C. K. Evans, Utah State University, Biology Department, Logan;
S. Bag, Department of Plant Pathology, Washington State University, Pullman;
E. Frank, Utah State University, Plant Pest Diagnostic Laboratory, Biology Department, Logan;
C. Ransom, and
D. Drost, Utah State University, Plants, Soils, and Climate Department, Logan; and
H. R. Pappu, Department of Plant Pathology, Washington State University, Pullman
Iris yellow spot virus (IYSV; family Bunyaviridae, genus Tospovirus) is a serious virus pathogen in onion bulb and seed crops in the United States and several parts of the world (1). The virus is exclusively transmitted by onion thrips (Thrips tabaci). Besides onion and other susceptible crops such as garlic, leek, chives, and several ornamentals, weeds could be serving as potential reservoir sources of virus inoculum. There are reports of several weeds found naturally infected with IYSV (1,2,4). However, there is no report of IYSV infection of a grass species. Leaves of green foxtail (Setaria viridis (L.) Beauv.) were collected from two naturally occurring plants approximately 30 m apart in a weed trial conducted in commercial onions grown in Box Elder County, UT on 24 September 2008. Notes of IYSV symptoms on green foxtail were made only on the two grass plants sampled. Density of green foxtail in the weed trial was low and was not recorded. Leaves on both plants displayed a range of symptoms that included streaking, purpling, and chlorotic and necrotic lesions along leaf margins oriented along the axis of longitudinal venation. Samples were positive for IYSV by double-antibody sandwich-ELISA with a commercially available kit (Agdia Inc., Elkhart, IN). ELISA values of the grass samples were 2.64 and 2.23 for each plant sampled. Negative and positive control readings were 0.24 and 4.33, respectively. All absorbance readings were made at 405 nm. To provide a contrast of the grass data in context to the onion field where the weed trial was located, final visual assessments of onions in the field were made on 4 September 2009. Approximately 300 onion plants were assessed for incidence and severity of disease. Incidence of the disease among onions was 100% and the severity of iris yellow spot on leaves was 20 lesions per leaf. The average ELISA value over 30 individual onions arbitrarily sampled from the field on the same day was 3.50, and the ELISA values among the samples ranged from 1.37 to 4.38. The negative and positive controls were 0.19 and 4.40, respectively. To further verify the presence of IYSV in the grass specimen, reverse transcription-PCR was performed on total nucleic acid extracts obtained from the symptomatic parts of the leaves. Primers specific to the nucleocapsid (N) gene coded by the small (S)-RNA of IYSV were used (3). The forward and reverse primer pairs, 5′-TCAGAAATCGAGAAACTT-3′ and 5′-CACCAATGTCTTCAACAATCTT-3′, respectively, amplify a 751-nt fragment of the N gene (3). An amplicon of expected size was obtained, cloned, and sequenced. The nucleotide sequence analysis and comparison with known IYSV S-RNA sequences showed that the amplicon from foxtail (GenBank Accession No. FJ652594) samples had the highest nucleotide sequence identity (98%) with the corresponding region of an IYSV isolate from Jefferson County, OR (GenBank Accession No. DQ233479). To our knowledge, this is the first report of natural infection of a grass species by IYSV and the first report of a Tospovirus infecting a grass species. The data suggests grasses may serve as a new host reservoir for IYSV. The increasing number of weed hosts of IYSV warrants further study on the role of these weeds as hosts for onion thrips and in IYSV epidemiology.
References: (1) D. Gent et al. Plant Dis. 90:1468, 2006. (2) C. Nischwitz et al. Plant Dis. 91:1518, 2007. (3) H. R. Pappu et al. Arch. Virol. 151:1015, 2006. (4) R. Sampangi et al. Plant Dis. 91:1683, 2007.