.

Date: 20 Sep 2007
Source: The American Phytopathological Society, Plant Disease 2007; 91(9), 1203 [edited]
<http://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-91-9-1203A>

[Reference: LJ du Toit, et al. _Iris yellow spot virus_ in onion seed crops in South Africa. Plant Dis 2007; 91(9): 1203. DOI: 10.1094/PDIS-91-9-1203A]

Iris yellow spot virus in onion seed crops in South Africa

In December 2006, symptoms typical of iris yellow spot caused by _Iris yellow spot virus_ (IYSV; genus _Tospovirus_, family _Bunyaviridae_) were observed on scapes (seed stalks) in an onion (_Allium cepa_ L.) seed crop in the Klein Karoo of the Western Cape Province, South Africa. Symptoms included diamond-shaped chlorotic or necrotic lesions on the scapes, some of which had 'green islands' with nested diamond-shaped lesions, as well as indistinct, circular to irregular, chlorotic or necrotic lesions of various sizes. At the time symptoms were observed, approximately 5 per cent of the scapes had lodged [fallen over] as a result of extensive lesions resembling those caused by IYSV. The crop was 2 to 3 weeks from harvest.

Symptomatic tissue from 2 plants (2 samples from one plant and 4 samples from the other plant) was tested for IYSV by reverse-transcriptase (RT)-PCR [polymerase chain reaction]. Total RNA was extracted from symptomatic scape tissue and RT-PCR was carried out amplifying the nucleocapsid (NP) gene of IYSV. An amplicon of expected size (approximately 750 bp) was observed for each of the symptomatic plants assayed and was sequenced revealing 95 per cent sequence identity with the NP gene of IYSV published in the GenBank database, with 8 amino acid differences. The known geographic distribution of IYSV in onion bulb or seed crops has increased rapidly in recent years in many areas of the world (1). To our knowledge, this is the 1st confirmation of IYSV in South Africa.

About 6100 ha [about 15 100 acres] of onion bulb crops are grown annually in South Africa in the Western Cape, KwaZulu Natal, Limpopo, and Northern Cape provinces, and 600 ha [about 1500 acres] of onion seed crops are grown primarily in the semi-arid regions of the Western Cape. Examination of an additional 10 onion seed crops in the Klein Karoo during January 2007 revealed the presence of iris yellow spot in 3 more crops at about 5 per
cent incidence in each crop. The 4 symptomatic crops had all been planted as bulb-to-seed crops, using vernalized bulbs produced on the same farm. This suggests that IYSV may have been disseminated into the seed crops on the vernalized bulbs, either as infected bulb tissue or in viruliferous thrips on the bulbs.

Reference:
DH Gent, et al. Plant Dis 2006; 90: 1468.

--
communicated by:
ProMED-mail
<promed@promedmail.org>

[IYSV has only recently been confirmed as a member of the genus _Tospovirus_ by the International Committee on the Taxonomy of Viruses (ICTV). It has been included in the Alert List of the European Plant Pathology Organisation (EPPO). Known hosts include onion, leek, iris, amaryllis, ornamental lisianthus (sporadically), and weeds including wild relatives of onion (_Allium_ species). IYSV has been reported from a number of locations in North and southern America, Europe, Asia, and Australia. The economic impact is low in some areas, but up to 100 per cent loss has been observed in onion crops, for example in Brazil. The virus is transmitted by the thrips species _Thrips tabaci_, which is widespread. Spread occurs via infected plant material and viruliferous vector insects. The virus appears not to be seed transmitted. Disease management includes the use of clean propagative material and vector control.

The isolate of IYSV reported above appears to be a different strain with some variation in nucleotide sequence compared to previously recorded isolates from other parts of the world.

Tospoviruses are an extremely important group of plant viruses capable of infecting a large range of important crops. The type member, _Tomato spotted wilt virus_ (TSWV), has had a serious impact on many crop species
worldwide. New tospoviruses are being found frequently. Tospovirus epidemiology is complex and natural resistance appears to be rare amongst their plant hosts. For more information on tospoviruses see ProMED-mail
posts 20070608.1872 and 20070524.1670.

Maps
South Africa:
<http://www.safarinow.com/destinations/South-Africa/south-africa-map.gif>
IYSV worldwide distribution (December 2005):
<http://www.eppo.org/QUARANTINE/Alert_List/viruses/maps/IYSV00_map.htm>
Pictures
IYSV symptoms on onion plant:
<http://www.columbiapublications.com/onionworld/marchapril2006/irisyellowspot%20K8.jpg> 
and
<http://www.apsnet.org/online/feature/tospovirus/images/figure4.JPG>
Lesions on onion leaf:
<http://www.bspp.org.uk/ndr/july2005/2005-33-2th.jpg>
Links
Information on IYSV:
<http://www.eppo.org/QUARANTINE/Alert_List/viruses/IYSV00.htm>
_Tospovirus_ taxonomy and species list:
<http://phene.cpmc.columbia.edu/ICTVdB/11050000.htm>
Current list of recognised tospoviruses:
<http://www.oznet.ksu.edu/tospovirus/tospo_list.htm>
_Tospovirus_ resource centre:
<http://www.oznet.ksu.edu/tospovirus/welcome.htm>
General information and background on tospoviruses:
<http://www.apsnet.org/online/feature/tospovirus/Top.html> . - Mod.DHA]

[see also in the archive:
Tomato spotted wilt virus, tomato - USA (CA) 20070608.1872
Groundnut ringspot tospovirus, peanut - Argentina (Cordoba): 1st report
20070524.1670
2006
---
Iris yellow spot virus, onion - Peru: 1st report 20060225.0612
2005
---
Iris yellow spot virus, onion - Chile 20051128.3444
Iris yellow spot virus, onion - Spain: 1st report 20051121.3384
Iris yellow spot, onion - India (Maharashtra): 1st rep. 20050507.1266
Iris yellow spot, onion - France (Reunion Island): 1st report 20050502.1219
Iris yellow spot, onion - USA (OR) 20050112.0110
2004
---
Tomato spotted wilt, Iris yellow spot, onion - USA (GA) 20041031.2938
Iris yellow spot virus, onion - USA (NM) 20040911.2529
Iris yellow spot virus, onion - USA (WA) 20040215.0492
2002
---
Iris yellow spot virus, onion - USA (Colorado) 20020614.4495
Iris yellow spot virus, onion - USA (Colorado) 20020613.4489]

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