Take home messages

• High levels of turnip yellows virus (TuYV) infection occurred in southern New South Wales canola crops in 2024 associated with green peach aphid (GPA) activity early in crop development
• TuYV is transmitted by the GPA in a highly efficient manner; so even small inconspicuous populations of aphids can cause significant TuYV spread
• In two field trials conducted in Western Australia, a neonicotinoid seed treatment was ineffective at suppressing TuYV
• A foliar spray of sulfoxaflor was moderately effective when applied during the early stages of GPA colonisation. With the aid of routine crop monitoring, if both GPA and TuYV are present, foliar insecticides should be targeted early in crop development when the crop is most vulnerable to TuYV
• Host resistance was highly effective at suppressing TuYV spread and offers a promising avenue for TuYV control
• Combining control measures in an integrated disease management approach is likely to be more effective long-term than relying on just one control measure.

Introduction

In Australian canola (Brassica napus), turnip yellows virus (TuYV) is the most widespread and economically damaging virus. Over the past decade, TuYV epidemics have commonly occurred in different regions causing considerable losses. Field experiments have demonstrated that TuYV causes seed yield losses of 10 to 50% and negative seed quality impacts including decreases in oil content and increases in erucic acid and glucosinolate content (Jay et al., 1999; Jones et al., 2007). Yield losses are higher when TuYV infection occurs earlier in crop development.  Once the crop has flowered, infection is less likely to cause direct seed yield losses. Yield losses will also vary depending on the TuYV strain, canola variety, and presence of abiotic stress (Congdon et al., 2020; Congdon et al., 2023a).

TuYV is particularly challenging to manage primarily because it is transmitted by thegreen peach aphid (GPA; Myzus persicae). GPA is a highly efficient TuYV vector (just 1-2 aphids required to transmit) meaning few aphids per plant are required to cause substantial spread (Congdon et al., 2023a). GPA can rapidly disperse through a crop at low population densities spreading TuYV through large areas of the crop in the process. Due to their green colour and relatively small size, GPA can be challenging to find on a canola plant, meaning they can spread TuYV before they are even noticed in the crop. Their tendency to colonise lower leaves makes them even more inconspicuous and can make adequate coverage of insecticides more difficult to achieve. Moreover, GPA continues to develop resistance to insecticides, limiting options for control (Ward et al., 2023). Symptoms of TuYV infection are easily confused with other stress e.g. plant stunting and reddening or yellowing of lower leaves. If symptoms are expressed, they may only appear weeks to months after transmission has occurred (infection latency) and so effectively monitoring thresholds based on virus symptoms is near impossible. Finally, viruses cannot be sprayed out of the crop like fungal pathogens, so once TuYV reaches high levels in the crop it is too late to control it.

Both GPA and TuYV have very wide host ranges, meaning they can survive across a broad variety of refuges over the non-cropping period. From these refuges, GPA populations expand as summer and early autumn rainfall supports the growth of host plants such as brassica weeds, volunteer canola and dual-purpose canola (Coutts et al., 2006; Henry et al., 2018). TuYV spread in a canola crop begins when viruliferous GPA fly into the crop, and land and feed on the phloem of uninfected plants (‘primary spread’). Once this first generation of GPA reproduces on newly infected plants, ‘secondary spread’ begins as the subsequent generations of winged and wingless aphids growing on that plant and move to other plants in the crop. Climatic factors greatly influence the movement and population growth of GPA and subsequent rate of TuYV spread (Maling et al., 2010).

