Screening of Onion Plant Introduction Accessions for Iris Yellow Spot Disease Severity

In the Western U.S., Iris yellow spot virus is a devastating foliar onion disease that is reducing bulb size and yield. The disease is spread by onion thrips that are difficult to control. 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 Iris yellow spot (IYS) and/or thrips damage. Certain onion foliar characteristics have shown nonpreferential feeding activity by thrips and may be the first step in developing IYS-tolerant onion cultivars. In the hopes of finding IYS-tolerant onion germplasm, seventy-five onion plant introduction accessions from the U.S. germplasm collection were selected and evaluated for IYS disease severity because they possessed a low amount of leaf bloom (degree of wax deposition) in a previous evaluation. 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. Ten plants per plot were rated individually at three separate times throughout the growing season for IYS severity on a scale of 0 to 4, where 0 = no symptoms, 1 = 1 to 2 small lesions per leaf, 2 = >2 medium-sized lesions per leaf, 3 = lesions coalescing on more than 25% of the leaf, and 4 = more than 50% leaf death. At 20 weeks post transplanting, PI 239633, PI 264320, PI 321385, PI 546100, PI 546115, PI 546188, and PI 546192 exhibited less severe IYS symptoms than other accessions. Four weeks later, IYS symptoms became more severe on plants of these accessions, however; plants of PI 546115 and PI 546192 exhibited less severe symptoms than most other accessions that had not matured by this time. Through the evaluation of these onion accessions for IYS disease severity, the potential exists to develop onion cultivars that have increased IYS tolerance.