Taking a Whole Farm Approach to Managing Onion Thrips and Iris Yellow Spot Virus.

Iris Yellow Spot Virus (IYSV) and onion thrips threaten sustainable, profitable onion production in Utah and the Western US. Onion growers in Utah rely on high-risk insecticides for thrips management, which has increased insecticide resistance and may lead to greater severity of IYSV outbreaks that are vectored by the pest. In order to maximize onion bulb size some Utah growers apply as much as 400 lb nitrogen (N) per acre per season coupled with up to nine applications of insecticide. Others apply 100 lb N per acre or less and have not sprayed for onion thrips in several years. A total of 30 onion fields were surveyed over the course of the 2008 and 2009 growing seasons. Fields were chosen to represent a range of grower practices. Soil and onion tissue N, bulb size, thrips populations and IYSV incidence were monitored from May through September.  IYSV was detected at low levels in all fields sampled even when plants were asymptomatic. Correlation analysis demonstrated positive relationships between total soil N, onion tissue N, onion thrips and IYSV. There was little relationship between N application rate and yield and size of onion, indicating factors other than N application rate were controlling yield. In one year IYSV was correlated with soil pH indicating a role of plant stress in expression of the disease. Soil nitrate and ammonium levels over the course of the season were often mismatched with maximum plant needs. Soil N was often low at rapid growth stages but high at harvest.  Some fields contained as much as 100 lb N per acre foot remaining at the end of the growing season. Economic and environmental sustainability of onion production in Utah could likely be enhanced through a reduction in N fertilization coupled with a decrease in insecticide use.