Location: Vegetable Crops Research2011 Annual Report
1a. Objectives (from AD-416)
We will expand close working relationships among breeders, extensionists, and growers of major Alliums in the U.S. to evaluate germplasms for prioritized pest resistances (thrips and Iris Yellow Spot Virus) and lay the foundation for the long-term translational genomics of the Allium vegetables. Workshops will be held at regional onion meetings. We will evaluate in the field onion populations for resistance or tolerance to thrips and Iris Yellow Spot Virus. Resistance or tolerant germplasms will be released to the onion breeders in the public and private sectors.
1b. Approach (from AD-416)
Work with extension professionals to empower growers to complete on-farm evaluations for resistances or tolerances to thrips and Iris Yellow Spot Virus. Self-pollinate and testcross selected plants and return seed for validation of phenotypes; Deliver validated germplasms to private and public sector breeders; Develop workshops for public and private-sector researchers, students, and regional grower and consumer groups for onion to illustrate the usefulness of genomics to solve high-priority research goals.
3. Progress Report
Studies were completed to identify, validate, and deliver resistance or tolerance in onion to Iris Yellow Spot Virus (IYSV) and/or thrips. A research field with a history of the two pests near Fort Collins, Colorado was used to evaluate 103 onion accessions, of which 18 were selected for acceptable levels of tolerance or resistance to these two pests. Of these, plant introductions 258956, 264320, 546140, 546188, and 5461952 were identified over two years for consistent acceptable responses to IYSV and thrips. Greenhouse studies were undertaken to provide information on the potential yield losses due to IYSV and thrips and to separate the effects of the two pests by investigating their effects on the growth, physiology and productivity of two onion cultivars: IYSV-field-tolerant Colorado 6 and IYSV-susceptible Talon. The effects of four treatments; Healthy Control (HC), IYSV Only (V), Thrips + IYSV (TV), and Thrips Only (T) were investigated. Growth rate was highest in HC, followed by V, TV, and then T. Colorado 6 showed significantly (P < 0.0001) higher growth rates than Talon. There was a significant treatment difference for weight (P < 0.0001), with HC giving the highest bulb weight, followed by V, T and finally TV. Distribution of IYSV within infected leaves of field-grown Colorado 6 and Talon were assessed using enzyme-linked immunosorbent assay. Entire leaves were divided into top, middle and bottom sections and each section was further divided into subsamples. Each treatment was significantly different (P < 0.0001) from the others in the increasing order of HC, V and TV. The three leaf sections were significantly different (P = 0.0095), with the bottom section having the highest optical density values, followed by the top, then the middle sections. Virus distribution among leaves of infected plants was determined using eight symptomatic onion plants at 9-leaf (pre-bulb) and 13-leaf (post-bulb) stages sampled from an onion field in northern Colorado. Virus titer steadily increased from a highly positive 0.71 in Leaf 1 to a highly positive 1.18 in Leaf 4, after which values declined steadily to a highly positive 0.72 in Leaf 8. Only the neck region of Leaf 1 tested highly positive for IYSV. Leaf 9 (youngest leaf) together with all non-leaf tissues, tested negative for the virus. All leaves of the post-bulb plants were highly positive for the virus, however, unlike the pre-bulb stage, virus distribution was considerably non-uniform, with no apparent trend. Project monitored by quarterly conference calls.