First report of Iris yellow spot virus on Allium cepa in New Zealand

L.I. Ward^1*, Z. Perez-Egusquiza¹, J.D. Fletcher², F.M. Ochoa Corona¹, J.Z. Tang¹, L.W.Liefting¹, E.J. Martin¹, B.D. Quinn¹, H.R. Pappu³ and G.R.G. Clover¹ 

¹ Plant Health and Environment Laboratory, MAF Biosecurity New Zealand, P.O. Box 2095, Auckland 1140, New Zealand
² Crop & Food Research, Canterbury Agriculture &  Science Centre, Gerald Street, Lincoln, 7608, New Zealand
³ Department of Plant Pathology, Washington State University, Pullman, WA 99164, USA 

*lisa.ward@maf.govt.nz

Accepted for publication 30/06/08

During a disease survey of Allium crops in New Zealand in 2007, characteristic diamond-shaped lesions, suggestive of Iris yellow spot virus (IYSV; genus Tospovirus, family Bunyaviridae) infection, were observed on a few brown onion seed plants (cv. Early Long Keeper) in the Blenheim area of the South Island (Fig. 1).

Figure 1: Diamond-shaped lesions characteristic of Iris yellow spot virus on the scape of an Allium cepa plant.

The plants with symptoms tested positive for IYSV using a DAS-ELISA assay (Agdia Inc., Elkhart, USA). Total RNA was extracted from the scape lesions and tested by RT-PCR using primers designed by Ochoa Corona et al. (2005), to amplify a 186 bp section of the nucleocapsid (N) gene of IYSV (forward 5'-TAGGGTGAAACCGTCAGAAA-3' and reverse 5'-TGTCTTGTAAATGCCTGCTC-3'), and primers published by Pappu et al. (2006), which amplify the whole (1.2 kb) of the N gene of IYSV. Amplicons of the expected size were cloned and sequenced (GenBank Accession No. EU555189 and EU477515). Sequence comparisons showed 98-100% nucleotide identity with other IYSV N gene sequences in GenBank (AF001387 and AB180921). A phylogenetic analysis of the deduced amino acid sequences of the N gene showed that the New Zealand isolate of IYSV grouped most closely with isolates from California and Texas, USA (Fig. 2.). 

Figure 2: Iris yellow spot virus phylogeny (Neighbor Joining Method) based on deduced amino acid sequences of the complete nucleocapsid gene. The New Zealand isolate of IYSV is indicated as NZ.

Samples from a further 28 Allium crops (100 plants per crop) and four ornamental Allium species (one plant of each: A. senesences, A. murryanum, A. caesium and A. moly), collected around New Zealand, were tested for IYSV by ELISA. The results showed that IYSV was widespread in onion (ten of twelve crops) and shallot (three out of three crops), in both the North and South Islands. However, all 13 garlic crops tested negative. The infected crops showed a significant amount of thrips damage, but most plants had no IYSV symptoms. Two of the ornamental Allium species (A. senesences and A. murryanum), which were also without symptoms, tested positive for IYSV by ELISA. Unfortunately attempts to amplify the viral genome by RT-PCR for sequencing were unsuccessful. 

Reports of new IYSV outbreaks continue to be made from different parts of the world and the virus is a major production constraint to onion bulb and seed production in some countries (Gent et al., 2006). This is the first report of IYSV in onion and shallot in New Zealand and its economic impact on these crops in New Zealand is still to be determined.

References

Gent D, du Toit LJ, Fichtner SF, Mohan SK, Pappu HR, Schwartz HF, 2006. Iris yellow spot virus: An emerging threat to onion bulb and seed production. Plant Disease 90, 1468-1480.

Ochoa Corona FM, Lebas BSM, Bootten TJ, Elliott DR, Tang JZ, Alexander BJR, 2005. Detection of Iris yellow spot virus by RT-PCR. Phytopathology 95, S963-964.

Pappu HR, du Toit LJ, Schwartz HF, Mohan SK, 2006. Sequence diversity of the nucleoprotein gene of Iris yellow spot virus (genus Tospovirus, family Bunyaviridae) isolates from the western region of the United States. Archives of Virology 151, 1015-1023. 

The British Society for Plant Pathology