Symptoms induced by Iris yellow spot virus (IYSV) in an onion seed crop (top left and right photos) and an onion bulb crop (lower photo). Characteristic symptoms include diamond-shaped chlorotic lesions on the scape (seed stalk) (top left photo) and leaves (lower photo), that may turn necrotic and coalesce (top right and lower photos). Photos courtesy of Lindsey J. du Toit and Gary Q. Pelter.

Lindsey J. du Toit, Gary Q. Pelter, and Hanu R. Pappu
Washington State University
Department of Plant Pathology, 16650 State Route 536, Mount Vernon, WA 98273-4768
dutoit@wsu.edu

Host: Allium cepa, onion seed crop and onion bulb crop
Disease name: Iris yellow spot
Pathogen name: Iris yellow spot tospovirus

Iris yellow spot virus has been endemic in the Treasure Valley of Idaho for over a decade, but was recently found in the region of onion production in eastern Oregon. The virus has become increasingly prevalent in onion crops in the western US, with serious outbreaks of iris yellow spot noted in Idaho in 2001 and 2002. While the epidemics have largely been confined to seed crops during the last decade, bulb crops were also detected as infected in 2001 and 2002. The disease can affect yield both in terms of quantity and quality in seed and bulb crops. More recently, the disease was reported in Arizona, California, Colorado, and Utah. Onion seed crops with symptoms suggestive of IYSV infection were observed in central Washington in the summer of 2002, and symptomatic seed crops and bulb crops were observed in the summer of 2003. Laboratory confirmation of the presence of IYSV in association with the lesions was achieved using a PCR assay and nucleotide sequence data.