TuYV management involves preventing high levels of spread during the vegetative stage of the crop. An integrated disease management approach is recommended combining cultural (manipulating plant density, sowing date, stubble retention and removing host reservoirs with herbicides prior to sowing), chemical (seed treatments and foliar insecticides) and genetic (host resistance) strategies. However, this approach has not been adopted by growers as some measures run contradictory to agronomic priorities and others are not widely available (host resistance). Instead, if management is undertaken at all, it is focussed on preventing high numbers of GPA using insecticides, ostensibly to prevent feeding damage which is a rare phenomenon (Micic et al., 2017). Neonicotinoid-based seed treatments are applied prophylactically to canola seed purchased in Australia and therefore have been the primary method of GPA control. These seed treatments have been a useful tool in broadacre systems as they require minimal labour from the grower, operate at scale against several pest species and are systemically active in the plant for many weeks after emergence. Imidacloprid, clothianidin and thiamethoxam are three neonicotinoids currently used in registered formulations. Over the past decade, GPA has developed metabolic resistance to neonicotinoids conferred by amplification of P450 gene CYP6CY3, which was shown to reduce the effectiveness of an imidacloprid-based and a thiamethoxam-based seed treatment when applied at recommended rates under semi-field conditions (Kirkland et al., 2023). Therefore, commercial seed treatments require fresh examination for their ability to control GPA and TuYV under field conditions.

Growers also have the choice of several foliar insecticides registered for GPA in canola crops including sulfoxaflor (registered as Transform^® in 2014), flonicamid (MainMan^®, 2021) and afidopyropen (Versys^®, 2021). These are systemically active and residually effective to differing degrees and therefore, have potential as virus control tools (Perring et al., 1999). In contrast to seed treatments, foliar insecticides require action on the part of the grower and are often applied in a reactive manner once significant populations of GPA have developed. Usually, if TuYV is present in these cases, it has already reached high incidences by the time the product is applied to the crop. Therefore, for foliar insecticides to be effective for TuYV control, the timing of application is crucial. Research in the United Kingdom suggests spraying during the early stages of GPA infestation for effective TuYV control (Stevens et al., 2008), however this has yet to be investigated in Australia.

Resistant varieties are considered the most cost-effective and reliable approach to managing plant viruses (Kang et al., 2005). In Australia, there have been no direct attempts to introduce TuYV resistance into canola varieties. When screened for TuYV resistance in 2020/2021, most varieties were highly susceptible, but resistance was present in some e.g. ATR Stingray (Congdon et al., 2021). TuYV resistance can be highly effective at suppressing TuYV spread under field conditions (Coutts et al., 2010). The effectiveness of TuYV resistance under field conditions compared to and in combination with traditional insecticide-based approaches requires further assessment to warrant investment by breeders.

This paper presents findings from GPA and TuYV monitoring sites across NSW, and additional testing of diseased canola crops growing in southern NSW in 2024 revealing high levels of TuYV infection. Furthermore, results are reported from two field experiments conducted by DPIRD in Western Australia which aimed to investigate the effectiveness of a neonicotinoid-based seed treatment, foliar insecticide (sulfoxaflor) applied at different timings, and TuYV-resistance in ATR Stingray, alone and in combination, to control TuYV.

Methods

Monitoring for GPA and TuYV in NSW in 2024

In 2024, six monitoring sites were established in NSW: Breeza, Boggabri, Cowra, Tamworth, Trangie, and Wagga Wagga. At each site in canola fields, three sticky traps were placed approximately 50 meters apart along the fence line. The traps were set early May to coincide with the canola germination period. Every 2 to 3 weeks, the traps were collected, and labelled with details such as site, placement date, collection date, and crop growth stage. Data collection was completed by early September, aligning with the end of flowering and the beginning of podding stage.

The collected traps were processed by counting aphids on both sides of each trap at Tamworth Agricultural Institute (TAI). The total aphid count was recorded, and GPA were identified under a microscope, noting their presence or absence. The aphids are currently being tested for the presence of TuYV.

The raw data for total aphids and GPA numbers were transformed using a logarithmic scale, to stabilize variance and normalize the distribution of the data for more reliable statistical analysis. The Analysis of Variance (ANOVA) was used to assess whether differences in aphid counts could be attributed to the time of year (month) or the location (site).

To detect the presence of TuYV at each site, plant samples, consisting of the top 10 cm of individual plants, were analysed using the Tissue-Blot Immuno Assay procedure (TBIA) in the TAI plant pathology laboratory.

