First report of Iris yellow spot virus in onion bulb and seed production fields in Reunion Island

In August 2003, leaf symptoms consisting of irregularly shaped, chlorotic or necrotic lesions, were observed on onion plants (Allium cepa) in Réunion Island. In 2004, a total of 84 symptomatic leaf samples were collected from two fields: one used for onion seed production and one for onion bulb production. Samples were screened for the presence of three 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). Forty five percent of the samples reacted positively (4 out of 6 and 34 out 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 IYSV 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% and 92% identity respectively). In 2004, a further survey of ten onion bulb production fields found that 75% of symptomatic leaves (n=221) and 27% of bulbs (n=64) tested positive for IYSV using ELISA. The virus was not detected in onion seed lots (n=59; tests using crushed seeds) but was present in 15% of 45-day old seedlings from a nursery (n=119). Symptomatic leaves from other Allium species growing in Réunion Island were screened for IYSV using ELISA. The virus was detected in leek (A. porrum) (9 out of 11 samples), garlic (A. sativum) (10 out of 11 samples) and shallot (A. cepa var. ascalonicum) (3 out 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.