First Report of Natural Infection of Garlic with Iris yellow spot virus in the United States 

S. Bag, Department of Plant Pathology, Washington State University, Pullman, WA 99164;  P. Rogers and R. Watson, Nunhems USA, 8850-59th Ave NE, Brooks, OR 97305, and H.R. Pappu, Department of Plant Pathology, Washington State University, Pullman, WA 99164. 

Iris yellow spot virus (IYSV, family Bunyaviridae, genus Tospovirus) is an important constraint to onion bulb and seed production in several onion growing regions of the United States (1, 3). While garlic (Allium sativum) was reported to be infected with IYSV in Réunion Island (4), there have been no confirmed reports of natural infection of garlic in the USA.  Garlic plants showing near-diamond shaped lesions were found in August 2008 in Marion County, Oregon. The one-acre field plot consisted of various true-seeded garlic varieties and was adjacent to three onion fields which showed IYSV symptoms. Symptoms were observed on 5% of the garlic plants with most of the symptomatic plants displaying small and diffuse straw-colored spots.  Seven of these symptomatic plants were selected for testing.  Of these, two showed characteristic diamond-shaped, elongated straw-colored lesions on garlic scapes. However, the lesions were more diffuse with less defined edges compared to the characteristic diamond-shaped lesions that are often associated with IYSV infection (1). All the symptomatic plants were positive for IYSV when tested by DAS-ELISA using a commercially available kit (Agdia Inc., Elkhart, IN). To verify IYSV infection, total nucleic acid extracts from the symptomatic parts of the leaves were prepared and tested for the presence of IYSV using reverse transcription-PCR (RT)-PCR with primers 5’TAA AAC AAA CAT TCA AAC AA 3’ and 5’CTC TTA AAC ACA TTT AAC AAG CAC 3’ which flank the nucleocapsid (N) gene coded by the small RNA of IYSV (2).  An amplicon of the expected ca. 1.2 kb was obtained from all symptomatic plants which was cloned and sequenced.  Nucleotide sequence comparisons using BLAST showed that a consensus of three clones derived from the amplicon from garlic (Accession No. FJ514257) was 85% to 99% identical with IYSV sequences available in GenBank (AF001387, AB180918, AB286063) confirming the identity of IYSV. This is the first report of natural infection of IYSV infection of garlic in the USA. Additional surveys and testing are needed to obtain a better understanding of IYSV incidence in garlic in order to evaluate its impact on garlic production.  

References(1
)   D. Gent et al. Plant Dis. 90:1468, 2006.
(2)   H.R. Pappu et al., Arch. Virol. 151:1015, 2006.
(3)   H.R. Pappu et al., Virus Res. 141:219, 2009.
(4)   I. Robène-Soustrade et al. Plant Pathol. 55:288, 2006.