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Fifteen Years of Verticillium Wilt of Lettuce in America's Salad Bowl: A Tale of Immigration, Subjugation, and Abatement

The Near East and the Mediterranean basin are hypothesized to be the center of origin of Lactuca sativa, the cultivated lettuce. Currently, lettuce is ubiquitously cultivated as a leafy salad vegetable. Globally, the United States ranks second in lettuce production, and the central coast of California produces nearly half of that total. Until the mid-1990s, lettuce was considered resistant to Verticillium wilt, caused by Verticillium dahliae. However, in 1994, several fields on a farm in southern Santa Cruz County, Pajaro Valley, reported a loss of the entire lettuce crop to an unknown disease. Verticillium wilt was dismissed as the potential causal agent of the disease, although it was the only pathogen isolated from infected plants and V. dahliae microsclerotia were recovered from soil samples. In 1995, V. dahliae was isolated from infected plants from the same fields and Koch's postulates were completed, proving V. dahliae to be the causal agent of wilt on lettuce. Since 1995, an increasing number of lettuce fields have shown varying levels of Verticillium wilt incidence. Following the initial appearance of Verticillium wilt, it wasn't until 1999 that it was first observed on lettuce in the neighboring Salinas Valley (Monterey County), where the majority of lettuce production in the United States occurs. Verticillium wilt was first observed to the north of Salinas City in 1999; and in 2003, the disease appeared on the south end of Salinas City. By 2006, the disease was recorded at the southern end of the Salinas Valley, more than 100 km south of Salinas in San Ardo, CA. While disease reports were confined to a small number of fields, by 2009 and 2010 most disease foci coalesced, and at the preparation of this manuscript, fields in an ~50 km stretch of the prime lettuce production area had developed Verticillium wilt.