In early September of 2001, we found evidence of a new viral disease of onion on greenhouse-produced transplants of 'Colorado 6' grown in a field in northern Colorado. Serological tests of Colorado samples were positive for presence of the virus.

The disease is caused by Iris Yellow Spot Virus (tospovirus) which is transmitted by various species of thrips, including onion thrips. The disease has a host range which includes onion, garlic, leek, and iris. It has been reported previously on onion in Israel, Brazil, Idaho, and Utah.

Disease symptoms include straw-colored, dry, tan, spindle- or diamond-shaped lesions on the leaves and scapes of onion plants. Some lesions have distinct green centers with yellow or tan borders; other lesions appear as concentric rings of alternating green and yellow/tan tissue. Infected plants may be scattered throughout a field as observed in Colorado, or generalized throughout an entire field as observed in Utah this year.

CSU personnel will monitor onion fields in 2002 for evidence of the virus, and urge growers to implement sound IPM practices which include sanitation and destruction of onion debris, onion culls and volunteers, crop rotation, selection of onion varieties that are less susceptible to thrips, planting clean seed and transplants, weed management in and around onion fields, and thrips control.

• Chemical Control of Onion Thrips - Fruita, CO
• Distribution and Incidence of Iris yellow spot virus in Colorado and its relation to onion plant population and yield. May, 2004, Plant Disease
• Iris Yellow Spot Virus, by Jensen, Geary, and Bijman, Oregon State University
  This is a pdf file and requires the free Acrobat Reader program.
• Iris Yellow Spot Virus: A New and Devastating Pathogen of Onion by K. Mohan, University of Idaho; J. W. Moyer, N. C. State University
• Western Colorado Thrips