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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. In the United States, IYS disease, caused by Iris yellow spot virus (IYSV) (family Bunyaviridae, genus Tospovirus), was first observed in bulb onion crops in the Treasure Valley of Idaho and Oregon (Hall et al., 1993) and later in Colorado (Schwartz et al., 2002). Since then, IYS has been confirmed in most other onion producing states in the United States, including Arizona, California, Florida, Georgia, Michigan, New Mexico, New York, Nevada, Oregon, Texas, and Washington (Bag et al., 2009; Creamer et al., 2004; Crowe and Pappu, 2005; du Toit et al., 2004a; Gent et al., 2006; Hoepting et al., 2007; Miller et al., 2006; Mullis et al., 2004; Pappu et al., 2009; Pappu and Matheron, 2008; Poole et al., 2007). An IYS epidemic in Colorado during 2003 was estimated to have cost growers $2.5 to $5.0 million in farm receipts alone (Schwartz and Gent, unpublished data). The virus is spread by onion thrips (Thrips tabaci L.). If the rate of spread and damage from IYS and onion thrips continues to be unchecked in the western United States, projected economic impacts could reach $60 million (10% loss) to $90 million (15% loss in farm gate value) per year in addition to environmental and economic costs incurred from an increase in numbers of pesticide sprays despite their limited efficacy ($7.5 to $12.5 million for three to five additional sprays on 48,500 ha per year) (Schwartz, unpublished data). IYS is characterized by irregular or diamond-shaped straw-colored lesions that develop on leaves and seedstalks (du Toit et al., 2004a; Gent et al., 2004; Schwartz et al., 2002). If IYS symptoms become severe, bulb size is reduced and the yield of larger bulb classes is reduced (du Toit et al., 2004b; Gent et al., 2004). Control of IYS requires an integrated approach including control of the vector, cultural practices, and genetic resistance/tolerance. IYSV is transmitted principally by onion thrips (Kritzman et al., 2001; Nagata et al., 1999) that acquire the virus during their larval stage and transmit the virus for their entire life. IYSV can also by transmitted by tobacco thrips (Frankliniella fusca) (Srinivasan et al., 2012). The primary means of controlling onion thrips is with insecticides. A promising and sustainable means for long-term thrips and IYS control is the development of cultivars that have an increased tolerance or resistance to the virus, thrips damage, and/or reduced thrips feeding injury. Several evaluations of onion cultivar suitability as thrips hosts have been reported (Coudriet et al., 1979; Jones et al., 1934; Saxena, 1977). These studies documented differences in thrips populations colonizing on different cultivars, but the differences in thrips numbers on different cultivars have generally been more modest. Al-dosari (1995) expanded the idea of thrips resistance in onion by considering cultivar response to injury along with cultivar suitability to thrips as a factor in resistance to thrips. These studies developed the idea that tolerance to thrips feeding, rather than host plant resistance to insect infestation, can be a very important resistance mechanism involved in reducing damage to onion crops. A wide range in tolerance to thrips feeding injury was demonstrated independent of thrips populations on plants and these were expanded into the idea of varietal-based action thresholds that have been promoted in Midwest onion production (Davis et al., 1995). The idea behind this is that some cultivars suffer high yield loss from thrips infestation; others sustain little, if any, yield loss at similar thrips infestation levels (Mahaffey, 2006). These types of studies suggest that some cultivars (e.g., ‘Aspen’) are more susceptible to the effects of thrips feeding than are other cultivars (e.g., ‘Yula’) and that tolerance to injury is an important thrips resistance mechanism (Mahaffey, 2006). In addition, cultivar differences for thrips damage are often associated with amounts of epicuticular waxes on leaves (Coudriet et al., 1979; Damon et al., 2014; Jones et al., 1934). Normal waxy onions possess copious amounts of epicuticular waxes (Damon et al., 2014) that build up