Iris yellow spot virus in onions: a new tospovirus record from India

During 2002-03, field-infected onion (Allium cepa) plants exhibited characteristic symptoms of chlorotic spindle or diamond shaped lesions on the leaves and scapes, with twisting or bending flower bearing stalks (Fig. 1-6), were observed in the Jalna and Nasik regions of Maharashtra. In the advanced stages, single spindle-shaped chlorotic lesions coalesced, leading to withering of leaves and flower-bearing stalks. The disease was transmitted to a number of virus indicator plants by mechanical inoculation using phosphate buffer (0.01 M sodium sulfate, pH 7.0, containing 0.1% sodium sulfite). Inoculated Nicotiana benthamiana plants produced systemic necrotic lesions, eventually resulting in die-back and wilting of the plants, while N. tabacum (varieties 'Xanthi NC', 'White Burley', 'Samsun' and 'GT-4') and N. clevelandii produced local chlorotic ring spots, 3 to 6 days after inoculation. In Vigna unguiculata necrotic local lesions developed 3-4 days post inoculation. N. rustica failed to produce any symptoms and did not become infected.

Field-infected onion samples and greenhouse-inoculated plants were tested by ELISA, using the following antisera: Tobacco streak virus (DSMZ AS-0615), Watermelon silver mottle virus (DSMZ AS-0118), Iris yellow spot virus (DSMZ AS-0528), Tomato spotted wilt Virus (DSMZ AS-0105), Impatiens necrotic spot Virus (DSMZ AS-0115), Chrysanthemum stem necrosis virus (DSMZ AS-0529), Peanut bud necrosis virus (ICRISAT) and Potato virus Y (DSMZ AS-0573); utilizing the respective positive control samples. Field-infected onion samples and the respective mechanically inoculated tobacco plants reacted strongly with IYSV antisera, but failed to react with any of the other antisera tested. RT-PCR using primers designed to the capsid gene and flanking sequences of IYSV (GenBank Acc. No. AF001387) produced the expected 925 bp amplicon from infected but not healthy onions. The results of symptomatology, host range, ELISA and RT-PCR indicate that the causal agent is a strain of Iris yellow spot virus, which is a new report from an important onion growing regions of Maharashtra state, India. This virus has been reported as a potentially devastating pathogen of onion in Europe (Cortes et al. 1998), Israel (Gera et al. 1998) and the United States (Gent et al. 2004).