Geospatial Relationships of Iris Yellow Spot Virus and Thrips to Onion Production in Colorado, 2004

Iris Yellow Spot Virus (IYSV) is a devastating pathogen on onion production in several US onion producing regions. Onion producers in Colorado are one of the most recent groups impacted by the IYSV pathogen. At this time no known alternate host has been identified in the state. Studies have found that the virus does arrive each year on transplants brought in from outside the state. The research we are conducting looks at the spatial and temporal distribution of IYSV in Colorado onion fields. Data collection and mapping were performed using Global positioning systems (GPS) and Geographical information systems (GIS). In 2004, three commercial onion fields along the Colorado Front-Range were chosen. Two of these fields did not produce visual symptoms of IYS. The third field was seeded with both red (Tango) and white (Sterling) cultivars. Six visual observations were made on approximately 5 to 10 day intervals beginning in early August. Initial symptom development appeared to coincide with harvest of infested onion transplants near our field location. The two cultivars were analyzed separately. The Moran’s I spatial autocorrelation coefficients for cultivar Tango (0.09) and Sterling (0.03) were significant (P<0.001 and P=0.03, respectively); although the very small values indicate little spatial dependency existed between sample locations at the half acre scale. Total yield as well as jumbo and medium market class onions were negatively correlated with IYS incidence in cultivar Tango (R2=0.3137, P=0.0024; R2=0.3923, P=0.0005; R2=0.1910, P=0.0227, respectively). Yield of medium market class onions of cultivar Sterling was negatively correlated to increasing IYS incidence (R2=0.1693, P=0.0457). Also, plant population of cultivar Sterling had a negative correlation to incidence of IYS (R2=0.2566, P=0.0115). Total yield and jumbo market class were not significantly correlated to disease incidence for this cultivar. The preliminary results of this study indicate that sampling must occur at a scale smaller than 0.5 acres to obtain strong positive spatial autocorrelation and determine whether secondary spread of IYSV plays a significant role in disease progression.