Barley Yellow Dwarf Virus management

Barley Yellow Dwarf Virus management

Take home messages

  • 2023 trial results do not inform industry regarding the threat BYDV-PAV poses to wheat yields in the HRZ.
  • 2022 trial results suggest even late infections of BYDV-PAV in susceptible wheat cultivars result in a significant loss.
  • Preliminary conclusions regarding management of BYDV-PAV using multiple aphicide applications need further validation.
  • Wheat breeders need to include BYDV tolerance traits into long-season cultivars.

Background

Past field trials in the southeast of SA suggest there is an opportunity in the high rainfall zone (HRZ) to lift productivity and close the gap between actual and potential crop yields. This may be achieved for wheat through choosing suitable cultivars, matching cultivar phenology to sowing date, and adopting appropriate agronomic management practices.

Barley yellow dwarf virus (BYDV) can be a devastating disease of wheat and other cereals, including barley and oats, especially in higher rainfall zones and other regions in wetter years. Yield losses in wheat can be up to 30 %. The virus restricts the flow of water and nutrients in the plant, which can lead to stunting, inhibited root development, delayed grain head development and poor seed filling. Foliar diseases can mask the presence of the virus, and symptoms of BYDV in wheat can also be confused with frost, waterlogging and nutrition deficiencies. There are two main sub-groups of BYDV, including the first subgroup containing BYDV-PAV, BYDV-MAV and BYDV-ORV, and the second subgroup of Cereal Yellow Dwarf Virus (CYDV).

Although insecticides, such as neonicotinoids, are currently available for treatment of seed to stop the spread of viruses by aphids, there is concern from growers around the reliance on seed treatments and that restrictive regulations of neonicotinoid use in Europe may be adopted in Australia. The timing and fit of post-emergent applications of insecticides into existing operations, the potential impact on beneficial species, and insecticide resistance, are issues raised by growers.

There has recently been a shift to sowing winter wheats earlier (mid-April) in the HRZ rather than the traditional later sowing (mid-May). Although this earlier sowing can increase the yield potential of winter wheats, it also might increase the risk of BYDV incidence due to warmer weather at sowing and likely higher aphid populations in green bridges. The phenology of these winter wheats is also not suited to current aphicide management regimes. Past observations of winter wheat field trials have indicated that seed treatments decrease in efficacy after 3–4 weeks post-seeding, leaving these wheat crops at risk of BYDV infection due to late aphid movement, up to the regional standard GS31 insecticide spray.

The aims of this project are to:

  • test if late BYDV infection in wheat can occur
  • determine yield loss due to late BYDV infections
  • test different cultivar responses to BYDV infection
  • establish an optimal strategy for preventing BYDV in winter wheat crops.

Method

Aphids and their natural enemies were monitored every 3–4 weeks using yellow sticky traps in both years. Insecticides applied as aphicides to control BYDV vectors:Gaucho® 600 Red Flowable Seed Treatment Insecticide (600 g/L imidacloprid) was the seed dressing applied at 240mL/100kg seed, and Trojan® Insecticide (150g/L gamma-cyhalothrin) applied at 15mL/ha was used in-crop. Grain quality tested included 1000 grain weight (g/1000 seeds), screenings (per cent below a 2.0mm sieve), test weight (kg/hl) and protein (per cent).

2022 trial

A replicated small plot trial was sown at Conmurra in the HRZ of the southeast of SA on 25 May 2022. The trial was harvested on 24 January 2023. Three wheat cultivars were sown: Manning , Accroc and RockStar. Four different insecticide management regimes were applied to each variety: Treatment 1 – an untreated control, Treatment 2 – seed dressing applied and no in-crop aphicides, Treatment 3 – seed dressing applied and in-crop aphicides applied at growth stages Z21 and Z31, and Treatment 4 – seed dressing applied and in-crop aphicides applied at growth stages Z21, Z21 + 4 weeks and Z31. Oat aphids infected with BYDV-PAV were released on all plots on 8 July 2022. Aphid presence was checked at growth stages Z21, Z21 + 4 weeks, Z31, Z39, and Z69. Visual BYDV symptoms were recorded at growth stages Z31, Z39 and Z69. Leaf tissue was tested for BYDV on 8 November 2022 and again on 28 November 2022.

