Moyer | IYSV: A New and Devastating Pathogen of Onion


For further information, contact: S. Krishna Mohan 208-722-6701 Ext. 218 kmohan@uidaho.edu University of Idaho Parma Research & Extension Center Parma, ID 83660 James W. Moyer 919-515-798 james_moyer@ncsu.edu Dept of Plant Pathology North Carolina State University Raleigh, NC 27695 Diane Ullman 530-752-3799 deullman@ucdavis.edu Dept of Entomology UC Davis Davis, CA
Literature Cited Cortes, I., Livieratos, I. C., Derks, A., Peters, D., and Kormelink, R. 1998. Molecular and serological characterization of Iris Yellow Spot Virus, a new and distinct tospovirus species. Phytopathology 88:1276-1282. Gera, A., Cohen, J., Salomon, R., and Raccah, B. 1998. Iris Yellow Spot tospovirus detected in onion (Allium cepa) in Israel. Plant Disease 82:127. Hall, J. M., Mohan, K., Knott, E. A. and Moyer, J. W. 1993. Tospoviruses associated with scape blight of onion (Allium cepa) seed crops in Idaho. Plant Disease 77:952. Kritzman, A., Beckelman, H., Alexandrov, S., Cohen, J., Lampel, M., Zeidan, M., Raccah, B., and Gera, A. Lisianthus leaf necrosis: a new disease of lisianthus caused by Iris yellow spot virus. Plant disease 84:1185-1189 Kritzman, A., Lampel, M. Raccah, B., and Gera, A. 2001. Distribution and transmission of Iris yellow spot virus . Plant Disease 85:838-842.

Iris Yellow Spot Virus: A New and Devastating Pathogen of Onion
K. Mohan, University of Idaho; J. W. Moyer, N. C. State University

A new disease of onion caused by a putative tospovirus was first reported in the United States in 1991 by the authors (Mohanl et al, 1991; Hall et al., 1993). In those reports we described the symptoms, and that a virus with enveloped virions characteristic of tospoviruses was isolated that did not react with either TSWV or INSV antisera. This new virus was determined to be an isolate of Iris Yellow Spot virus (IYSV) (Cortes et al., 1998). They demonstrated that the virus was indeed a tospovirus with typical enveloped virion morphology and a tripartite RNA genome. Comparisons of the IYSV N gene amino acid sequence with the N gene from other representative tospoviruses revealed only a 34-44 % similarity. It was also distinct from TSWV in that it had a very narrow host range. It was only reported to infect Datura stramonium and Nicotiana benthamiana outside of Allium spp. In the Netherlands it was detected in leek as well as Iris. A very similar virus was isolated from onions in Brazil with symptoms of the "sapeca" disease. Symptoms consist of chlorotic and necrotic eye-like or diamond shaped lesions on the stalk bearing the seed head. This isolate also had a narrow host range, but would also infect N. rustica (Pozzer et al., 1999). At about the same time the virus was also reported from Isreal to cause significant losses in onion production (Gera et al, 1998) and cause leaf necrosis in Lisianthus (Kritzman et al., 2000). Interestingly, IYSV has only been transmitted by Thrips tabaci and not by Frankliniella occidentalis, an efficient vector of many other tospoviruses (Cortes, et al., 1998; Kritzman, et al., 2001).

In the United States. IYSV has been endemic in southwestern Idaho and eastern Oregon onion, leek and chive seed production fields for over 10 years. It has also been detected in onion seed plants from California and Arizona . Symptoms include chlorotic and necrotic diamond shaped lesions on the scape (seed stalk) (Fig 1 & 2). These lesions are often aggregated around the bulb-like swelling about a third of the way up the scape. Only rarely have symptoms been observed on the leaves, although the virus was detected in symptomless leaves of onion seed plants showing symptoms on seed stalk. However, during the 2001 growing season, severe chlorotic and necrotic lesions have been observed on the leaves of onion bulb and seed crops, and these were confirmed to be infected with IYSV. Fields of onion seed and bulb production are often located adjacent, or close to each other, and yet there has been little evidence of spread from seed fields into neighboring bulb production fields until 2001. During the summer of 2001, symptomatic plants were observed in commercial bulb production fields in Colorado, Idaho, Utah and Washington. We have obtained serological confirmation of the presence of IYSV in all states but Washington. Studies are currently underway to compare isolates from the 2001 outbreak to those previously collected from infected seed crops.