Epidemiological and molecular aspects of Iris yellow spot virus in the Pacific northwest
Pappu HR1, duToit L2, Schwartz H3
1Department of Plant Pathology, PO Box 646430, Washington State University, Pullman, WA
2WSU - Northwestern Washington Research & Extension Center, Mount Vernon, WA
3Colorado State University, Department of Bioagricultural Sciences and Pest Management, Fort Collins, CO.
Correspondence: hrp@wsu.edu
Iris yellow spot virus (IYSV), of genus Tospovirus, family Bunyaviridae, has been endemic in onion crops in the Treasure Valley of Idaho for over a decade (Hall JM et al. 1993. Plant Disease 77: 952), where the virus was largely confined to seed crops. Epidemics of IYSV were observed in bulb crops beginning in 2000, and the virus has since been detected in Arizona, California, Colorado, New Mexico, Oregon, Utah and Washington in the western US, and in Georgia in the southeastern US. In Washington, IYSV was first observed in one county with a few symptomatic plants in 2002. By 2004, the virus was detected in all major counties for onion production, with some fields showing 90% incidence of symptomatic plants. Severe outbreaks in seed crops in central Oregon (Crowe F, Pappu HR. 2004. Plant Disease 89:105) and seed and bulb crops in Washington during 2004 suggest favorable conditions for the virus and vector buildup. The rapid spread of the virus and its emergence as a serious disease constraint raises several questions about the epidemiology of the disease and the factors that contributed to the shift towards increased incidence. Rapid spread and the increased impact of IYSV in northwest may be attributable to: 1) prevalence of the vector, Thrips tabaci, 2) the ｀green bridge¨ effect from overlapping biennial seed and annual bulb crops, and 3) limited resistance in commercial cultivars. Nucleoprotein (NP) gene sequences of several isolates from the Pacific Northwest of the US were determined. Onion samples showing symptoms indicative of IYSV infection were collected from commercial seed and bulb crops in Colorado, Idaho, and Washington in 2003 and 2004. Using a RT-PCR assay, the NP gene was amplified and cloned from selected isolates. Dendrograms based on the translated amino acid sequences of the NP gene showed that isolates from Colorado, Idaho, and Washington were highly similar to one another and formed a tight cluster and shared a high degree of sequence identity with the Lisianthus isolate from Japan. Isolates from several counties within the Washington state were closer to each other and with the Idaho isolate. Isolates from Australia, Brazil, Israel, Japan (Alstroemeria isolate), the Netherlands, and Slovenia diverged from the isolates sequenced in this study. ELISA, PCR, and real-time PCR assays are being developed for improved detection of IYSV. Management options that are being explored include vector management, and use of SAR-inducers such as acibenzolar-S-methyl.
