November 21, 2005
From: American Phytopathological Society, Plant Disease Notes, November 2005 [edited] <http://www.apsnet.org/pd/searchnotes/2005/PD-89-1243A.asp>

Iris yellow spot virus: a new onion disease in Spain
C. Cordoba-Selles and L. Martinez-Priego, Instituto Agroforestal del Mediterraneo, Grupo de Virologia, Universidad Politecnica de Valencia, Camino de Vera s/n, CP: 46022, Valencia, Spain; R. Munoz-Gomez, ITAP, Instituto Tecnico Agronomico Provincial de Albacete. Apdo 451, CP: 02080, Albacete, Spain; and C. Jorda-Gutierrez, Instituto Agroforestal del Mediterraneo. Grupo de Virologia. Universidad Politecnica de Valencia, Camino de Vera s/n,  CP: 46022, Valencia, Spain. Plant Dis. 89:1243, 2005; published on-line as DOI: 10.1094/PD-89-1243A. Accepted for publication 5 Jul 2005.

So far, only 3 viral diseases have been identified in onion crops grown in Spain. These are Tomato spotted wilt virus (TSWV), Onion yellow dwarf virus (OYDV), and Leek yellow stripe virus (LYSV). In September 2003, unusual virus-like symptoms, including straw-colored, dry, tan, diamond-shaped lesions on the leaves and stalks, sometimes with necrotic lesions, curled leaves, and bulbs of reduced size, were observed on several onion plants (_Allium cepa_ L.) in commercial fields in Albacete, Spain. Severely affected plants eventually died.

To verify the identity of the disease found in the Spanish onions, double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) was performed on leaf extracts of symptomatic onions using specific polyclonal antibodies against OYDV, LYSV, Cucumber mosaic virus (CMV) (Biorad Phyto-Diagnostics, Marnes-La Coquette, France), Iris yellow spot virus (IYSV), and TSWV (Loewe Biochemica, Sauerlach, Germany). All samples of infected onion tissue were positive for IYSV and negative for the other viruses tested.

To confirm the ELISA results, viral RNA was extracted from 5 of the ELISA-positive onion samples, a healthy onion plant, and a positive control for IYSV (DSMZ, Braunschweig. Germany). The extracted RNA was used in a One-Step reverse transcription-polymerase chain reaction (RT-PCR) assay using SuperScript Platinum Taq (Invitrogen Life
Technologies, Barcelona, Spain) in the presence of the IYSV1S and IYSV1A primers for the nucleocapsid gene of IYSV (1). The RT-PCR assay produced an amplicon of the expected size of 790 bp. No amplification products were observed when healthy plants or a water control were used as templates in the RT-PCR reaction.

To establish the authenticity of the virus from onion, the PCR products were purified (High Pure PCR Product Purification Kit, Roche Diagnostics, Mannheim, Germany), sequenced, and the nucleotide sequences obtained were analyzed and compared with the published sequences in GenBank. The PCR product was 97 percent identical to the sequence of the IYSV nucleocapsid gene (Genbank Accession No. AB121026).

IYSV, an emerging tospovirus that is potentially a devastating pathogen of onion, has been reported in many locations in Brazil, Japan, the Netherlands, Israel, Australia, the western United States, Slovenia, and Iran (2). IYSV is included in the European and Mediterranean Plant Protection Organization alert list of viruses (2), and to our knowledge, this is the 1st report of IYSV in Spain.

This tospovirus is transmitted in a propagative manner by _Thrips tabaci_. Although the vector is present in large populations in the onion-growing areas in Spain, the efficiency of the Mediterranean ecotype in transmitting IYSV is not known.

References:
(1) B. A. Coutts et al. Australas. Plant Pathol. 32:555, 2003.
(2) European and Mediterranean Plant Protection Organization. EPPO on-line publication at
<http://www.eppo.org/QUARANTINE/Alert_List/Viruses/irysxx.html>.

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

[IYSV has been reported from North America for several years and recently in Australia. The virus is mainly transmitted by onion thrips (_Thrips tabaci_) and to some extent by the western flower thrips (_Frankliniella occidentalis_), which causes considerably more damage to the crop. Onion seed, bulbs, and roots are not known to carry the virus, but volunteer onions are often symptomatic in early spring in Colorado. The virus likely over-winters in perennial and winter annual weeds, over-wintering onion, and adult thrips.

To my knowledge, there is no biological control for IYSV. Disease management depends upon use of thrips-free transplants, utilization of crop rotation (at least 3 years between crops), elimination of culls and weed hosts of the
vector, and avoidance of plant stress by providing appropriate irrigation, and avoidance of soil compaction and saline soils. There are no completely IYSV-resistant onion cultivars available, but there are some less resistant ones that can be used. Thrips control may provide some reduction in Iris yellow spot, but thrips control alone is not sufficient to economically control the disease. Thrips resistance to commonly applied insecticides is widespread in Colorado and other onion production regions of the High Plains in the USA.

Links:
<http://www.eppo.org/QUARANTINE/Alert_List/viruses/irysxx.htm>
<http://westernfarmpress.com/news/5-3-05-onion-thrips/>
<http://mtvernon.wsu.edu/path_team/newsjune03.htm#pest>  
- Mod.DH]

[see also in the archive:
Iris yellow spot, onion - USA (OR) 20050112.0110
Iris yellow spot, onion - India (Maharashtra): 1st rep. 20050507.1266
Iris yellow spot, onion - France (Reunion Island): 1st report 20050502.1219
2004
----
Iris yellow spot virus, onion - USA (WA) 20040215.0492
Iris yellow spot virus, onion - USA (NM) 20040911.2529
2002
----
Iris yellow spot virus, onion - USA (Colorado) 20020613.4489
Iris yellow spot virus, onion - USA (Colorado) 20020614.4495]