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First Report of Onion (Allium cepa) Naturally Infected with Iris yellow spot virus in Peru

Onions have become an important export crop for Peru during the last few years. The onions produced for export are primarily short-day onions and include Grano- or Granex-type sweet onions. The first of two growing seasons for onion in Peru occurs from February/March until September/October and the second occurs from September/October to December/January. Iris yellow spot virus (IYSV [family Bunyaviridae, genus Tospovirus]), primarily transmitted by onion thrips (Thrips tabaci), has been reported in many countries during recent years, including the United States (1,2). In South America, the virus was reported in Brazil during 1999 (3) and most recently in Chile during 2005 (4). During 2003, an investigation of necrotic lesions and dieback in onions grown near the towns of Supe and Ica, Peru led to the discovery of IYSV in this region. Of 25 samples of symptomatic plants collected from five different fields near Supe, 19 tested strongly positive and an additional three tested weakly positive for IYSV using double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA) (Agdia Inc., Elkhart, IN). None of the samples tested positive for Tomato spotted wilt virus (TSWV). A number of onions with necrosis and dieback symptoms were also observed during 2004 and 2005. During September 2005, 25 plants with symptoms suspected to be caused by IYSV or TSWV in the Supe and Casma valleys were collected and screened for both viruses using DAS-ELISA. All plants screened were positive for IYSV. There was no serological indication of TSWV infection in these samples. The positive samples were blotted onto FTA cards (Whatman Inc., U.K.) to bind the viral RNA for preservation and processed according to the manufacturer's protocols. The presence of IYSV was verified by reverse transcription-polymerase chain reaction (RTPCR) using (5′-TCAGAAATCGAGAAACTT-3′) and (5′-TAATTATATCTATCTTTCTTGG-3′) as forward and reverse primers (1), respectively. The primers amplify the nucleocapsid (N) gene of IYSV, and the RT-PCR products from this reaction were analyzed with gel electrophoresis with an ethidium bromide stain in 0.8% agarose to verify the presence of this amplicon in the samples. Subsequent to the September 2005 sampling, 72 additional samples from regions in northern and southern Peru were analyzed in the same manner. The amplicons obtained were cloned, sequenced, and compared with known IYSV isolates for further verification. Onions have become a significant export crop for Peru, and more research is needed to determine the impact of IYSV on the Peruvian onion export crop. To our knowledge, this is the first report of IYSV in onion in Peru.

References: (1) L. du Toit et al. Plant Dis. 88:222, 2004. (2) S. W. Mullis et al. Plant Dis. 88:1285, 2004. (3) L. Pozzer et al. Plant Dis. 83:345, 1999. (4) M. Rosales et al. Plant Dis. 89:1245, 2005.