Iris yellow spot virus (IYSV) is an emerging and potentially devastating disease of onion that was recently detected in Colorado and other onion producing regions in the western United States. In annual surveys, IYSV was confirmed in one of 18 fields (5.6%) in 2001, four of 24 (16.7%) in 2002, and 41 of 56 (73.2%) in 2003. IYSV was confirmed on volunteer onions in 2003 at all four locations where IYSV was observed in the onion crop the previous year. The disease was detected in six of seven western Colorado onion fields surveyed in 2003, but was not observed any year in southern or northeastern Colorado. The spatial variability of disease incidence, yield, and plant population also was mapped in two fields in 2003 using the global positioning system and a geographic information system. Disease incidence varied among cultivars, plant population, fields, and location in the field. Distinct disease gradients were observed in both fields with susceptible cultivars Teton and Granero, but not in the moderately resistant cultivar Sterling. In fields planted to the susceptible cultivars, disease incidence was highest on the field edges and lowest near the field centers. Plant population was negatively correlated with IYSV incidence in cultivar Sterling (R2 = 0.56, P = 0.003), but not with the susceptible cultivars. Yield of jumbo market class onions, but not total yield, was negatively correlated with increasing IYSV incidence (R2 = 0.37, P = 0.012) in cultivar Teton. Colossal market class yield, but not other yield components, was negatively correlated with IYSV incidence in cultivar Sterling (R2 = 0.28, P = 0.061). The results of these studies indicate the distribution of IYSV is rapidly expanding in Colorado and is associated with a general reduction in bulb size.