|dc.description.abstract||Onion (Allium cepa L.) is the most economically important monocot outside of the grasses. Onion thrips (Thrips tabaci), a cosmopolitan and polyphagous insect, infests and damages onion crops grown between sea level and 2,000 m. In recent years, onion thrips has emerged as the principal vector of the economically important Iris yellow spot virus (IYSV, family Bunyaviridae). Any attempt to significantly reduce the economic impact of this pest/pathogen will require a multifaceted approach designed in an IPM framework. Host plant resistance is an important foundation to the success of such approaches, and in an effort to find sources of resistance to IYSV and onion thrips, 137, 104 and 84 onion germplasms, respectively, were evaluated in 2009, 2010 and 2011 in northern Colorado, near Fort Collins. Sixteen, 18 and 11 germplasms, respectively, were selected in 2009, 2010 and 2012 for exhibiting acceptable level of tolerance/resistance to the two pests. Of these, PIs 264320, 546140, and 546192 were selected in both 2009 and 2010, and PIs 258956 and 546188 were selected in all three years of the evaluation. Selected germplasms were included as candidates for the onion translational genomics component of this national endeavor. Greenhouse experiments were carried out to study the effects of five treatments; Healthy Control (HC), Spray (S), Thrips Only (T), IYSV Only (V) and Thrips + IYSV (TV), on the growth, physiology and productivity of Colorado 6, Talon and Salsa Red onion cultivars. Seasonal mean net photosynthesis (A) and late season light response curves were higher in Colorado 6 than in Talon. Late season curves were significantly lower in TV, V and T than in HC and S in all cultivars. Seasonal mean growth rate was significantly higher in Colorado 6 than in Talon. Among the treatments, seasonal mean growth rate was in the decreasing order of HC, V, TV and T in 2009; and S, HC, V, T and TV in 2011. In one of three years, yield was significantly higher in Colorado 6 than in Salsa Red and Talon. Biomass partitioning pattern revealed that Talon had the highest harvest index among the cultivars. TV had the lowest bulb yield among the treatments in all three years of the study, and caused 14-60% yield loss in Colorado 6, 14-46% loss in Talon, and 17-48% loss in Salsa Red. Seasonal dynamics of IYSV titer in leaves of Colorado 6 and Talon indicated that virus titer for TV and V were consistently higher in Talon than in Colorado 6. Titer was in the increasing order of middle, top and base leaf sections. In naturally infected pre-bulb plants, virus titer increased from Leaf 1 (outer leaf) to Leaf 4 (inner leaf) after which it declined in subsequent younger leaves. Virus was not detected in dead leaves, bulb scales, basal plates or roots. In post-bulb plants, virus titer distribution was considerably non-uniform, with no apparent trend within or between leaves. Sources of resistance to IYSV and onion thrips exist in the onion gene pool. The search for these vital sources should continue so that the sustainability of the U.S. onion industry may be achieved through the use of efficient, reliable and environmentally safe integrated disease management strategies.