Progress towards Developing Onion Cultivars Tolerant to Iris Yellow Spot Virus

In the Western U.S., Iris yellow spot virus is a devastating foliar onion disease, that is reducing bulb size and yield, and is difficult to control. The disease is spread by onion thrips and disease symptoms are exacerbated by hot, dry climatic conditions that also favor rapid thrips multiplication and reduced control efficacy. Currently, no onion cultivar is resistant or tolerant to the virus and/or its symptoms. In the hopes of finding Iris yellow spot (IYS)-tolerant onion germplasm, seventy-five onion plant introduction accessions from the U.S. germplasm collection were selected and evaluated for leaf color, waxiness (bloom), and axil pattern; thrips number per plant, and IYS disease severity. IYS-infected onion bulbs were placed on the borders of the disease evaluation field to ensure that the virus was present for evaluation. No thrips control means were used during the growing season. Individual plants, that exhibited few IYS disease symptoms, were selected at bulb maturity from 14 different accessions for a total of 181 bulbs. These bulbs were self-pollinated and testcrossed to male-sterile lines in the hopes of finding individual progeny that possess a higher level of IYS tolerance.