Impatiens necrotic spot virus (INSV; family Bunyaviridae, genus Tospovirus) is an important pathogen of ornamental plants in North America and Europe, particularly in the greenhouse industry (2,3). However, INSV is now emerging as a pathogen of vegetable crops. During the 2006 and 2007 growing seasons, lettuce (Lactuca sativa) in Monterey County, CA showed necrotic spotting, leaf chlorosis, and plant stunting typical of symptoms induced by Tomato spotted wilt virus (TSWV). Significant and damaging outbreaks of these disease symptoms were found in numerous romaine, greenleaf, redleaf, butterhead, and iceberg lettuce fields in Monterey and San Benito counties. Samples from symptomatic plants from 21 of 27 fields in Monterey County were negative when tested with TSWV immunostrips (Agdia, Elkhart, IN); however, tests of the TSWV-negative samples with INSV immunostrips were positive. In most fields where INSV was detected, disease development was limited to the edges of fields and disease incidence was <5%; however, some fields had incidences >50% and crop loss was experienced. The virus causing the tospovirus symptoms in the TSWV-negative lettuce was sap transmitted to Nicotiana benthamiana and lettuce, where it induced chlorosis and necrosis. Symptoms in N. benthamiana were consistent with INSV infection, and those in lettuce were similar to symptoms observed in the field. Immunostrip tests confirmed that symptomatic N. benthamiana and lettuce plants were infected with INSV. To further confirm the identity of this virus, reverse transcription (RT)-PCR analysis was conducted with an INSV primer pair that directs the amplification of a ~1.3-kb fragment from the small RNA of INSV (4). The 1.3-kb fragment was amplified from RNA from symptomatic lettuce plants that were INSV positive with immunostrips, and not from asymptomatic lettuce. A total of 38 of 54 samples showing tospovirus-like symptoms were confirmed to be infected with INSV by RT-PCR. Sequences of two representative 1.3-kb DNA fragments were 98 to 99% identical with sequences of INSV isolates from Japan, Italy, and The Netherlands (GenBank Accession Nos. AB109100, DQ425096, and X66972). Taken together with the previous identification of the INSV vector, the western flower thrips (Frankliniella occidentalis), in central California lettuce (1), these results confirm that INSV induced tospovirus symptoms in lettuce fields in Monterey County in 2007. To our knowledge, this is the first report of the occurrence of INSV infecting lettuce in California.
References: (1) W. E. Chaney. Annu. Rep. California Lettuce Res. Board. 2006. (2) M. Daughtrey et al. Plant Dis. 81:1220, 1997. (3) M. D. Law and J. W. Moyer. J. Gen. Virol. 71:933, 1990. (4) R. A. Naidu et al. Online publication. doi: 10.1094/PHP-2005-0727-01-HN. Plant Health Progress, 2005.
