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New routes tested for lettuce disease control

New approaches for control of two of the California lettuce industry's major disease pests, lettuce drop and lettuce wilt, are being evaluated by University of California researchers.

Krishna Subbarao, UC plant pathologist based at Salinas and project team leader, described the new trials in a recent report at Seaside to the California Lettuce Research Board.

Lettuce drop is caused by the fungal pathogens Sclerotinia minor, mainly along the coast, and S. sclerotiorium in other areas of the state. Infected plants develop a brown, soft decay that eventually destroys the crown.

Lettuce wilt, brought on by the fungus Verticillium dahliae that attacks several other crops, has been expanding in coastal fields since 1995. Plants infected with it develop yellow outer leaves, wilt, and die, and they show a discolored streak in vascular tissue of the tap root.

Subbarao noted that trials to control S. minor with Contans, a Sylvan Bioproducts formulation containing the fungus Coniothyrium minitans, were unsuccessful first in tests in the Salinas area a decade ago and later in the Imperial Valley.

However, the manufacturer's research on both Sclerotia pathogens in France showed encouraging results, and Subbarao, with funding from the company, began evaluating a new, four-step approach with the product this fall in Salinas Valley fields.

The trial includes treatments immediately after planting, one week before thinning, one week after thinning, and finally, before disking under the crop.

Subbarao said he is particularly interested to see how Contans will perform before disking when the disease is in its mycelial stage and most vulnerable to the beneficial fungus.

Meanwhile, Subbarao and his colleagues continued CLRB-supported research on lettuce drop, including gauging S. minor populations before and after the advent of use of the fungicide Endura to compare them for potential resistance to that product.

He is also collaborating with USDA plant breeder Ryan Hayes to develop sources of plant resistance to S. minor.

Subbarao earlier found that populations of both Sclerotinia species were increased on 80-inch bed widths under biweekly irrigation when compared with 40-inch beds under the same irrigation frequency. The disease responds to the additional moisture from the wider pattern.

“Thus far,” he said, “by our measurements, it looks like the airborne phases of S. sclerotiorum are encouraged by this practice.”

Subbarao warned that his surveys of commercial fields in the southern part of the Salinas Valley show that fields infected by airborne infections by S. sclerotiorum are “incrementally increasing.”

His project on monitoring Verticillium wilt, Subbarao said, shows it is was quite active this year and continues to spread. The fungus has a wide range of host crops, including several grown in the Salinas and San Joaquin valleys.

While other on-going research continues, he recently began a new avenue with greenhouse studies on Syngenta's Actigard, a systemic compound that induces host plant resistance by mimicking the resistance response found in most plant species.

The product is registered for lettuce, and if found to be successful, it could be immediately adapted to Verticillium wilt control in commercial lettuce fields, he said.

Other parts of the Verticillium research identified eight new fields, struck by losses of 30 percent to 80 percent, and soil assays are being done on each of them.

The disease was previously confined to fields west of Highway 101, but this year, a new field was found east of the highway. During the past 10 years the wilt has appeared in three clusters, one near Watsonville and two near Salinas.

Subbarao has been observing fields with high levels of the disease to determine the efficacy of fumigation with methyl bromide plus chloropicrin. Depending on the crops grown, the fungus is more active at certain soil depths. Lettuce leads to high populations, while cabbage reduces them.

The only means of preventing recolonization of fumigated soil by the wilt's microsclerotia is fallowing, which is not practical for coastal California fields.

On the plant resistance front, he said research continues on the second race of Verticillium on lettuce discovered in mid-2005. Progress has been made by USDA breeders searching for potential plant resistance to Race 1, but none has been found for Race 2.

A new test to rapidly distinguish the two races has been developed by USDA scientists, and a wilt-infested field near the USDA station at Salinas is being used exclusively for uninterrupted resistance evaluation.

In his research for the CLRB on another lettuce disease that is spreading, Fusarium wilt, Thomas R. Gordon, plant pathologist at UC Davis.

Fusarium has been most severe in California's desert growing region and Arizona, likely because higher temperatures favor its development. Growers have also found it in the Salinas and San Joaquin valleys during warmer planting periods.

Using a Fusarium-infested field at the UC, Davis campus, Gordon conducted trials where the disease inoculum is uniformly distributed. Trials were planted June 27, July 25 and Sept. 9. Disease progression, he said, was most rapid in the first planting and slowest in the third.

“This correlated with mean temperatures, which were highest following the June planting and progressively lower thereafter.”

He went on to say the results confirmed prior research in Japan and “these effects on temperature are potentially important as they could allow growers minimize damage by selecting appropriate planting windows in areas where the disease is present.”

Most head lettuce varieties are susceptible, but varieties such as Salinas and Grand Max are relatively resistant, and higher levels of resistance are seen in some leaf and romaine varieties.

He said he has learned that the only practical way soil populations of the disease can be reduced is by crop rotation.

“In order to determine how much time is required for a meaningful reduction in inoculum to occur, we are studying the durability of pathogen propagules in microplots of naturally infested soil.”

For example, he found that a sample of 3,482 propagules per gram of soil declined after seven months to 1,022, a reduction of about 70 percent. After an additional nine months, the count was down to 285 or about 8 percent of the original level.

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