Cultural Practices to Reduce the Expression of Iris Yellow Spot Virus In Onion

Iris yellow spot virus (IYSV) is known to cause onion (Allium cepa) seed stalk and leaf tissue necrosis. Iris yellow spot virus has recently been recognized as a serious disease of onion. It was first found locally in the Treasure Valley of Idaho and Oregon infecting onion seed crops in 1989 and is now known to occur in 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). The incidence of IYSV can be increased by inadequate control of onion thrips. Thrips control has become more difficult through increased thrips resistance to insecticides. A certain degree of varietal tolerance to thrips and IYSV has been shown. It is not known if management factors such as irrigation and fertilization that reduce plant stress might reduce IYSV expression. Research at the Malheur Experiment Station in 2007 and 2008 evaluated the combined effects of variety, irrigation system, irrigation criterion, and N rate on IYSV expression and onion yield and grade. Each irrigation treatment had split plots with two N fertilizer rates (112 and 224 kg N/ha). Within each N split plot, four varieties were tested as split-split plots. This report focuses on the four water stress levels tested with drip irrigation. The water stress levels were achieved by automatic and independent drip irrigation of each plot to maintain soil water tension at 10, 15, 20, or 30 kPa and examine the interactions of irrigation criteria with variety and N rate.
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 2008, but failed to influence IYSV expression either year. Iris yellow spot virus symptoms were low 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 supercolossal bulb yield than the wetter irrigation criteria in both 2007 and 2008, 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 either year.