With Verticillium wilt now infecting more than 2,200 acres of lettuce ground in California’s Salinas and Pajaro valleys, plant pathologists and breeders are intensifying efforts to bridle the disease’s rapid spread.

The fungus, Verticillium dahliae, is a costly pest of many crops, and it first appeared in coastal lettuce fields in the mid-1990s. At first, it turned up in two or three new lettuce fields each year, but in 2009 it skyrocketed, being confirmed in 43 new fields making up more than half the acreage stricken since 1995.

Identified by wilting of lower leaves at the rosette stage, followed by yellowing and dark discoloration of taproots and crowns, and death of the plant, the wilt remains indefinitely in the soil. Rates of infection range from as little as 5 percent to total crop loss. The pathogen has two races.

The outbreaks are associated with the region’s increased acreage of another host, spinach, which is harvested before the disease can claim the crop. Mysteriously, it appears in lettuce fields with and without a history of spinach.

Krishna Subbarao, University of California plant pathologist at Salinas, who has been working with the disease since its discovery in coastal lettuce, reported on his 2009 findings at the annual meeting of the California Leafy Greens Research Board (CLGRB) near Coalinga.

As first reported last fall, he learned that one new infection occurred in a field that had been fumigated with methyl bromide and chloropicrin and planted to strawberries before going into lettuce.

Several new fields again showed development of infection from a corner, suggesting introduction by equipment or personnel. However, he added, in other new fields infections were found randomly throughout.

Although rotation to small grains was previously thought to discourage the wilt, Subbarao said “a body of literature has clearly established” that vert can colonize and reproduce on several grass species, including wheat, rye, and barley, plus onion and tulips. These carriers show no symptoms as the disease builds up its inoculum in the soil.

“That means on ground infected with Verticillium, you need to choose your cover crop with care,” he said.

Sunflower, widely planted as a windbreak in coastal lettuce fields, has recently been found to be another host. Subbarao said reports indicate that sunflower seed from Argentina was linked to the spread of the disease in potato in North Dakota.

Plots of infected fields earlier pointed to three main clusters, two in the Salinas Valley and one in the Pajaro Valley around Watsonville. With new outbreaks, the clusters are closing together. Subbarao’s investigation of lettuce seed as a source of the spread has shown that vert can be carried both externally and internally. Tests thus far of commercial seed lots from the U.S., Chile, China, and the Netherlands showed levels of the fungus insufficient to trigger an infection.

But preliminary experiments during 2009 point to a possible airborne transmission. The pathogen turned up in the pollen of one spinach germplasm line. Other similar fungal species have occasionally been found on seeds of hosts, but their influence on the vert problem is not known.

Another development is a computer simulation model to predict the number of infected seeds and other conditions needed to set off the disease in lettuce.

According to the model, for an epidemic to occur a minimum of 5 percent of the seed has to be infected with vert, infected seeds have to be planted over six or seven lettuce crops, the infected plants must produce at least 3 million microsclerotia each, and at least 80 percent of the microsclerotia need to survive between seasons.

Subbarao said that shows that it takes several years of exposure to the pathogen before an outbreak can take place. He plans to use the model in the future to learn what role infected spinach seed plays where lettuce crops follow.

Lindsey Du Toit, plant pathologist at Washington State University, told the board she has screened a group of fungicides, including both conventional and organic, that reduces seedborne Verticillium in spinach. Her trials were done at the university’s research and education center at Mount Vernon. Washington and Oregon are major suppliers of spinach seed used in the U.S.

However, of the group, only Thiram had a federal label for spinach seed and Topsin had a special local needs registration for the crop in 2009. Du Toit said a product with systemic properties will be needed.

Planting non-treated seed into semi-pasteurized and non-pasteurized field soil instead of sand, she added, resulted in transmission of the pathogen as low as that of the best fungicide seed treatments.

“This demonstrates the need to evaluate the potential effects of different planting media and soil properties on seed transmission rates of V. dahliae in spinach,” she concluded.

Jim Correll, plant pathologist at the University of Arkansas in Fayetteville, who has been working with Subbarao and Du Toit, continued his greenhouse observations of the pathogenicity and virulence of the disease.

In addition to learning that lime treatments to soil appear to increase disease severity, he found that other Verticillium species and related pathogens can be recovered from spinach.

“These results indicated that Verticillium and related species associated with spinach display substantial variability in parasitic ability, virulence and pathogenicity,” said Correll.

Another plant pathologist working on the vert problem, Steven Klosterman with USDA-ARS in Salinas, described a polymerase chain reaction assay he developed for use on DNA taken from the fungus in spinach and lettuce.

The test, he explained, is not only more sensitive than the conventional plating assay in defining Verticillium, it accomplishes the job in a single day, opposed to a 10- to 14-day period. It is expected to hasten collection of information on levels of infestation of the disease in seed.

Continuing with his board project to locate plant resistance to vert in iceberg, romaine, and leaf lettuces, is Beiquan Mou, plant breeder with USDA-ARS in Salinas. During 2009 he worked on material resistant to race 1 Verticillium dahliae for release to commercial seed companies and selected 48 possible sources of resistance to race 2 from more than 600 examples.

Another part of his search for resistance was collaboration with breeders at the University of California, Davis to evaluate seed from some 200 plants from four wild Lactuca species collected in Armenia and the Republic of Georgia.