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Christopher S. Cramer, Neel Kamal and Narinder Singh

Iris yellow spot (IYS) disease, caused by Iris yellow spot virus (IYSV), results in irregular and diamond-shaped, chlorotic, and necrotic lesions on the leaves and seedstalks of onions (Allium cepa L.). These lesions reduce leaf photosynthetic area and ultimately reduce onion bulb size and yield from larger bulb classes. IYSV is vectored by onion thrips (Thrips tabaci L.) that are difficult to control under certain environmental conditions. Currently, no onion cultivar is resistant to the disease symptoms, virus, and/or thrips. Twenty-one cultivars and 17 germplasm lines were evaluated in the field for IYS disease severity and thrips densities at multiple times during the season as well as leaf color, waxiness, and axil openness of these entries. Plants were grown under conditions that favored thrips populations (high temperatures, low moisture, and no insecticidal spray applications), IYSV presence and distribution, and IYS development. Plants of New Mexico State University (NMSU) 07-10-1 had fewer thrips than several entries later in the season in both 2009 and 2010. Several entries exhibited a lower number of thrips per plant early or later in the season; however, these results were not consistent across years and were not associated with a particular foliage characteristic. Lighter leaf color and/or a lesser amount of epicuticular wax did not always result in the fewest number of thrips per plant as has been reported in the literature. Plants of NMSU 09-58 tended to exhibit fewer and less severe IYS symptoms early in the season as compared with plants of other entries.

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Christopher S. Cramer, Narinder Singh, Neel Kamal and Hanu R. Pappu

Iris yellow spot is an economically important disease of onion that reduces bulb size and yield and is difficult to control. The disease is spread by Thrips tabaci (onion thrips) and disease symptoms are exacerbated by hot, dry climatic conditions that also favor rapid thrips multiplication and make control programs less effective. Currently, no onion cultivar is resistant to the disease and/or thrips. Certain onion foliar characteristics have shown nonpreferential feeding activity by thrips and may be the first step in developing Iris yellow spot (IYS)-tolerant onion cultivars. Seventy-five onion PI accessions from the U.S. germplasm collection were evaluated for leaf color, waxiness (bloom), and axil pattern; thrips number per plant; and IYS disease severity under conditions that favored thrips and disease buildup. Plants of PI 289689 were less attractive to thrips and had a lower number of thrips per plant than plants of most other accessions. These plants were rated as having light green to green-colored foliage and a relatively low amount of epicuticular leaf wax. Plants of PIs 239633 and 546192 generally exhibited less severe IYS disease symptoms than those of other accessions. Individual plants, that exhibited less leaf area exhibiting IYS disease symptoms, were selected at bulb maturity from 22 different accessions with PI 546140 producing the largest number of selected bulbs. Physiological plant development, environmental conditions, and tolerance to plant stress may influence the degree of disease symptom expression. Further work that examines the role of plant maturity and host plant tolerance to stress with respect to disease expression is needed.