Verticillium wilt infections accelerated in lettuce fields during the 2009 season in the Salinas and Pajaro valleys of California’s Central Coast, and researchers say grass cover crops may be contributing to the spread.

Krishna Subbarao, University of California plant pathologist stationed at Salinas, reported on his investigation of the disease during a meeting in Seaside of the California Leafy Greens Research Board (CLGRB).

Verticillium dahliae, which claims a toll on several crops, first appeared in coastal lettuce in the mid-1990s, and in the following several years was most severe in two separate areas of the Salinas Valley and one in the Pajaro Valley.

According to Subbarao, it has been identified thus far in at least 104 fields comprising some 2,200 acres, some of it on prime land in the Salinas Valley.

Typically, two or three new fields became infected each year, but by 2008 the disease was found in 13 additional fields distributed across the entire Salinas Valley and near Watsonville as the former clusters of infection began to merge.

In 2009, even before the fall lettuce season was completed, it struck at 33 new fields, the most ever and an almost 50 percent increase over the confirmed fields between 1995 and 2008. Losses were complete in at least five fields and ranged from 5 percent to 60 percent in the others.

Subbarao said growers with newly infected fields may be reluctant to report them, but it would be helpful for them to let him know so he can document the sites in the effort to reduce spread of the disease.

The outbreak of the wilt has been associated with the increased acreage of spinach, which is also a host. Although symptoms of the pathogen appear when spinach plants flower and set seeds, they are not expressed when the crop is harvested and infections go unnoticed. Its inoculum, however, accumulates in the soil and can be moved by contaminated vehicular and human traffic in fields. French marigolds grown for seed on the coast are another host.

“We saw several unique features among the new fields discovered this year,” Subbarao said. It was found in both fields which had been planted to spinach and those which had not. Some fields showed randomly spaced and low incidence of infection, without the customary concentration in corners associated with vehicle or personnel movement, while others did develop infections from corners.

In a few fields, he added, the disease reappeared in lettuce that followed fumigation with methyl bromide plus chloropicrin and a crop of strawberries. “It was unlike anything we have seen before.”

Although at a low level, infections broke out in the first lettuce crop planted after fumigation. Subbarao then learned that one of the fields had been planted to a grass cover crop between the strawberries and lettuce.

“A body of literature,” he said, “has clearly established that although V. dahliae is a pathogen of dicots, it is able to colonize and reproduce on several grass species, including wheat, rye and barley, and onions and tulips that are monocots. These monocots can serve as symptomless carriers while also dramatically increasing the soil inoculum levels.”

That means, he suggested, growers with vert-infected ground should “think hard before they plant a grass cover crop before planting lettuce.”

Subbarao is continuing his investigation of commercial seed lots of lettuce cultivars used for salad mixes. Included in the findings was an instance of a vert infection appearing even when soil infestation was too low to trigger extensive development of the disease, suggesting a possible airborne spread.

Using data collected in previous research, Subbarao and his team have developed a simulation model to show how many infected lettuce seed and what other conditions are required to trigger the pathogen. Further use of the model will also hopefully provide information on the role of infected spinach seed in following lettuce crops.

The group has not found a definite cause of the spread of wilt in lettuce during the past 15 years. However, Subbarao said there might be three: a local adaptation of the disease toward lettuce, a sudden increase in population numbers in the area, or recurrent introductions of the pathogen into the area.

In another project funded by the board on vert wilt, Subbarao collaborated with Steve Koike, Monterey County farm advisor, and Lindsey Du Toit, plant pathologist at Washington State University, Mount Vernon, Wash.

The overall purpose of their research is to develop management guidelines for the disease in lettuce and spinach.

Specifically, they have been searching for a standardized test for the pathogen in spinach seed. Du Toit, who has a background in spinach seed production, explained that when they sent samples to various laboratories, the results were quite variable.

“This really highlighted the need to come up with a standardized assay,” she said, adding that the disease shows up on spinach seed, regardless of where it is produced.

They have evaluated three types of assays, and although each has advantages, no single one solves all the issues of sensitivity, time required, and possible influence on seed treatments. Du Toit said plans are to hold a demonstration of the three types for the spinach seed industry in the spring of 2010.

In a separate project, in 2008 Du Toit compared seed treatments for seedborne Verticillium. The top three fungicide treatments were Topsin M 70 WP, Mertect 340F (alone or with Farmore D300), and BAs 595 XG F. Also highly effective were three proprietary organic treatments: Seedgard, Seed Support I, and Seed Support II.

“Very few of the treatments evaluated in this trial,” Du Toit said, “were registered for use on spinach seed in the U.S. in 2008.”

However, she added, Topsin 70 WP was granted a Special Local Needs 24(c) seed treatment registration by the Washington State Department of Agriculture for management of vert in spinach crops in that state in 2009.

Work on breeding for resistance to the wilt continues at the USDA station in Salinas. In lettuce, scientists are working with the gene identified for resistance to Race 1 of the disease. They are also screening a selection of lines that have resistance to Race 2.

Meanwhile, using selections from USDA’s spinach germplasm collection and nine commercial cultivars, breeders are sorting plants showing resistance traits.