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United States Department of Agriculture

Agricultural Research Service

Research Project: Strategies to Improve Soil and Pest Management in Organic Vegetable Production Systems Title: Characterization and epidemiology of outbreaks of Impatiens necrotic spot virus on lettuce in coastal California

Authors
item Kuo, Yen-Wen -
item Gilbertson, Robert -
item Turini, Tom -
item Brennan, Eric
item Smith, Richard -
item Koike, Steven -

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 20, 2014
Publication Date: August 1, 2014
Citation: Kuo, Y., Gilbertson, R.L., Turini, T., Brennan, E.B., Smith, R.F., Koike, S.T. 2014. Characterization and epidemiology of outbreaks of Impatiens necrotic spot virus on lettuce in coastal California. Plant Disease. 98:1050-1059.

Interpretive Summary: Since 2006, lettuce in the coastal region of California has been affected Impatiens necrotic spot virus. Disease surveys conducted in 2008 and 2009 confirmed that INSV was the predominant tospovirus associated with disease outbreaks in lettuce in the coastal region and revealed incidences as high as 27%. This virus can be spread by thrips insects. Monitoring of thrips revealed moderate to high populations in all surveyed lettuce fields, and the majority of thrips present in these lettuce fields were western flower thrips. There was significant positive correlation between thrips numbers and INSV incidence in seeded, conventional romaine lettuce. It is likely that INSV was introduced into lettuce fields via thrips that initially acquired the virus from local sources such as weeds. This study provides information that can help growers devise disease management strategies for INSV in lettuce in California.

Technical Abstract: Beginning in 2006, outbreaks of diseases caused by tospoviruses in lettuce have occurred in California with greater frequency. Lettuce in the coastal region has been affected primarily by Impatiens necrotic spot virus (INSV), though Tomato spotted wilt virus (TSWV) was also present. In contrast, TSWV was the only tospovirus associated with disease outbreaks in Central Valley lettuce. Disease surveys conducted in 2008 and 2009 confirmed that INSV was the predominant tospovirus associated with disease outbreaks in lettuce in the coastal region and revealed incidences as high as 27%. Molecular characterization of INSV isolates associated with these disease outbreaks revealed little genetic diversity and indicated that lettuce-infecting INSV isolates were nearly identical to those previously characterized from ornamental or other hosts. Monitoring of thrips revealed moderate to high populations in all surveyed lettuce fields, and the majority of thrips present in these lettuce fields were western flower thrips (WFT), Frankliniella occidentalis. There was significant positive correlation between thrips numbers and INSV incidence in seeded, conventional romaine lettuce. An RT-PCR assay was developed that allowed for detection of INSV in thrips, and this assay shows promise as a monitoring tool in the field. Surveys for other INSV hosts revealed that the weeds little mallow (Malva parvifolia) and shepherd’s purse (Capsella bursa-pastoris) were commonly infected with INSV in the coastal area. Naturally field-infected M. parvifolia did not show obvious symptoms, though little mallow plants inoculated in the laboratory with INSV by sap transmission developed necrotic spots and chlorosis. A number of other weeds were symptomless INSV hosts but were infrequently detected, whereas symptomatic infections were detected in commercial plantings of basil, bell pepper, calla lily, fava bean, radicchio, and spinach. Thus, it is likely that INSV was introduced into lettuce fields via viruliferous thrips that initially acquired the virus from local sources. Results of this study provide a better understanding of INSV epidemiology in coastal California and can help growers devise disease management strategies.

Last Modified: 11/24/2014
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