Measuring Selection Progress in Onion Germplasm after One Selection Cycle for Reduced Iris Yellow Spot Symptom Expression

Onion thrips and Iris yellow spot virus (IYSV) cause huge adverse qualitative and quantitative losses to onion bulb and seed production throughout the world. An increased resistance to several insecticides by onion thrips along with elevated costs of pesticide sprays restricts onion grower’s options to effectively control thrips population and IYSV. Due to the absence of resistant cultivars to thrips and IYSV; the objective of this study was to find if any progress was made in onion germplasm for thrips and IYSV resistance after selection was performed. Plants with fewer Iris yellow spot (IYS) disease symptoms were selected in Summer 2009 and self-pollinated in the following year to produce seed. In October 2011, IYSV-infected bulbs were placed on the first and last bed of the study and at the front and back borders to obtain maximum exposure of thrips and IYSV to the test plots. In addition, to act as disease spreader rows, IYSV-susceptible cultivar, NuMex Dulce, was sown in Autumn 2011 after every two rows of the test plots. The original and selected onion germplasm were evaluated in 2012. Data were collected on number of onion thrips per plant from ten plants per plot at three times 4 weeks apart, starting at 17 weeks after sowing. Plants were rated for IYS severity based on a rating scale of 0–4 at three intervals, starting at 17 weeks after sowing and 4 weeks apart. To confirm the presence of IYSV in plants through ELISA, leaf samples from 10 plants in each plot were collected. NMSU 10-807 had a significantly fewer number of thrips from its original material of NMSU 07-53-1 and susceptible check Vaquero at week 25 after sowing. NMSU 10-785, NMSU 10-799, and NMSU 10-813 all had a significantly lower disease severity from its original materials, NMSU 07-32-2, NMSU 07-52-1, and NMSU 07-53-1, respectively, along with from the susceptible checks, ‘Rumba’ and ‘Vaquero’ at 21 weeks after sowing. NMSU 10-785 had a lower disease incidence from its original material, NMSU 07-32-2, and the susceptible check, ‘Rumba’ at week 17 after sowing. No significant differences among different entries were observed for thrips count, IYS disease severity and incidence at two out of three intervals after data evaluation. On the whole, some progress was observed in some of the improved populations for better resistance for thrips and IYS after one selection cycle.