September 3, 2004
From: ProMED-mail<promed@promedmail.org>
Source: American Phytopathological Society, Plant Disease Notes [edited]

Iris yellow spot virus on onion in New Mexico

R. Creamer, S. Sanogo, A.Moya, and J. Romero, Department of Entomology, Plant Pathology, and Weed Science, New Mexico State University, Las Cruces 88003; and R. Molina-Bravo and C. Cramer, Department of Agronomy and Horticulture, New Mexico State University, Las Cruces, 88003. Plant Dis. 88:1049, 2004; published on-line as
D-2004-0715-01N, 2004. Accepted for publication 29 Jun 2004.

Onions are an important crop for New Mexico, with 7700 acres (3116 ha) harvested in the state in 2003 (3). In 2002, onions of several cultivars were first noticed with diamond-shaped chlorotic or bleached lesions on seed stalks or leaves, typical of those reported for Iris yellow spot virus (IYSV). A more widespread survey of breeding stocks and commercial onion fields revealed similar symptoms on thrips-infested onions in Dona Ana and Rio Arriba counties. Incidence of disease symptoms ranged from less than 0.5 to nearly 30 percent.

Symptomatic leaves were assayed for the presence of IYSV using enzyme-linked immunosorbent assay (ELISA; Agdia, Elkhart, IN) and antisera acquired from Agdia. Symptomatic leaves from breeding and commercial fields tested positive for IYSV. The virus was transmitted by _Thrips tabaci_ from symptomatic onions to 3 onion cvs. New Mex Mesa, New Mex Vado, and New Mex Cryspy in growth chamber tests.

All 3 cultivars showed symptoms of IYSV and tested positive for the disease using ELISA. However, New Mex Vado and New Mex Cryspy cultivars each showed 24 percent infection (4 infected plants of 17 tested) compared with 59 percent infection (10 infected plants of 17 tested) for New Mex Mesa, suggesting that not all cultivars are equally susceptible to the virus.

To our knowledge, this is the 1st report in onions in New Mexico of IYSV, which has also been reported in the western United States in Idaho, Oregon, Colorado, and Washington (1,2,4).

References:
(1) L. J. du Toit et al. Plant Dis. 88:222, 2004.
(2) J. M. Hall et al. Plant Dis. 77:952, 1993.
(3) National Agricultural Statistics Service, On-line publication. USDA, 2004.
(4) H. F. Schwartz et al. Plant Dis. 86:560, 2002.

[Iris yellow spot tospovirus (IYSV) is transmitted by onion thrips (_Thrips tabaci_) and has been reported from over 50 countries worldwide. Recommended disease management options include removal of culled onions so as to reduce sources of infection, reduction of plant stressors so that the crop has sufficient soil and fertility, reduction of soil compaction and irrigation stresses, and management of thrips populations by appropriate chemical insecticides. Natural enemies, including predaceous mites, minute pirate bugs, and lacewings, are often found feeding on thrips. These beneficial insects are very susceptible to insecticidal sprays, however, and are thus
unlikely to be effective in biological control of insects in fields treated with insecticides.

Useful references:
<http://www.ento.csiro.au/thysanoptera/Symposium/Section4/13-Kritzman-et-al.pdf>
<http://www.ipm.ucdavis.edu/PMG/r584300111.html>
- Mod.DH]