2023 trial

A replicated small plot trial was sown on SARDI’s Struan Farm, Bool Lagoon SA, on 24 May 2023. The trial was harvested on 5 January 2024. Nine cultivars were sown: Anapurna, Manning, BigRed, Vixen, RGT_Cesario, RGT Calabro, Accroc, RGT Waugh and RockStar. Three different insecticide management regimes were applied to each variety with three replicated plots per variety and treatment: nil, seed treatment only, seed treatment plus three foliar aphicides at monthly intervals from first week of July. Oat aphids infected with BYDV-PAV were released on 19 June 2023. Wheat plants were sampled and tested for BYDV on 26 September 2023 and on 31 October 2023.

Results and discussion

2022 results

Aphids and other insects

Almost no aphids were detected, however large numbers of hoverflies were detected.

BYDV infection

BYDV-PAV was not detected in leaf tissue samples from plots at the trial site in 2022, despite infected oat aphids being released. ‘Visual assessment of what is considered ‘Typical’ BYDV symptoms (as per GRDC BYDV Factsheet: Barley Yellow Dwarf Virus Fact Sheet: Management tips to avoid yield penalties, grdc.com.au) were observed late in the season. Samples of the leaves thought to be infected were tested for the virus. Of the eleven samples tested, only one tested positive to BYDV-PAV. These findings raise questions about the ability to visually identify BYDV in-crop, the reliance on ‘typical’ BYDV symptoms to make insecticide decisions, the adequacy of the test used in the laboratory to confirm BYDV, restrictions of using BYDV-PAV infected aphids, and, if BYDV is not causing the ‘typical’ symptoms, what is the cause (for example, another virus, nutrition) and does it require management.

Yield

Overall, there was a significant difference in grain yield between cultivars: Accroc averaged 8.25t/ha, greater than Manning at 7.71t/ha and RockStar at 7.04t/ha (P value <0.001, Lsd 0.238). The treatments with in-crop applications of aphicides had significantly higher grain yields compared to no in-crop insecticide treatments. Treatment 4 (8.03t/ha) and Treatment 3 (7.87t/ha) increased grain yield significantly compared to Treatment 2 (7.47t/ha) and Treatment 1 (7.29t/ha). Although BYDV-PAV was not detected within plots, Accroc yield was responsive to the application of insecticides, with a 13.5% increase in yield with the application of a seed dressing and three in-crop sprays, compared to an untreated control. The addition of an in-crop insecticide spray between the regional standard of growth stage 21 and growth stage 31 increased the yield of RockStar by 6.9%. The yield of Manning, which has tolerance to BYDV, was not as responsive to in-crop insecticides as the other two cultivars. Manning, being the check, did not display a yield response, which indicates the increased yield responses observed was due to the control of BYDV.

2023 results

Aphids and other insects

Low aphid numbers were detected by yellow sticky traps from June–August, with oat aphids being the dominant species (Figure 1). There was a high number of generalist predators and aphid-specific parasitoid wasps, with more spiders early compared to hoverflies, which became the dominant natural enemy. The increase in natural enemies corresponded with a decline in aphids as expected. Despite the low number of BYDV vectors, that is, oat aphids, and their regulation by natural enemies, BYDV was detected in the trial and surrounding the trial. These observations suggest low aphid numbers may transmit BYDV.

Figure 1. Oat aphid, aphid, parasitoid and predator numbers for 2023.

Figure 1. Oat aphid, aphid, parasitoid and predator numbers for 2023.

