February 13, 2003
From: American Phytopathological Society, Plant Disease Notes [edited]

Iris yellow spot virus in onion bulb and seed crops in Washington

LJ du Toit, Washington State University-Mount Vernon Research and Extension Unit, Mount Vernon 98273; HR Pappu, KL Druffel, Department of Plant Pathology, Washington State University, Pullman 99164; GQ Pelter, Grant/Adams Counties, Washington State University Cooperative Extension, Ephrata 98823. Plant Dis 2004; 88: 222, published online as D-2003-1201-03N, 2004. Accepted for publication 30 Oct 2003.

The geographic distribution of Iris yellow spot virus (IYSV, genus Tospovirus, family Bunyaviridae) in onion (_Allium cepa_ L.) crops in the western United States has increased with the most recent report in Colorado (1,2). Furthermore, the incidence of IYSV has increased significantly in onion crops in the Treasure Valley of southern Idaho and eastern Oregon, where the disease was first detected in the United States (1,3).

Surveys of onion seed crops in Washington during the past 2 years showed the presence of plants with symptoms characteristic of IYSV infection, including distinct diamond-shaped chlorotic or necrotic lesions, as well as indistinct circular to irregular, chlorotic or necrotic lesions of various sizes on the scapes of flowering plants. To date, symptomatic plants have been observed in 5 seed crops in Washington, at incidences ranging from 1 per cent to about 20 per cent in individual seed crops.

Enzyme-linked immunosorbent assays carried out directly on symptomatic onion samples collected in July 2002, and on _Nicotiana benthamiana_ plants mechanically inoculated with sap from these symptomatic plants, did not detect the presence of IYSV. In late July 2003, symptomatic plants were collected from an onion seed crop in Grant County and tested for IYSV infection by reverse transcription-polymerase chain reaction (RT-PCR). Total nucleic acid was extracted from symptomatic areas of the scapes with the procedure described by Presting and colleagues (4).

Primers specific to the nucleocapsid (NP) gene of IYSV were designed based on sequences in GenBank: 5'-TCA GAA ATC GAG AAA CTT-3' and 5'-TAA TTA TAT CTA TCT TTC TTG G-3' (sense and antisense polarity, respectively. The RT-PCR assay produced an amplicon of the expected size (about 700 bp) that was cloned and sequenced. Comparison with the GenBank IYSV gene sequences showed 98 per cent sequence identity of the NP gene.

In August 2003, symptoms of IYSV infection were observed in 2 onion bulb crops, each located within 2 miles of the symptomatic onion seed crop in Grant County. The presence of IYSV in these crops was confirmed by RT-PCR
with cloning and sequencing of the amplicon, as described for the seed crop samples.

To our knowledge, this is the first confirmation of IYSV in onion bulb and seed crops in Washington, where 16 000 to 18 000 acres of onion bulb crops and 700 to 900 acres of onion seed crops are grown annually (USDA National
Agricultural Statistics Service). The increase in prevalence of IYSV in the Pacific north west highlights the need for additional research to clarify the epidemiology of this potentially significant pathogen and to develop a regional management program for iris yellow spot.

References:
(1) JM Hall, et al. Plant Dis 1993; 77: 952.
(2) HF Schwartz, et al. Plant Dis 2002; 86: 560.
(3) JW Moyer, et al. (abstr.) Phytopathology 2003; 93(suppl): S115.
(4) GG Presting, et al. Phytopathology 1995; 85: 436.

[In addition to infected onion crops in Washington and Colorado, IYSV is present in Brazil, Iran, Israel, the Netherlands, and Slovenia. IYSV has been endemic in south western Idaho and eastern Oregon onion, leek, and
chive seed production fields for over 10 years. It has also been detected in onion seed plants from California and Arizona. The virus is vectored by onion thrips (_Thrips tabaci_) but not by western flower thrips (_Frankliniella occidentalis_), which are an efficient vector of the virus. Although there is no cure, diseased plants can still produce reasonable yields through irrigation and good soil fertility. Not much is known about resistance to IYSV in onion.

Additional references:
<http://www.eppo.org/QUARANTINE/Alert_List/Viruses/irysxx.html> 
<http://www.cals.ncsu.edu/plantpath/Faculty/moyer/moyer_jw/posters/iysv/iysv.html> - Mod.DH]