Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2005
Publication Date: 3/1/2006
Citation: Pappu, H.R., Hellier, B.C., Dugan, F.M. 2006. Wild allium ssp. as natural hosts of iris yellow spot virus. Plant Disease. 90:378.
Interpretive Summary: Iris Yellow Spot is a damaging disease of onion caused by a virus, IYSV. The disease negatively impacts both bulb and seed production. The vector of the disease is a minute and common insect, onion thrips. The disease has been spreading regionally in Washington State and the Pacific Northwest, and is a major concern of onion growers. In 2005, symptoms of the disease were noted on three species of wild onion from Asia. These onions were being grown at the USDA-ARS farms in Pullman and Central Ferry, Washington. Some onions at these locations had displayed similar symptoms in prior years. The presence of the virus was confirmed by use of the polymerase chain reaction to amplify viral DNA. The sequence of the amplified DNA matched that of IYSV. Another molecular technique, ELISA, also confirmed the presence of the virus. Records such as this are useful for tracking spread of the disease, and for noting any potential sources of resistance in onion germplasm.
Technical Abstract: The incidence of Iris yellow spot virus (IYSV) in a commercial onion crop was first confirmed in WA state in 2003. First found in Adams County, IYSV has rapidly spread to all the onion-producing counties in the state. The USDA-ARS Western Regional Plant Introduction Station (WRPIS) collects, maintains, and distributes various Allium (garlic and onion) accessions. As part of the regeneration process, accessions are grown under field conditions at the WRPIS farms in two locations: Pullman and Central Ferry, WA. In June 2005, leaf and scape tissue were collected from WRPIS accessions of wild onions (Allium pskemense, A. vavilovii, A. altaicum in Central Ferry and A. stipitatum in Pullman) that had symptoms indicative of IYSV infection. Some onions had shown similar symptoms in prior years. IYSV infection was confirmed by ELISA using a commercially available kit. Virus infection was further verified by RT-PCR using primers derived from the small RNA of IYSV. The primers flank the IYSV N gene (5-TAA AAC AAA CAT TCA AAC AA-3’ and 5’-CTC TTA AAC ACA TTT AAC AAG CAC-3’). RT PCR gave a PCR product of expected size (ca. 1.2 kb). The DNA amplicon was cloned and sequenced. Nucleotide sequence comparisons with known IYSV NP gene sequences showed 95 to 98% sequence identity. The prevalence of the vector, onion thrips, in the state combined with the widespread incidence of IYSV in both seed and bulb production areas of the state may have resulted in natural infection of wild relatives of cultivated onion.