Mutant lettuce lines show glyphosate resistance

University of California weed scientists have discovered several hundred lines of lettuce germplasm that show moderate resistance to glyphosate herbicide and suggest potential avenues for simpler and more effective weed control.

The findings were announced by John Rachuy, UC, Davis staff research assistant stationed at Salinas, during the recent annual reports of the California Lettuce Research Board (CLRB) at a meeting near Coalinga, Calif.

The project, led by Steve Fennimore, UC, Davis weed scientist, in collaboration with other UC specialists, is working with herbicide-resistant lettuce material developed solely by classical breeding techniques and not by genetic modifications.

Rachuy delivered the report prepared by Fennimore, who was abroad at the time, and said they first sought lettuce plants resistant to Caparol herbicide used on celery. After screening seedlings of 3,000 lines they found no examples that survived exposure to Caparol.

However, Rachuy added, the greenhouse project recently identified several hundred mutant lines having moderate resistance to glyphosate, the active ingredient in Roundup, as verified by sophisticated molecular marker tests at UC, Davis.

In other continuing research projects Fennimore’s team pursued during 2006, Rachuy said their 2006 trials showed Kerb herbicide applied by chemigation via surface or buried drip tape to have a greater than tenfold effect in controlling shepherd’s purse and total weed densities as opposed to spraying the material and irrigating by sprinkler.

“The Kerb applied by chemigation,” Rachuy said, “caused no decrease in lettuce stand and none of the Kerb treatments, regardless of method of application, injured the crop.” They observed no differences in lettuce yields.

In addition to using less water, he added, the chemigation method eliminates labor costs associated with sprinkler line movement.

Kerb via chemigation has a 24-C registration in Riverside, Imperial, and Fresno counties, and Dow AgroSciences officials say they hope the registration will be expanded throughout the state later this year.

In a separate but related trial with drip irrigation near Salinas, the researchers learned that counts of nettle and other weeds were less with Kerb or Prefar applied though lines buried at 1 inch than with those buried at 2 inches.

In examining the soil types best suited for Kerb chemigation, Fennimore’s laboratory studies have indicated that the herbicide is less mobile, and stays closer to the weed seed germination level, in Chualar loam, Salinas clay, Guadalupe clay-loam, and Cropley series soils.

Updating the CLRB on his research on Sclerotinia species, the fungal causes of lettuce drop, Krishna Subbarao, UC, Davis plant pathologist based at Salinas, warned that 80-inch lettuce beds irrigated twice a week are increasing disease potential.

Pointing to recent soil assays, he said they show “this treatment returns a significantly greater number of sclerotia to the soil.

“By every metric measured under experimental conditions, it is clear that the establishment of the airborne phase of S. sclerotiorum in the Salinas Valley is a distinct possibility.”

Subbarao added that data from surveys of commercial fields during the past five years suggest that in the lower part of the Salinas Valley (below 36.41 latitude) the number of lettuce fields with the airborne S. sclerotiorum “is incrementally increasing.”

In his research with methods to curb S. minor, Subbarao is continuing work begun last fall with Contans, a biological product from Sylvan Bioproducts containing the fungus Coniothyrium minitans.

Although the material was unsuccessful in evaluations in the Salinas Valley ten years ago and later in the Imperial Valley, Sylvan’s recent research in France shed new light on timing to realize Contans’ potential.

In trials funded by Sylvan and its marketers, Subbarao is evaluating a four-step treatment with Contans applied immediately after planting, at one week before thinning, at one week after thinning, and finally just before disking the crop residue.

“Since the mycelial state of the pathogen is the most susceptible phase for attack by C. minitans, the final application is expected to lower the number of sclerotia formed on infected plants,” he said.

Other dimensions of Subbarao’s continuing investigations of Sclerotinia include studies on the potential of S. minor to develop resistance to the fungicide Endura and collaboration with USDA breeders to screen for resistant germplasm.

Some new germplasm examples, he said, have “slow-dying” resistance to S. minor, collapsing after two weeks after becoming infected, while susceptible plants fail after only two days.

Another serious lettuce disease, Verticillium wilt, caused new alarm in the Salinas Valley in 2006, he said, when it appeared for the first time in one field east of Highway 101 and in another near King City.

In all, the wilt, which was first found in clusters in the Pajaro Valley and the northern part of the Salinas Valley, showed up in nine new lettuce fields last year, having incidence of from 30 to 80 percent. It was later detected in unknown amounts in five other fields.

Although resistance to Race I of the wilt exists and breeders are incorporating it in new germplasm, none is yet available for Race II. Subbarao is working with UC colleagues and area growers to first develop tests to distinguish the two races and then search for resistance to Race II.

One of his difficulties in field screening materials for resistance is finding a field that has not been fumigated where the disease can be observed.

However, Subbarao said, the plant defense elicitor Actigard (also known as Blockade) may have potential against the wilt in lettuce. The Syngenta product is in the class of compounds that stimulate the crop’s natural defense system against fungal, bacterial or viral pathogens. It has been promising in other hosts and diseases.

If Actigard, which is already registered for use in lettuce, is effective, he noted, “then it could be immediately integrated into current production practices on commercial lettuce varieties.”

His greenhouse studies thus far on transplants of the cultivars Salinas and Pacific showed that the material applied at two, four and six weeks after transplanting was not significantly different from controls. He is continuing evaluations with earlier and more frequent applications.

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