In Search of Onion Germplasm Tolerant to Iris Yellow Spot Virus

Iris yellow spot virus is a relatively new foliar disease of onion that causes bulb size reduction. Disease symptom expression is exacerbated by hot, dry climatic conditions that also favor the virus vector, onion thrips. Through their feeding on leaf tissue, onion thrips reduce bulb size and yield. They are attracted to onion leaves that are blue to bluish green in color and have a thick waxy coating. Some research suggests that they are less attracted to leaves that are light green in color and possess little or no wax on their surface. Onion germplasm, that is less attractive to thrips feeding because of certain leaf characteristics, would aid in the development of disease tolerant cultivars. Thirty-two entries were evaluated for the thrips number/plant, leaf characteristics, and Iris yellow spot (IYS) disease symptoms. IYS-infected onion bulbs were placed on the borders of the disease evaluation field to ensure that the virus was present for evaluation. Thrips number was counted from ten plants/plot at three separate times. Entries were evaluated for leaf color (scale of 1-4 where 1=light green color and 4=blue) and leaf waxiness (scale of 1-4, where 1=glossy and 4=waxy) when grown in Las Cruces, NM. 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=>50% leaf death). At 16 weeks post seeding, plants of NMSU 07-54-1 had fewer thrips/plant than plants of other entries. At 20 weeks, plants of ‘Early Red Burger’, NMSU 07-56-2, NMSU 07-57-2’, ‘Stockton Early Yellow’, and ‘Stockton Red’ averaged less than 10 thrips/plant and had fewer thrips/plant than plants of most other entries. Most entries had leaves that were light to dark green in color. Plants of 07-54-1 had glossy leaves while 39 entries had semi-glossy leaves. At 20 weeks, plants of NMSU 08-43, NMSU 07-54-1, and NMSU 09-58 expressed less severe IYS symptoms than plants of other entries. The disease incidence at 16 weeks was lower for NMSU 07-33-1, NMSU 09-58, and ‘NuMex Centric’ than other entries. Seven individual plants, that exhibited few IYS disease symptoms, were selected at bulb maturity from 4 different entries. These plants are being self-pollinated in the hopes of finding individual progeny that possess a higher level of IYS tolerance.