October 29, 2004
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

First report of Vidalia onion naturally infected with tomato spotted wilt virus and iris yellow spot virus (Family Bunyaviridae, Genus Tospovirus) in Georgia
S. W. Mullis, D. B. Langston, Jr., R. D. Gitaitis, J. L. Sherwood, and A. C. Csinos, Department of Plant Pathology, University of Georgia, P.O. Box 748, Tifton 31793; D. G. Riley, Department of Entomology, University of Georgia, Coastal Plain Experiment Station, Tifton 31793; and A. N. Sparks, R. L. Torrance, and M. J. Cook, IV, Cooperative Extension Service, University of Georgia, Tifton 31793. Plant Dis. 88:1285, 2004; published on-line as D-2004-0909-01N, 2004. Accepted for publication 12 Aug 2004.

Vidalia onion is an important crop in Georgia's agriculture with worldwide recognition as a specialty vegetable. Vidalia onions are short-day, Granex-type sweet onions grown within a specific area of southeastern Georgia.

Tomato spotted wilt virus (TSWV) has been endemic to Georgia crops for the past decade, but has gone undetected in Vidalia onions. Tobacco thrips (_Frankliniella fusca_) and Western flower thrips (_Frankliniella occidentalis_) are the primary vectors for TSWV in this region, and a number of plant species serve as reproductive reservoirs for the vector or virus.

Iris yellow spot virus (IYSV), an emerging tospovirus that is potentially a devastating pathogen of onion, has been reported in many locations in the western United States (2,4). _Thrips tabaci_ is the known vector for IYSV, but it is unknown if non-crop plants play a role in its epidemiology in Georgia.

During October 2003, a small (n = 12) sampling of onions with chlorosis and dieback of unknown etiology from the Vidalia region was screened for a variety of viruses, and TSWV and IYSV infections were serologically detected. Since that time, leaf and bulb tissues from 4424 onion samples were screened for TSWV and IYSV using double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA) with commercial kits (Agdia Inc., Elkhart, IN).

Samples were collected from 53 locations in the Vidalia region during the growing season between November 2003 and March 2004. Plants exhibiting stress, such as tip dieback, necrotic lesions, chlorosis or environmental damage were selected. Of these, 306 were positive for TSWV and 396 were positive for IYSV using positive threshold absorbance of 3 times the average plus 2 standard deviations of healthy negative onion controls.

Positive serological findings of the onion tissues were verified by immunocapture-reverse transcription-polymerase chain reaction (IC-RT-PCR) for TSWV (3) and RT-PCR for IYSV (1). In both instances, a region of the viral nucleocapsid (N) gene was amplified. The PCR products were analyzed with gel electrophoresis with an ethidium bromide stain in 0.8 percent agarose.

86 percent (n = 263) of the TSWV ELISA-positive samples exhibited the expected 774-bp product and 55 percent (n = 217) of the IYSV ELISA-positive samples exhibited the expected 962-bp product. The reduced success of the IYSV verification could be attributed to the age and deteriorated condition of the samples at the time of
amplification.

_Thrips tabaci_ were obtained from onion seedbeds and cull piles within the early sampling (n = 84) and screened for TSWV by the use of an indirect-ELISA to the nonstructural (NSs) protein of TSWV. Of the thrips sampled, 25 were positive in ELISA.

While the incidence of IYSV and TSWV in the Vidalia onion crop has been documented, more research is needed to illuminate their potential danger to Vidalia onions.

References:
(1) I. Cortes et al. Phytopathology 88:1276, 1998. (
(2) L. J. du Toit et al. Plant Dis. 88:222, 2004.
(3) R. K. Jain et al. Plant Dis. 82:900, 1998.
(4) J. W. Moyer et al. (Abstr.) Phytopathology 93(suppl.):S115, 2003.

[The gist of this report is that TSWV and IYSV have been detected for the 1st time in the Vidalia region despite its undetected presence in onion fields. Disease incidences were 7 percent and 9 percent for TSWV and IYSV, respectively. Disease management requires weed control and use of alternating applications of different chemical
insecticides. Insecticide resistance is often a factor in disease management. 4 thrips species are the major vectors of TSWV: _Frankliniella occidentalis_ (western flower thrips); _F. schultzei_, _F. fusca_ (tobacco thrips); and _Thrips tabaci_ (onion thrips). TSWV is now common in temperate, subtropical, and tropical regions around
the world and is a major constraint to onion production.

Regarding the Granex-type of onion, it is a short-day variety. To be marketed as a true "Vidalia," an onion must be grown in the Vidalia area and conform to the Yellow-Granex type.

Links: <http://vegetablemdonline.ppath.cornell.edu/factsheets/Virus_SpottedWilt.htm>
<http://www.ipm.msu.edu/CAT03_veg/V07-02-03.htm#1>
<http://www.coopext.colostate.edu/TRA/PLANTS/index.html >
- Mod.DH]