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

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).

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.).

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.