 to 1.03 g/cc.

However, he added, “PAM improved the lateral movement of moisture in a commercial lettuce field located on Pico fine sandy loam with beds that had a bulk density greater than 1.2 g/cc.”

He said the project also showed that total water use could be reduced sharply with drip for germination. Drip for the entire crop used 10 to 13 inches, while sprinklers used 32 inches. Run-off was eliminated with drip.

The greatest benefit of germination by drip, Cahn concluded, would be on decomposed granite soils that allow run-off when sprinklers are used.

Lettuce weeds

In his continuing work for the board on new weed management strategies in lettuce, Steve Fennimore, University of California Extension weed specialist at Salinas, screened sulfonylurea herbicides for their effects on a butterhead lettuce germplasm.

His objective is to find herbicides that are compatible with current production practices and do not cause serious injury to the germplasm, ID-BR1, developed through conventional breeding from prickly lettuce and Bibb lettuce at the University of Idaho.

Using Kerb as a standard, he found that ID-BR-1 was tolerant, with no stand reductions, to pre-emergence treatments of Maverick and Upbeet. Maverick was effective on most weeds, but Upbeet was less effective.

For postemergence materials applied at the three to four-leaf stage, Harmony appears to be a good possibility, and Fennimore said he likes it because it has a very short soil residual activity of less than 10 days, eliminating rotational restrictions.

Some stand reduction was seen with Express, and it will probably be discarded from trials.

Fennimore evaluated the Valent experimental, V-10142, and although it was shown to be safe on lettuce in 2004, trials during 2005 proved the opposite. Stinger was found to be harmful to lettuce in the same trials.

Kerb by chemigation

Trials by Fennimore and others on chemigation with Kerb through sprinklers indicate it may have promise. The method has been successful in the Yuma area, and they are evaluating it for other areas. Laboratory studies are focused on factors of soils and organic matter associated with the herbicide’s mobility.

“None of the Salinas Valley applications demonstrated any difference between applications of Kerb by chemigation or ground,” he said.

Trials in western Fresno County are in progress, although preliminary results indicated chemigation gives some improved control compared to ground applications.

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