on leaf surfaces and allow thrips to adhere to the plant and incite damage. Glossy foliage accumulates only sparse amounts of epicuticular waxes, appears lighter green in color, slows the growth of thrips populations, and experiences less damage from thrips feeding (Alimousavi et al., 2007; Damon et al., 2014; Jones et al., 1934; Molenaar, 1984; Mote and Sonone, 1977; Pawar et al., 1975). Some heirloom cultivars of onion (such as ‘Colorado #6’ and many ‘Sweet Spanish’ cultivars) are ‘‘semi-glossy’’ (Damon et al., 2014), meaning that the plants accumulate an intermediate amount of epicuticular waxes relative to glossy and waxy plants. As suggested by some studies, thrips also prefer blue-colored foliage to light green and green-colored foliage (Czencz, 1987; Kirk, 1984; Lu, 1990; cited in Gent et al., 2006). Plants with a higher amount of epicuticular leaf wax tend to have blue-colored foliage. Diaz-Montano et al. (2012) observed that two cultivars, Yankee and Nebula, had Received for publication 30 June 2014. Accepted for publication 31 July 2014. This research was funded in part by the USDA-NIFA Specialty Crop Research Initiative grant number 2008-51180-04875, a germplasm evaluation grant from the National Plant Germplasm System, USDA-ARS, the New Mexico Agricultural Experiment Station, and the New Mexico Dry Onion Commission. Professor of Horticulture. To whom reprint requests should be addressed; e-mail cscramer@nmsu.edu. HORTSCIENCE VOL. 49(10) OCTOBER 2014 1253 significantly higher numbers of thrips larvae as compared with other cultivars. Both of these cultivars possessed blue-colored foliage. Plant architecture has been suggested to influence a cultivar’s susceptibility/resistance to thrips. Onion thrips typically reside in the tight junction (axil) of the basal portion of the visible leaves. Jones et al. (1934) suggested that the tight leaf axil of ‘White Persian’ might be a possible mechanism of thrips resistance in that cultivar. Although numerous evaluations have been conducted, no onion cultivar or breeding line has been found to be highly resistant to IYS (Boateng et al., 2014; du Toit et al., 2004b; Gent et al., 2004, 2006; Mohseni-Moghadam et al., 2011; Multani et al., 2009; Schwartz et al., 2005; Shock et al., 2008). Creamer et al. (2004) reported differences in the IYS incidence of three onion cultivars grown in a growth chamber. du Toit et al. (2004b) rated 46 onion cultivars for IYS severity and observed significant differences for IYS susceptibility. IYS incidence ranged from 58% to 97% among different cultivars. Schwartz et al. (2005) reported significant differences for IYS incidence among onion cultivars when IYS incidence ranged from 16% to 100% in 2003 and 13% to 61% in 2004. Shock et al. (2008) found that ‘Joaquin’, ‘Charismatic’, and ‘Affirmed’ expressed some of the lowest IYS ratings over a 2-year period. Multani et al. (2009) observed that among 18 winter-sown entries, NMSU 03-52-1, NMSU 04-41, NMSU 04-44-1, and ‘NuMex Jose Fernandez’ exhibited the fewest IYS symptoms. NMSU 05-33-1 exhibited a delayed symptom expression when compared with 12 other entries (Mohseni-Moghadam et al., 2011). In another study, ‘Cometa’ and NMSU 05-35-1 exhibited a delay in symptom expression and lower IYSV levels relative to 20 other entries (Cramer et al., 2012). The objective of this research was to evaluate onion PI accessions in the current collection for foliage color, visible amount of epicuticular leaf wax, number of thrips per plant, and IYS disease severity and incidence and to select individual plants with reduced disease symptom expression in the hopes of developing disease-resistant populations. Materials and Methods Using the Genetic Resources Information Network (USDA, ARS, National Genetic Resources Program, 2014a), the U.S. germplasm collection was searched for onion (Allium cepa L. and A. cepa var. cepa) PI accessions. Accessions were selected from the collection based on their availability and their descriptor score of 1, 2, or 3 for leaf bloom that indicated a lesser amount of epicuticular leaf wax (Tables 1 and 2) (USDA, ARS, National Genetic Resources Progr

Screening Onion Plant Introduction Accessions for Tolerance to Onion Thrips and Iris Yellow Spot