California is the leading producer of lettuce (Lactuca sativa) for the United States and grows 77% of the country's supply. Prior to 2006, coastal California lettuce was only periodically and incidentally infected by a single tospoviruses species: Tomato spotted wilt virus (TSWV). However, beginning in 2006 and continuing through 2012, severe outbreaks of disease caused by Impatiens necrotic spot virus (INSV) have affected the coastal lettuce crop, though TSWV was also present. In contrast, TSWV was the only tospovirus associated with disease outbreaks in Central Valley lettuce during this period. Disease surveys conducted over two seasons (2008 and 2009) in 10 commercial fields (acreage of 6 to 20 ha) indicated that INSV was the only tospovirus associated with economically damaging disease outbreaks in lettuce in the coastal region, with incidences of 0.5 to 27% (mean = 5.7%). 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 from different locations in the United States and the world. Monitoring of thrips revealed moderate to large populations in all surveyed lettuce fields, and the majority of thrips identified from these fields were western flower thrips, Frankliniella occidentalis. There was significant positive correlation (r2 = 0.91, P = 0.003) between thrips populations and INSV incidence in the most commonly encountered type of commercial lettuce (romaine, direct seeded, conventional) included in this study. A reverse-transcription polymerase chain reaction assay developed for detection of INSV in thrips showed promise as a monitoring tool in the field. Surveys for INSV reservoir hosts in the coastal production area revealed that the weeds little mallow (Malva parvifolia) and shepherd's purse (Capsella bursa-pastoris) were commonly infected. M. parvifolia plants infected in the field did not show obvious symptoms, whereas plants of this species inoculated in the laboratory with INSV by sap transmission developed necrotic spots and chlorosis. Eleven other weed species growing in the lettuce production areas were found to be hosts of INSV. Coastal crops found to be infected with INSV included basil (Ocimum basilicum), bell pepper (Capsicum annuum), calla lily (Zantedeschia aethiopica), faba bean (Vicia faba), radicchio (Cichorium intybus), and spinach (Spinacia oleracea). Thus, it is likely that INSV was introduced into coastal California lettuce fields via viruliferous thrips that initially acquired the virus from other local susceptible plant species. Results of this study provide a better understanding of INSV epidemiology in coastal California and may help growers devise appropriate disease management strategies.
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