The 2011 ASHS Annual Conference

Onion leaf color, waxiness, and axil pattern has been associated with onion thrips feeding preference. Plants, that possess a closed leaf axil pattern, blue to dark green leaf color, and waxy leaves, are preferred by onion thrips while plants, that possess an open pattern, light green color, and glossy leaves, are not. Onion thrips cause drastic yield losses, are difficult to control, and spread Iris yellow spot virus, that is a devastating foliar disease. Disease symptoms are exacerbated by hot, dry climatic conditions that also favor rapid thrips multiplication and reduced control efficacy. Currently, no onion cultivar is resistant or tolerant to Iris yellow spot (IYS) and/or thrips damage. Onion germplasm, that is less attractive to thrips feeding, would aid in the development of disease-tolerant cultivars. In the hopes of finding IYS-tolerant onion germplasm, 48 onion plant introduction accessions were evaluated for leaf characteristics, onion thrips number/plant, and IYS disease severity. Accessions were evaluated for leaf color (scale of 1-4 where 1 = light green color and 4 = blue), leaf waxiness (scale of 1-4, where 1 = glossy and 4 = waxy), and leaf axil pattern (scale of 1-4, where 1 = very open and 4 = tight) when grown in Las Cruces, NM. Thrips number was counted from ten plants per plot at three separate times throughout the growing season. Ten plants/plot were rated individually at three separate times for IYS severity (scale of 0-4, where 0 = no symptoms, 1 = 1 to 2 small lesions per leaf, 2 = >2 medium-sized lesions per leaf, 3 = lesions coalescing on more than 25% of the leaf, and 4 = more than 50% leaf death). Twelve accessions were rated as having light to dark green leaf color, three were rated as having semi-glossy to glossy leaves, and one possessed an open leaf axil pattern. PIs 165498, 258956, and 264320 possessed semi-glossy foliage that was dark green in color. Thrips number per plant was highest at 12 weeks post transplanting while the number decreased afterwards up to 16 weeks.  At 12 weeks, plants of PI 165498 had fewer thrips per plant than plants of other accessions. At 16 weeks, PIs 249899, 288073, and 391509 exhibited less severe IYS symptoms than other accessions. Through this evaluation, accessions have been identified that possess characteristics favorable for the development of thrips and/or IYS disease tolerant/resistant cultivars.