Turnip yellows virus variants differ in host range, transmissibility, and virulence.

Turnip yellows virus (TuYV; family Solemoviridae, genus Polerovirus, species Turnip yellows virus) is a genetically diverse virus that infects a broad range of plant species across the world. Due to its global economic significance, most attention has been given to the impact of TuYV on canola (syn. oilseed rape; Brassica napus). In Australia, a major canola-exporting country, TuYV isolates are highly diverse, with the most variation concentrated in open reading frame 5 (ORF 5), which encodes the readthrough domain (P5) component of the readthrough protein (P3P5), which plays an important role in host adaptation and aphid transmission. When analysing ORF 5, Australian TuYV isolates form three phylogenetic groups with just 45 to 49% amino acid sequence identity: variants P5-I, P5-II, and P5-III. Despite the possible implications for TuYV epidemiology and management, research examining phenotypic differences between TuYV variants is scarce. This study was designed to test the hypothesis that three TuYV isolates, representing each of the Australian P5 variants, differ phenotypically. In particular, the host range, vector species, transmissibility, and virulence of isolates 5414 (P5-I5414), 5509 (P5-II5509), and 5594 (P5-III5594) were examined in a series of glasshouse experiments. Only P5-I5414 readily infected faba bean (Vicia faba), only P5-II5509 infected chickpea (Cicer arietinum), and only P5-I5414 and P5-III5594 infected lettuce (Lactuca sativa). Myzus persicae transmitted each isolate, but Brevicoryne brassicae and Lipaphis pseudobrassicae did not. When using individual M. persicae to inoculate canola seedlings, P5-I5414 had significantly higher transmission rates (82%) than P5-II5509 (62%) and P5-III5594 (59%). As indicated by enzyme-linked immunosorbent assay absorbance values, P5-I5414 reached higher virus titers in canola than P5-II5509, which, in turn, reached higher titers than P5-III5594. P5-I5414 was also more virulent in canola than P5-II5509 and P5-III5594, inducing more severe foliar symptoms, stunting, and, in one of two experiments, seed yield loss. Results from this study compared to those of previous studies suggest that analysis of ORF 5 alone is insufficient to assign isolates to coherent strain categories, and further sequencing and phenotyping of field isolates is required.

Vector species, pasture legume host range, and impact on grain legumes of an Australian soybean dwarf virus isolate.

Soybean dwarf virus (SbDV; family Tombusviridae, genus Luteovirus, species Soybean dwarf virus) can cause damaging disease epidemics in cultivated plants of the family Fabaceae. The biological characteristics of SbDV isolate WA-8, including its vector species, host range, and impact on Australian grain legume cultivars, were investigated in a series of glasshouse experiments. Isolate WA-8 was classified as the YP strain, as it was transmitted by Acyrthosiphon pisum (pea aphid) and Myzus persicae (green peach aphid) and infected known strain indicator species. Of the 18 pasture legume species inoculated with SbDV, 12 were SbDV hosts, including eight that had not been identified previously as hosts. When inoculated with SbDV, field pea (Pisum sativum), faba bean (Vicia faba), lentil (Lens culinaris), and narrow-leafed lupin cv. Jurien were the most susceptible (70 to 100% plant infection rates), and albus lupin (Lupinus albus), chickpea (Cicer arietinum), and narrow-leafed lupin cv. Mandelup were less susceptible (20 to 70%). Over the course of three experiments, chickpea was the most sensitive to infection, with a > 97% reduction in dry above-ground biomass (AGB) and a 100% reduction in seed yield. Field pea cv. Gunyah, faba bean, and lentil were also sensitive, with a 36 to 61% reduction in AGB. Field pea cv. Kaspa was relatively tolerant, with no significant reduction in AGB or seed yield. The information generated under glasshouse conditions in this study provides important clues for understanding SbDV epidemiology and suggests that it has the potential to cause damage to Australian grain legume crops in the field, especially if climate change facilitates its spread.

Impact of Turnip yellows virus infection on seed yield of an open-pollinated and hybrid canola cultivar when inoculated at different growth stages.

Turnip yellows virus (TuYV; family Luteoviridae, genus Polerovirus) is the most economically damaging virus infecting canola (Brassica napus) in the south-west Australian grainbelt. However, the impact of TuYV infection at different growth stages on canola seed yield has not been examined. This information is vital for implementing targeted management strategies. Four glasshouse experiments were conducted to examine seed yield losses incurred by an open-pollinated (ATR Bonito) and hybrid (Hyola® 404RR) canola cultivar when aphid-inoculated with TuYV at GS12 (two leaves unfolded), GS17 (seven leaves unfolded), GS30 (beginning of stem elongation) and GS65 (full flowering). When inoculated at GS12 and GS17, cv. Bonito plants incurred 30 % and 36 % seed yield losses, respectively, compared to healthy plants. Similarly, cv. 404RR incurred 41 % and 26 % seed yield losses at GS12 and GS17, respectively. However, when inoculated at GS30, whilst cv. Bonito plants incurred a 26 % seed yield loss, cv. 404RR incurred no significant loss. Neither cultivar incurred seed yield losses from inoculation at GS65. Additional information was collected from these experiments to improve sampling protocols to enhance TuYV detection, with a molecular and serological technique. When canola plants were at pre-flowering growth stages, TuYV was reliably detected 7-14 days after inoculation (DAI) in the youngest leaf. Once flowering had begun, TuYV was consistently detected 7-14 DAI in petals and flower buds. In contrast, regardless of growth stage, testing the oldest leaf regularly resulted in delayed detection or false negatives. Information generated in this study helps to quantify the value of management strategies targeted at preventing TuYV spread in pre-flowering canola crops and ultimately increase the efficiency of resource use.