Influence of Selected Cultural Practices On the Expression of Iris Yellow Spot Virus In Onion

Iris yellow spot virus (IYSV) is a major constraint to the production of onion (Allium cepa) bulb and seed crops in the Pacific Northwest of the US.  First found locally in the Treasure Valley of Idaho and Oregon infecting onion seed crops in 1989, the virus is now reported from many onion-producing areas around the world. Onion plants infected with IYSV can progressively loose leaf area, resulting in reduced yield and reduced bulb size. The virus is transmitted by onion thrips (Thrips tabaci). Thrips control has become more difficult because of the increased thrips resistance to insecticides.  It is not known whether irrigation and fertilization practices that reduce plant stress might also reduce the impact of IYSV. The combined effects of variety, irrigation system, irrigation criterion, and nitrogen (N) rate on IYSV expression and onion yield and grade were evaluated in 2007, 2008, 2009 and 2010.  Fertilization at 112 kg N/ha resulted in a higher onion yield and grade in 2007 than 224 kg N/ha.  There were no differences in onion yield or grade between N rates in the other three years, and N failed to influence disease incidence with the exception of slightly higher IYSV in 2010 at 224 kg N/ha.  Symptoms of IYSV were fewer in 2007 and no significant differences between treatments were observed. In 2008, averaged over varieties and N rates, drier irrigation criteria (higher SWT) resulted in significantly more severe IYSV symptoms. Averaged over varieties and N rates, drip irrigation at 30 kPa resulted in significantly lower marketable, colossal, and colossal plus super-colossal bulb yield than the wetter irrigation criteria all four years, but the differences were more pronounced in the presence of IYSV in 2008. There were no significant interactions between variety, irrigation criteria, and N rate.  Sprinkler irrigation failed to suppress thrips or IYSV.