Iris yellow spot virus in Onion in Nevada and Northern California.

The disease caused by thrips-transmitted Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae) has become a major constraint to bulb and seed onion crops in several parts of the country and the world (1,3). As part of an ongoing survey for IYSV incidence in onion in the western United States, commercial fields in Lyon County, Nevada and several commercial fields in the northern Californian counties of Colusa, San Benito, Sutter, and Yolo were surveyed during the summer of 2008. Symptomatic plants were found widespread in northern California, especially in seed-production fields. In Lyon County, NV, symptoms were observed only on volunteer onions in one commercial field. Symptoms on leaves and scapes included characteristic diamond-shaped lesions with or without green islands. Four samples from Nevada and fourteen from northern California were tested by double-antibody sandwich (DAS)-ELISA using a commercially available kit (Agdia Inc., Elkhart, IN). All tested samples were found positive in ELISA. IYSV infection was verified by reverse transcription (RT)-PCR. Total nucleic acids were prepared from symptomatic tissue, and primers specific to the small (S) RNA of IYSV were used to amplify an approximate 1.2-kb region of the S-RNA. This region included the complete nucleoprotein (N) gene (2). The amplicons from one sample each from Nevada and northern California were sequenced (GenBank Accession Nos. FJ713699 and FJ713700, respectively). Sequence analysis showed that the amplicons contained a single open reading frame of 822 bp, coding for a 273-amino acid N protein, and the gene shared 96 to 98% identity with known IYSV N gene sequences. To our knowledge, this is the first report of IYSV in onion in Nevada. In California, outbreaks of IYSV had been reported earlier in Imperial Valley and Antelope Valley in southern California (4), and the disease has been increasing in incidence in bulb and seed crops in northern California, as well. California and Nevada are major onion-producing states in the United States and regular surveys to determine the incidence and impact on yield are needed to develop an integrated disease management program. References: (1) D. H. Gent et al. Plant Dis. 90:1468, 2006. (2) H. R. Pappu et al. Arch. Virol. 151:1015, 2006. (3) H. R. Pappu and M. E. Matheron. Online publication. doi:10.1094/PHP-2008-0711-01-BR. Plant Health Progress, 2008. (4) G. J. Poole et al. Online publication. doi:10.1094/PHP-2007-0508-01-BR. Plant Health Progress, 2007.