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Iris yellowspot (IYS) disease in onion (Allium cepa L.) is caused by onion thrips (Thrips tabaci L.) vectored Iris yellow spot virus (IYSV). The absence of cultivars that are resistant/tolerant to thrips and/or IYS is a challenge for onion bulb and seed production worldwide. To measure selection progress for reduced/delayed IYS symptom expression in onion breeding lines after two selection cycles, selections were performed in 2011 on previously evaluated lines that exhibited a reduced symptom expression after one selection cycle. Selected plants from each line were massed in a cage and the resulted progenies were evaluated in 2013 and 2014 along with their original populations and a susceptible check--'Rumba'. In some comparisons, the selection progress for delayed/reduced IYS symptom expression was observed for some breeding lines. Plants of most selected breeding lines exhibited less disease expression than plants of 'Rumba'. For some selections, a low disease severity was observed even with a relatively high number of thrips per plant. These results suggest that further improvement might be achievable with additional cycles of selection. [ABSTRACT FROM AUTHOR]

Onion thrips (Thrips tabaci Lindeman)-vectored Iris yellow spot virus (IYSV) causes the disease Iris yellow spot (IYS), which is a major threat to the sustainability of onion production worldwide. An increase in thrips resistance to various insecticides, high costs, and the limited efficacy of insecticides under hot and drier conditions found in various onion-growing regions restrict grower's options for effective control of thrips and spread of IYSV. Because cultivars resistant to thrips and IYS are lacking, this study was undertaken to measure selection progress for IYS resistance after one selection cycle. In 2009, selections were performed on previously evaluated New Mexico State University (NMSU) breeding lines that showed some reduced IYS disease symptoms, and the selected plants self-pollinated the following year. In 2011 and 2012, plants from the original and selected populations along with a susceptible check, 'Rumba', were evaluated under field conditions when onion thrips and IYSV were present. Plants were rated for IYS disease severity and the number of thrips per plant was recorded three times during the study in each year. First-generation material, NMSU 10-776, NMSU 10-782, NMSU 10-785, NMSU 10-807, and NMSU 10-813, had fewer thrips number per plant, lower disease severity, and disease incidence than their original breeding lines on at least one or two rating times in both years. Some first-generation breeding lines performed better with a lower thrips number and disease severity than their original population in 1 year or the other. Most entries exhibited fewer thrips, lower IYS disease severity, and less incidence than the susceptible check 'Rumba' at most rating times. Overall, some progress was observed in this first-generation material for reduced IYS disease symptom expression when compared with their original populations. [ABSTRACT FROM AUTHOR]