In addition to collecting results from our monitoring sites, growers and advisors in southern NSW started noticing virus-like symptoms in canola crops early in the season and submitted samples to the TAI laboratory for virus testing. Agronomists and growers were encouraged to submit paired samples from the same paddock: An equal number of symptomatic plants showing clear virus symptoms and healthy, non-symptomatic plants as controls for comparison.

Assessing TuYV management strategies in WA

Two field experiments were sown in WA at Muresk Field Research Station (medium-high rainfall zone); one in 2023 and the other in 2024. These were conducted to examine the effectiveness of three different timings of sulfoxaflor application (two timings examined in each trial) and a neonicotinoid-based seed treatment to control GPA and TuYV in a susceptible (ATR Bonito) and TuYV-resistant (ATR Stingray) variety. Seedlots of ATR Bonito and ATR Stingray were commercially treated either with imidacloprid at 120 g a.i/100 kg + clothianidin at 180 g a.i./100kg and fungicide, or fungicide only.

Canola seed was sown at 2.5 kg/ha with a plot seeder. Each experimental plot was 8 x 10 m, consisting of four plot seeder runs and a bare earth gap of 2 m between plots. Plots were organised in a randomised complete block design, with amendments made to ensure spatial optimisation. In 2023, the experiment was sown on 10^th May. In 2024, the experiment was initially sown on the 26^th April but did not germinate adequately and so was resown on the 7^th June.

GPA-infested TuYV-infected canola plants were generated in the glasshouse and then transplanted equidistantly throughout the experiment so that each plot had a virus source plant on each corner. In 2023, these aphids were introduced at the three-leaf growth stage (3-4 weeks after emergence) and in 2024 at the one-leaf growth stage (2 weeks after emergence). The GPA clone used had a 3x copy number of P450 gene CYP6CY3 suggesting it carries resistance to neonicotinoids. The TuYV strain used was TuYV-5509 originating from an infected canola crop growing in Jerramungup, WA in 2017.

Plots were sprayed with sulfoxaflor at 50 g a.i./ha either four days prior to aphid release (2023 only), two-weeks after aphid release (2023 and 2024) or four-weeks after aphid release (2024 only). An equal number of plots were left unsprayed as a control.

The total number of aphids were counted by whole plant visual search on four plants down the centre rows of each of the four-plot seeder runs that constituted each plot (16 plants total) at two, four and six weeks after aphid introduction (WAAI). In the same manner, the youngest fully formed leaf from 16 plants were taken from each plot at two, four, six, eight and ten WAAI and tested for the presence of TuYV infection by enzyme-linked immunosorbent assay.

Following plant senescence, each plot was harvested with the mean of two samples taken per plot used to obtain total seed yield. Seed oil and protein content was measured on a 500 g sample using a Perten IM9500 NIR machine.

For statistical analysis, aphid numbers underwent log transformation, whilst for TuYV spread an area under the disease progress curve (AUDPC) was calculated. Analysis of variance (ANOVA) was used to test whether each management strategy and the interactions between them influenced the aphid population and the AUDPC.

Results and discussion

Monitoring for GPA and TuYV in NSW in 2024

Aphid movement in canola fields during 2024 showed variability across monitoring sites in NSW, with generally low GPA counts. The highest mean aphid count in May was recorded at Cowra, with 1.33 (±0.89 SE) GPA per trap. In June, most sites recorded low aphid numbers, with Wagga Wagga having the highest count at 9.5 (±3.11 SE) GPA per trap. Overall, GPA numbers remained low across all sites during winter, although a brief peak was observed in June at Wagga Wagga, coinciding with the 4-leaf growth stage. Significant aphid movement did not occur until August and September, aligning with the flowering growth stage. Wagga Wagga experienced the earliest movement and recorded the highest peak of 66.67 (±22.66 SE) GPA in August.

