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First Report of Iris yellow spot virus in Spiny Sowthistle (Sonchus asper) in the United States

Iris yellow spot virus (IYSV) is a member of the genus Tospovirus in the family Bunyaviridae. Its known host range is very limited, and the currently known hosts include onion, leek, lisianthus, and alstroemeria (2). The virus is vectored by onion thrips (Thrips tabaci). Onion (Allium cepa) is grown as a winter crop in Georgia from September to April and is the only known host commercially grown in the region. However, the virus has been found across the onion-growing region in the state every year since its first occurrence during 2003 (3). Consequently, the virus must oversummer in other host(s) or its insect vector. Accordingly, samples of weeds were collected in the vicinity of onion fields and cull piles in the Vidalia region and tested for the presence of IYSV by a double-antibody sandwich (DAS)-ELISA (Agdia, Inc., Elkhart, IN). One of three nonsymptomatic spiny sowthistle samples tested positive by ELISA for IYSV. Total RNA was extracted from the leaf using the RNeasy Plant Mini Kit (Qiagen, Valencia, CA) following the manufacturer's protocol. Two microliters were used for reverse transcription (RT)-PCR with the forward primer (5′-TCAGAAATCGAGAAACTT-3′) and reverse primer (5′-TAATTATATCTATCTTTCTTGG-3′) for the IYSV nucleocapsid gene (1). A band of the expected size (approximately 800 bp) was obtained and sequenced. The sequence from the sowthistle (GenBank Accession No. EU078327) matched IYSV sequences from Georgia and Peru in a BLAST search in GenBank (closest matches with Accession Nos. DQ838584, DQ838592, DQ838593, and DQ658242). This is to our knowledge, the first confirmed report of IYSV infecting spiny sowthistle. The distribution of IYSV in sowthistle and its role as an oversummering host for IYSV is currently an on-going study.

References: (1) L. du Toit et al. Plant Dis. 88:222, 2004. (2) D. H. Gent et al. Plant Dis. 90:1468, 2006. (3) S. W. Mullis et al. Plant Dis. 88:1285, 2004.