Thrips vectors and resistance to tomato spotted wilt virus (TSWV) in potato

Field: Agricultural and Biological Sciences (Life Sciences)

This study was formulated to examine the efficiency of Tomato spotted wilt virus (TSWV) transmission by onion thrips (Thrips tabaci Lindeman) and factors associated with host resistance in potato; in particular to investigate the suggestion that potato cv. Bismark has a high level of resistance to thrips, and to examine why onion thrips have failed to transmit TSWV in laboratory experiments in previous studies. Three field trials were conducted in Tasmania and South Australia to evaluate differences in potato cultivar resistance to thrips and TSWV (Chapter 2). TSWV-infection levels were moderate in two trials, with TSWV-incidence varying from 9-26 percent in Tasmania and 3-22 percent in South Australia, but only 0-6 percent in the second Tasmanian trial. Thrips counts showed the highest numbers of T. tabaci on Bismark and lowest thrips numbers were found on Shepody. There were no significant differences in TSWV foliar or tuber infections between cultivars, and no correlation between thrips numbers and TSWV incidence. A population of T. tabaci was subjected to choice experiments to test for colour preference (Chapter 3), and host preference and oviposition choice (Chapter 4), using a number of commercial potato cultivars and coloured cards. Populations of western flower thrips (Frankliniella occidentalis Pergande) and tomato thrips (Frankliniella schultzei Trybom) were also tested for colour and host preference alongside onion thrips in separate experiments. Colour preference tests showed strong colour preferences amongst all three thrips species tested. Western flower thrips and tomato thrips strongly preferred green to red, blue and white; but preferred yellow to green. Onion thrips preferred green and yellow equally and over the other three colours. Onion thrips showed a strong preference for light-green over darker shades of green. Host preference tests showed differences in potato cultivar preference by onion thrips, with higher attraction to cultivars with lighter green foliage: Shepody and Russet Burbank. Oviposition choice tests showed almost the opposite, with higher numbers of hatched juvenile thrips on darker green potato cultivars: Atlantic, Bismark, Royal Blue and Tasman. Several female-only, parthenogenetic populations of T. tabaci were collected from Tasmania, New South Wales and South Australia from potato, onion and Chrysanthemum. These populations were tested for their ability to transmit TSWV to potato and other hosts, and subjected to a phylogenetic analysis following DNA extraction and PCR amplification of mitochondrial gene cytochrome c oxidase subunit 1 (COI) (Chapter 5). Vector competence was associated with the host from which the populations were collected, with three populations collected from potato transmitting TSWV, but three populations collected from onion failing to transmit the virus. This ability to transmit TSWV was also associated with differentiation in COI, with vector competent and non-competent populations separating into subgroups within the ‚ÄövÑvªL2‚ÄövÑvº European clade of Brunner et al. (2004). This is the first study to link genetic differentiation of T. tabaci to both source host and vector competence, and provides a credible explanation for why many studies have failed to achieve any transmission of TSWV by this species. Strong colour preferences and some host preferences were also demonstrated, however field experiments suggest that potato cultivar resistance to thrips is unlikely to provide a reliable method for reducing TSWV infection levels in commercial potato crops.