Spatial and Temporal Distribution of Iris yellow spot virus and Thrips in Colorado Onion Fields

Iris yellow spot virus and its onion thrips vector (Thrips tabaci) are yield limiting pests of onion in the western United States. This two-year project investigated the relationship of iris yellow spot to thrips populations, soil properties, and grower management practices in six onion fields during 2005 and 2006 in Colorado. Thrips density (average number of thrips per plant per day) was negatively associated with soil pH (r = -0.61). The incidence of iris yellow spot at harvest was positively associated with organic matter, zinc, potassium, percent sodium, percent hydrogen, copper, iron, and soil pH. Thrips-days had a clear non-linear association with disease incidence at harvest. Onion plant population was positively associated with levels of iron, phosphorus, potassium, and zinc; and negatively associated with calcium, salts, percent potassium, and cation exchange capacity (CEC). Marketable (medium or larger sized bulbs) yield was negatively associated with phosphorus and percent potassium, and positively associated with salts, calcium, and CEC. Results from this project could contribute to the development of a risk index to predict potential risk from Iris yellow spot virus and its onion thrips vector.