The ANOVA analysis demonstrates that both month (F = 14.02, p < 0.001) and site (F = 7.95, p < 0.001) significantly influence GPA, with notable interactions between the two (F = 2.95, p = 0.009) (Figure 1). The months of August and September show a marked increase in GPA, particularly at sites like Wagga Wagga, which records the highest values. The Bonferroni test further confirms that GPA is significantly higher in the later months (August and September), which are grouped separately ('b'), compared to May, June, and July, which are grouped together ('a'). The significance of month aligns with the biological expectation that pest populations fluctuate over time. Peaks and reductions in pest abundance correspond to specific growth stages of the crop, environmental conditions, or pest reproductive cycles.

Figure 1. Logged (log₁₀ (x+1)) GPA counts per site during the monitoring period. Data within columns sharing the same lowercase letter are not significantly different at the 5% level of significance.

Although GPA numbers increased in August and September, they were not the primary cause of TuYV infections. Aphid flights in late autumn and early winter, especially those recorded in June at Wagga Wagga around the 4-leaf growth stage, likely contributed to the spread of TuYV at that site. While our data showed no significant difference in GPA levels across the autumn and winter months (May, June, and July) at the monitoring sites, a brief peak in GPA numbers in June likely played a role in the primary TuYV infections observed at Wagga Wagga. This was further supported by TBIA test results from July, which revealed 95% virus presence in symptomatic samples and 50–85% infection in asymptomatic samples from Wagga Wagga. In contrast, other sites had lower virus infection rates, such as Trangie, where 40% of symptomatic plants and 7% of non-symptomatic plants were affected. Tamworth showed virus presence ranging from 0% to 5% in randomly selected plants, while Breeza and Boggabri recorded no TuYV in either symptomatic or asymptomatic samples.

Additional testing of TuYV by TBIA of submitted samples from growers and agronomists showed high levels of TuYV already in June (Table 1), an indication that first crop infections would have occurred shortly after crop emergence.

Table 1. Canola samples from southern NSW tested for presence of turnip yellows virus (TuYV)

Out of the 174 canola samples submitted, 104 were paired samples, i.e. samples of symptomatic and non-symptomatic plants taken from the same paddock. Out of the 52 non-symptomatic samples, 15 still showed an infection of >90% TuYV and only 20 of the paired samples showed an infection level in the non-symptomatic samples that was 10% less than the infection level of the symptomatic samples. These results point at continuous re-infection in the paddocks and less symptom development in later infections.

Very few samples were received from northern NSW and no TuYV was detected. The regional differences were also shown during a short survey in late June; random sampling of three paddocks in the Coonamble region (western NSW) showed TuYV infection levels of 15 to 35%, while four paddocks near Temora (southern NSW) all had over 70% TuYV. The regional differences are also clearly demonstrated by the random sampling done in the aphid trapping sites (see above).

All canola samples were also tested for the presence of Turnip mosaic virus (TuMV), but only low levels of infection were found in three samples submitted in June. However, surprisingly high TuMV levels (> 40%) were found in three samples submitted in October. This high infection level was likely related to the proximity of a paddock of grazing canola that showed an 85% TuMV infection level.

Assessing TuYV management strategies in WA

General observations

The number of aphids per plant in the plots across the first 6 WAAI was very low (<6) in both seasons. However, plots with at least 2 GPA per plant had >50% plant infestation rate with no significant spatial trends in aphid infestation illustrating the high dispersal rate of GPA. Infection levels were relatively low in 2023 and high in 2024 despite a similar level of aphid infestation which could have been due lower temperatures in 2023, reducing plant susceptibility and aphid transmission efficiency. Furthermore, aphids were released at a later growth stage in 2023 than 2024 (3 leaf vs 1 leaf stage), and it is possible plants are more susceptible the earlier they are inoculated.

There were no consistent interactions between the different management strategies (meaning there was no increased GPA or TuYV control when they were used in combination), so each strategy is discussed in isolation below.

