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Vegetable growers helped with damaging thrips-vectored viruses

The Issue

During the past few years, vegetable growers in California’s central coast have seen their crops affected by outbreaks of a mysterious virus disease. For crops such as lettuce, pepper and basil, these problems were new and caused significant losses in quality and yield. The virus outbreaks were particularly extensive in lettuce, with numerous fields affected in a number of counties (Monterey, San Benito, Santa Cruz). For the Salinas Valley, this problem developed in fields in the north (Salinas, Chualar, Gonzales) as well as in the south (Soledad, Greenfield, King City). Disease losses ranged from minimal (less than 1 percent) to over 65 percent. Affected lettuce plants were stunted, yellowed, and developed extensive brown, dead spots and lesions that rendered the plant unmarketable. Iceberg, romaine, leaf and butterhead lettuce were all affected. Resistant lettuce cultivars are not currently available. In addition to this coastal situation, similar symptoms have been observed in lettuce grown in the San Joaquin Valley.

What Has ANR Done?

A collaborative research team consisting of county-based farm advisors (Steven Koike, Tom Turini, Richard Smith) and campus-based researchers (Bob Gilbertson, Yen-Wen Kuo) initiated a multi-year investigation of the problem. They discovered that the disease was caused by the Impatiens necrotic spot virus (INSV), a virus that had not previously been documented on lettuce. Similar disease outbreaks on other crops were also caused by INSV. Because this virus is spread to plants by thrips insects, the team conducted intensive surveys to collect and identify which thrips were vectoring the virus. They discovered that the western flower thrips (Frankliniella occidentalis) was virtually the only thrips found on lettuce in the Salinas Valley. The campus-based team members developed molecular tests that confirmed that these thrips contained INSV. The San Joaquin Valley problem was attributed to another thrips-vectored virus, Tomato spotted wilt virus (TSWV), which has long been known to occur in California. These California findings support the worldwide trend in which this group of thrips-vectored viruses (the tospoviruses) is becoming more important, and is spreading into previously unaffected crops, such as lettuce.

The Payoff

UC researchers document new virus problem in lettuce.

The UC team was able to rapidly confirm that the virus outbreaks on lettuce and other coastal crops were due to INSV. This information enabled farmers and pest control advisors to specifically target control of western flower thrips in their integrated pest management programs. The thrips survey information revealed that certain flowering plants such as iceplant, alyssum, and other groundcovers were significant sources of thrips. In some cases, growers would be advised to remove such plantings if INSV problems are persistent in the area. The new finding of INSV on lettuce enabled other researchers to initiate breeding programs that could eventually develop INSV-resistant lettuce cultivars. Finally, the UC results served an important forensics function. Because INSV symptoms closely resemble burn damage from pesticide or fertilizer applications, spray companies were being blamed for the problem. The finding that INSV was causing the damage dismissed such concerns.


Supporting Unit:

Monterey and Fresno counties, UC ANR Cooperative Extension, and UC Davis Department of Plant Pathology.
Steven T. Koike and Richard Smith, UCCE Farm advisors, (831) 759-7350, stkoike@ucdavis.edu, rifsmith@ucdavis.edu

Tom Turini, UCCE farm advisor, (559) 456-7157, taturini@ucdavis.edu

Bob Gilbertson, professor, (530) 752-3163, rlgilbertson@ucdavis.edu.