2013 ASHS Annual Conference

Onion thrips and Iris yellow spot virus (IYSV) are two major pests of onions worldwide for which host plant resistance is absent. In Summer 2009, plants with fewer Iris yellow spot (IYS) disease symptoms were selected from plant introduction accessions and the resulting progeny were evaluated in Summer 2012. These selected lines were compared with their respective original population and ‘Rumba’ to see if any progress had been made for reduced thrips number and IYS symptom expression. In order to ensure that every plant in the field screening had the potential to be infected with IYSV, the experiment was design with the spread of thrips and IYSV in mind. In October 2011, border rows were planted with thrips-containing onion bulbs selected from the previous year’s IYSV study.  Spreader rows were sown at the same time with a known IYS susceptible cultivar. Plants of the test entries were transplanted in March 2012. As bulbs of the border rows flowered, thrips and IYSV moved to the plants of the spreader rows. When those plants matured, thrips moved to the test plants and moved the virus from the periphery of the field to the interior. The number of thrips per plant was determined from 10 plants/plot at 9, 12, and 15 weeks after transplanting (WAT). Since thrips exhibit feeding differences based upon leaf color and epicuticular wax amounts, plants were rated for leaf color and waxiness at 9 WAT. At 12, 14, 16, and 18 WAT, plants were rated on a scale of 0 to 4 for disease severity, where 0 indicated no symptoms and 4 represented more than 50% of leaf tissue was necrotic. NMSU 10-575-1, which possessed waxy, bluish green leaves, exhibited significantly fewer thrips at 15 weeks when compared to its original population. NMSU 10-577-1 and NMSU 10-582-1 did not exhibit any reduction in thrips number, but showed significant less IYS severity when compared to their original populations. NMSU 10-575-1 and NMSU 10-577-1 exhibited less severe symptoms when compared to ‘Rumba’. The results from this evaluation suggest that the progress can be made toward the development of cultivars with reduced IYS symptoms.