Seed treatment insecticide

In both experiments, the neonicotinoid-based seed treatment offered only mild reduction in GPA population and infestation rate relative to untreated plots, and little to no reduction in TuYV spread (Figure 2). Partial or complete GPA control failures could be caused by metabolic resistance carried by the GPA present (Kirkland et al., 2023) and/or substandard coverage of insecticide achieved by commercial seed dressing methods (Coutts et al., 2010). Abiotic factors that impact active ingredient uptake into the germinating seed such as soil moisture and temperature may also play a role (Sekulic and Rempel 2016; Stamm et al., 2016).

Figure 2. The neonicotinoid-based seed treatment (120 g a.i/100 kg imidacloprid + 180 g a.i./100kg clothianidin) was ineffective at significantly reducing turnip yellows virus (TuYV) spread in  ATR Bonito plots.

Foliar insecticide

In 2023, compared to the nil insecticide control, the four days pre-aphid introduction spray moderately reduced aphid numbers, but this was only sufficient to reduce (but not significantly) TuYV spread to a modest degree in ATR Bonito plots (Figure 3). This spray timing treatment has been used in previous field experiments with similarly low effectiveness (Congdon et al., 2023b). The sulfoxaflor spray at two WAAI was effective at suppressing subsequent GPA numbers, especially in 2023. The two WAAI spray was also moderately effective at reducing early TuYV spread; in 2023 significantly reducing AUDPC by 68% with just 7% infection by ten WAAI vs 23% in the nil spray control, and in 2024, reducing AUDPC by 46% although infection level at ten WAAI was still 55% vs 93% in the nil spray control. The four WAAI spray only reduced the amount of late TuYV spread (between six and ten WAAI) and infection by ten WAAI was almost 70% as it failed to suppress aphid numbers during the first four weeks after their introduction.

It is important to acknowledge some obvious differences between experimental plots and paddocks. When 80 m² field plots are exposed to TuYV infection sources on each corner, the contribution of primary spread to total spread is very high. In a 50-ha paddock, primary spread may only originate from one direction with subsequent secondary spread being the main contributor to total spread throughout the vast area of crop. Foliar insecticides are known to be less effective for virus control when the contribution of primary spread to an epidemic is high and more effective for controlling secondary spread by interrupting crop colonisation and production of secondary vectors (Perring et al., 1999). Therefore, moderate levels of control achieved with the foliar insecticide in these plot experiments may translate to greater levels of control when applied at the paddock scale.

Figure 3. Foliar application of sulfoxaflor at 50 g a.i./ha was most effective at suppressing turnip yellows virus spread (TuYV) in ATR Bonito plots when sprayed two weeks after aphid introduction into the plots. Spraying four days prior to, or four weeks after aphid introduction provided only modest TuYV control.

Host resistance

In contrast to insecticides, host resistance in canola variety ATR Stingray was highly effective at suppressing TuYV spread with a >90% reduction in AUDPC in both trials (Figure 4). Even in 2024 when mean virus incidence across all plots of ATR Bonito exceeded 70% at 10 WAAI, there was just 8% infection in ATR Stingray plots. This supports findings from DPIRD trials presented at the 2023 GRDC Research Updates (Congdon et al., 2023b). ATR Stingray has been consistently outperformed in seed yield by ATR Bonito in NVT trials in every season in medium-high rainfall zones from 2018 to 2022 (Power et al., 2024). In contrast, ATR Stingray had 27% higher seed yield and 3% higher oil content than ATR Bonito in 2024 (high virus pressure), no significant difference in seed yield and oil content in 2023 (low virus pressure), and 7% higher seed yield in a similar trial conducted in 2022 (moderate virus pressure) (Figure 5).

Figure 4. Turnip yellows virus (TuYV) resistance in ATR Stingray was highly effective at suppressing TuYV spread.

Figure 5. ATR Bonito (black) consistently outperformed ATR Stingray (grey) in NVT trials in medium to high rainfall zone from 2018 to 2022 (A). In contrast, seed yield was higher in ATR Stingray than ATR Bonito in 2022 under moderate TuYV pressure and in 2024 under high TuYV pressure, and no different in 2023 under low TuYV pressure (B). Furthermore, oil content was higher in ATR Stingray than ATR Bonito in 2024 and no different in 2023 (C).

