IYSV SYMPTOMS AND ETIOLOGY: OBSERVATIONS SINCE 1989.

S.K. Mohan, University of Idaho, Parma, ID 83660; and J. W. Moyer North Carolina State University, Raleigh, NC 27695.

Onion seed crops in the Treasure Valley region of Idaho and Oregon were observed during 1989 with hitherto unknown symptoms, mainly on the scapes (flower stalks). The dry, chlorotic or necrotic lesions were of varying sizes, mainly oval, circular or elongate, eye- or diamond-shaped. No fungus, bacterium or chemical damage could be associated with these symptoms. Initial studies with indicator plants and electron microscopy revealed association of a Tospovirus with the symptomatic tissues. Further studies using serological tests and bioassay, confirmed occurrence of the disease in onion, bunching onion, leek and chive seed crops in Idaho and on onion seed crops in Arizona and California. Disease incidence and severity varied widely among fields and yield losses ranged from negligible to complete loss of the seed crop. Tests with seeds from symptomatic plants did not show transmission of the virus to the seedlings. A disease with similar symptoms on irises was described from The Netherlands in 1998 and the causal agent was named Iris Yellow Spot Virus. Occurrence of the virus in onion bulb crops was first observed in 2001 and has since been confirmed from several states in the U.S.

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