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Iris yellow spot virus in Onion Seed Crops in South Africa

In December 2006, symptoms typical of iris yellow spot caused by Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae) were observed on scapes (seed stalks) in an onion (Allium cepa L.) seed crop in the Klein Karoo of the Western Cape Province, South Africa. Symptoms included diamond-shaped chlorotic or necrotic lesions on the scapes, some of which had ‘green-islands’ with nested diamond-shaped lesions, as well as indistinct, circular to irregular, chlorotic or necrotic lesions of various sizes. At the time symptoms were observed, approximately 5% of the scapes had lodged as a result of extensive lesions resembling those caused by IYSV. The crop was 2 to 3 weeks from harvest. Symptomatic tissue from two plants (two samples from one plant and four samples from the other plant) was tested for IYSV by reverse-transcriptase (RT)-PCR. Total RNA was extracted from symptomatic scape tissue with the SV Total RNA Isolation System (Promega, Madison, WI) according to the manufacturer's instructions. First strand cDNA was synthesized with the RevertAid H Minus First Strand cDNA Synthesis kit (Fermentas Inc., Hanover, MD), followed by PCR amplification with primers IYSV-For (TGG YGG AGA TGY RGA TGT GGT) and IYSV-Rev (ATT YTT GGG TTT AGA AGA CTC ACC), which amplify the nucleocapsid (NP) gene of IYSV. An amplicon of expected size (approximately 750 bp) was observed for each of the symptomatic plants assayed and was sequenced. Comparison of the sequence (GenBank Accession No. EF579801) with GenBank sequences revealed 95% sequence identity with the NP gene of IYSV GenBank Accession No. EF419888, with eight amino acid differences. The known geographic distribution of IYSV in onion bulb or seed crops has increased rapidly in recent years in many areas of the world (1). To our knowledge, this is the first confirmation of IYSV in South Africa. Approximately 6,100 ha of onion bulb crops are grown annually in South Africa in the Western Cape, Kwazulu Natal, Limpopo, and Northern Cape provinces, and 600 ha of onion seed crops are grown primarily in the semi-arid regions of the Western Cape. Examination of an additional 10 onion seed crops in the Klein Karoo during January 2007 revealed the presence of iris yellow spot in three more crops at approximately 5% incidence in each crop. The four symptomatic crops had all been planted as bulb-to-seed crops, using vernalized bulbs produced on the same farm. This suggests that IYSV may have been disseminated into the seed crops on the vernalized bulbs, either as infected bulb tissue or in viruliferous thrips on the bulbs.

Reference: (1) D. H. Gent et al. Plant Dis. 90:1468, 2006.