Conclusions

The early and rapid spread of TuYV in southern NSW canola crops highlights the need for a deeper understanding of early-season movement and expansion of GPA into canola crops and a more proactive monitoring and management approach.

The aphid movement data highlights a clear influence of both time of year and location on GPA dynamics. In general, aphid levels across NSW were relatively low during autumn and winter 2024. These levels likely reflected a combination of local environmental and agronomic factors, such as temperature, rainfall, and pest control practices. However, the GPA levels observed in Wagga Wagga at the end of autumn and beginning of winter, particularly during the sensitive vegetative stage, likely played a critical role in facilitating virus infections. The data suggests that monitoring efforts should focus not only on aphid counts but also on understanding the timing and conditions under which populations surge. Integrating these insights into regional pest management programs could improve the effectiveness of control measures and minimise the risks of virus transmission in crops.

The key findings from the management field experiments were (i) the neonicotinoid-based seed treatment was ineffective at suppressing TuYV spread, (ii) the sulfoxaflor application was moderately effective when sprayed during the early stages of GPA infestation, and (iii) host resistance was highly effective even under high virus pressure.

The role of current neonicotinoid-based seed treatments in control of GPA and TuYV may require reconsideration. There is now evidence from research and field observations (including in southern NSW in 2024) to suggest these treatments as they are currently formulated and applied cannot be relied upon to control GPA infestation to a level that adequately suppresses TuYV. Further field experiments testing the different commercial seed treatments against susceptible and resistant GPA clones under different growing conditions will provide further insights.

This study supports the use of routine monitoring of canola crops using whole plant visual inspection in high-risk areas early in crop development to enable timely application of foliar insecticide. If laboratory diagnostic services are available, plants and colonising GPA can be tested for TuYV to further aid decision support. If both GPA and TuYV are present early in crop development, then the foliar insecticide application should be strongly considered. If a single foliar spray is used, it needs to be applied during the early stages of viruliferous GPA infestation before GPA populations can expand too far through the crop. In high virus pressure scenarios with significant primary spread (large numbers of aphids flying into the crop for longer periods of time), a second spray with a different mode of action 2-3 weeks after the first spray may be required to adequately suppress TuYV spread e.g. sulfoxaflor followed by afidopyropen or flonicamid. Continued monitoring of crops following an insecticide application is important to assess its effectiveness and inform the need for any follow up applications.

In the long term, host resistance offers a more efficient and sustainable basis of TuYV control to and will reduce the requirement for insecticide application. Further research is being undertaken to understand the TuYV resistance phenotype and its economic benefits in GRDC project DAW2305-003RTX ‘Effective virus management in grains crops’. For instance, resistance in ATR Stingray is overcome by different strains of TuYV under glasshouse conditions. However, several other sources of resistance have been identified that offer a broader spectrum of resistance and offer potential for breeding commercially available TuYV-resistant varieties.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. We wish to acknowledge the support of growers, agronomists and DPIRD Wagga Wagga staff who generously collaborated with us during the aphid survey. We would like to thank Living Farm for field experiment sowing, harvest and maintenance, Muresk Field Research Station for hosting field experiments, Nuseed Australia for providing seed, and casual and technical staff that assisted with field experiment data collection.

Useful links

Insecticidal control of green peach aphid and turnip yellows virus – resistance threats, limitations and future alternatives

GRDC Green peach aphid – best management guide

Turnip yellows virus and its management in canola

GRDC Western Australian Crop Sowing Guide

Insecticide spray guides for crops in Western Australia

Aphid management in pulse crops

IPM Checklist

PODCAST: Disease Update with Steve Simpfendorfer & Joop van Leur

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Contact details

Ben Congdon
DPIRD, Perth WA
Ph: 0488 904 480
Email: Benjamin.Congdon@dpird.wa.gov.au

Date published
February 2025

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