Distribution and incidence of Iris yellow spot virus in Colorado and its relation to onion plant population and yield
David H. Gent and Howard F. Schwartz, Department of Bioagricultural Sciences and Pest Management, and Rajiv Khosla, Department of Soil and Crop Sciences, Colorado State University, Fort Collins 80523. Plant Dis. D-2004-0225-01R, 2004 (online).
Accepted for publication 15 December 2003.

Iris yellow spot virus (IYSV) is transmitted by thrips and has emerged as a potentially devastating and widespread disease of onion in the western United States. In annual surveys in Colorado, 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 spatial variability of disease incidence, yield, and plant population also were 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 cv. Sterling. In fields planted to the susceptible cultivars, disease incidence was highest on the field edges and lowest near the field centers. Yield of jumbo market class onions, but not total yield, was negatively correlated with increasing IYSV incidence in cultivar Teton. Colossal market class yield, but not other yield components, was negatively correlated with IYSV incidence in cultivar Sterling. 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.

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