May 2, 2005
Source:  British Society for Plant Pathology, New Disease Reports, Vol. 11 [edited] <http://www.bspp.org.uk/ndr/july2005/2005-33.asp>

First report of Iris Yellow Spot Virus in onion bulb and seed production fields in Reunion Island
I. Robene-Soustrade, UMR PVBMT, Cirad, Pole de Protection des Plantes, Ligne Paradis, 97410 St-Pierre, La Reunion, France; B. Hostachy, DAF - Service de la Protection des Vegetaux, Pole de Protection des Plantes, Ligne Paradis, 97410 St-Pierre, La Reunion, France; M. Roux-Cuvelier (as for Robene-Soustrade); J. Minatchy, FDGDON, Pole de Protection des Plantes, Ligne Paradis, 97410 St-Pierre, La Reunion, France; M. Hedont (as for Robene-Soustrade); R. Pallas (as for Minatchy); A. Couteau (as for Robene-Soustrade); N. Cassam (as for Hostachy); and G. Wuster (as for Hostachy). Accepted for publication 18 Apr 2005.

In August 2003, leaf symptoms consisting of irregularly shaped, chlorotic or necrotic lesions were observed on onion plants (_Allium cepa_) in Reunion Island. In 2004, a total of 84 symptomatic leaf samples were collected from 2 fields: one used for onion seed production and one for onion bulb production.

Samples were screened for the presence of 3 tospoviruses, Iris yellow spot virus (IYSV), Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV), by double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA) using commercial antisera (PRI, Wageningen, The Netherlands; BIORAD, Marnes la Coquette, France and LCA, Blanquefort, France, respectively). 45 percent of the samples reacted positively (4 of 6 and 34 of 78 respectively for bulb and seed production fields) with the antiserum directed against IYSV; all samples tested negative for TSWV and INSV.

The presence of IYSV in the serologically-positive samples was verified using reverse transcription-polymerase chain reaction (RT-PCR). Total nucleic acid was extracted using an RNeasy Plant Extraction kit (Qiagen, Courtaboeuf, France). Primers based on published sequences for the nucleocapsid (CP) gene for various YSV
isolates (Cortes et al., 1998; Kritzman et al., 2000) produced negative results using RT-PCR. New primers were designed based on sequences flanking the CP gene: IYSV56U (5'-TCCTAAGTATTCACCAT-3') and IYSV917L (5'-TAAAACTTAACTAACACAAA-3') (sense and antisense polarity, respectively). These produced a 896 bp amplicon of the expected size, that was cloned and sequenced.

The amplicon sequence was compared with other IYSV sequences using BLAST. Best matches of nucleotide identity were obtained with the CP gene of IYSV from Japan (AB121026) and Brazil (AF067070) (93 percent and 92 percent identity respectively). In 2004, a further survey of 10 onion bulb production fields found that 75 percent of symptomatic leaves (n=221) and 27 percent of bulbs (n=64) tested positive for IYSV using ELISA.

IYSV was not detected in onion seed lots (n=59; tests using crushed seeds) but was present in 15 percent of 45-day old seedlings from a nursery (n=119). Symptomatic leaves from other _Allium_ species growing in Reunion Island were screened for IYSV using ELISA. The virus was detected in leek (_A. porrum_) (9 of 11 samples), garlic
(_A. sativum_) (10 of 11 samples) and shallot (_A. cepa var. ascalonicum_) (3 of 3) leaves. The potential vector, _Thrips tabaci_, was widespread in all _Allium_ crops surveyed, whereas _Frankliniella occidentalis_ was only occasionally observed.

This is the first record of IYSV in the Mascarenes archipelago.

Acknowledgements:

We thank seed and bulb growers for providing onion samples and M.J.B. Grondin from SEMOI. We thank ARMEFHLOR and Chambre d'Agriculture de la Reunion for technical assistance during field surveys.

References:

Cortes I, Livieratos IC, Derks A, Peters D, Kormelink R, 1998. Molecular and serological characterization of Iris yellow spot virus, a new distinct Tospovirus species. Phytopathology 88, 1276-1282.

Kritzman A, Beckelman H, Alexandrov S, Cohen J, Lampel M, 2000. Lisianthus leaf necrosis: a new disease of Lisianthus caused by Iris yellow spot virus. Plant Disease 84, 1185-1189.

--
ProMED-mail
<promed@promedmail.org>

[IYSV attacks a number of vegetable, fruit, and flower crops causing considerable economic damage. Disease management employs several cultural control options, including avoiding planting thrips-susceptible crops following small grains, managing vegetation in fields and field edges, using colored mulches, and avoiding high
nitrogen levels. Some cabbage and onion varieties are somewhat resistant to thrips attack. Several beneficial insects suppress thrips levels. Organically-acceptable pesticides are available for thrips control.

It would be interesting to know how IYSV reached Reunion Island. Perhaps our colleagues in Reunion Island could provide information on this point.

Link: <http://www.attra.org/attra-pub/thrips.html>
- Mod.DH]