BYDV infection

Plant samples from all plots were collected on 31 October 2023, with 4–7 plants/plot were removed. Plant stems were tested for BYDV-PAV using tissue blot immunoassay (TBIA). A percentage of stems infected by BYDV-PAV was calculated (number of stems positive for BYDV-PAV/total number of stems tested). There was a significant difference between the treatments and percentage of stems that tested positive (P <0.001, Lsd 14.6) (Figure 2). In the untreated plots, 60% of stems tested positive to BYDV-PAV, while in the seed-dressing only plots, 42% of stems tested positive to BYDV-PAV, and in the seed-dressing plus three aphicides plots, 22% of stems tested positive to BYDV-PAV.

Figure 2. Percentage of BYDV-PAV infection on sampled stems from treatments (Treatment 1 – untreated, Treatment 2 – seed dressing only, Treatment 3 – seed dressing plus three aphicides), Bool Lagoon SA, 31 October 2023. Error bars denote significance (P <0.001, Lsd = 14.6).

Figure 2. Percentage of BYDV-PAV infection on sampled stems from treatments (Treatment 1 – untreated, Treatment 2 – seed dressing only, Treatment 3 – seed dressing plus three aphicides), Bool Lagoon SA, 31 October 2023. Error bars denote significance (P <0.001, Lsd = 14.6).

Figure 3. Percentage of BYDV-PAV infection on sampled stems from nine cultivars, Bool Lagoon SA, 31 October 2023. Error bars denote significance (P <0.001, Lsd = 25.3).

Figure 3. Percentage of BYDV-PAV infection on sampled stems from nine cultivars, Bool Lagoon SA, 31 October 2023. Error bars denote significance (P <0.001, Lsd = 25.3).

Yield and grain quality

The yield difference observed in 2023 between cultivars was not due to BYDV-PAV, as the grain yield did not correlate to infection observed (see Figure 3) and the check (BYDV tolerant) cultivar Manning achieved one of the highest yields (Treatment 1 = 5.42t/ha, Treatment 2 = 5.57t/ha, Treatment 3 = 5.74t/ha). There was no significant difference in grain yield between treatment groups (P = 0.331). Harvested grain was quality tested (1000 grain weight (g/1000 seeds), screenings (per cent below a 2.0mm sieve), test weight (kg/hl) and protein (per cent)). Only protein per cent varied between cultivars.

Discussion

This study raises more questions than providing growers with answers. Large numbers of natural enemies were present during both trials, which indicates reasonable biological control of aphids, yet transmission of BYDV did occur. The cold wet conditions may have impeded aphid activity in 2022, hence less transmission of virus and observation of symptoms later that year. It should be noted that yellow sticky traps only catch flying aphids, so there could have been plenty of non-winged aphids in the crop transmitting virus. This is suspected to have confounded results in 2023, where BYDV-MAV was detected in the surrounding crop and a significant block effect on yield was observed. However, the aphid data do highlight the need to consider not only the virus, host, and environment, but also the vector when assessing whether or not a virus will cause loss.

Yellow Dwarf Virus (YDV) field experiments, just like the virus-plant-vector pathosystem itself, are really complex. In some years, things will be very straightforward (2022), in other years (2023), background factors can have a substantial impact. In the 2023 trial, we might have observed background infection with BYDV-MAV from the surrounding crop, but it might not have been there either. There is not really any way of knowing this without non-inoculated control plots for each cultivar. Different virus species, the different aphid species that transmit the different virus species, and host differences could relate to virus or aphids or both, but nobody knows. Some BYDV papers report on two paddocks of different host types right next to each other (wheat next to oat, for example) and the most prevalent YDV species in each crop was different, even though the crops are right next to each other. Future experiments are needed that control for background factors to tease apart the relationships between host, virus, vector, and environment; noting that management is nested within environment.

In all of the BYDV prevalence and incidence studies that have been done in Victoria, BYDV-PAV occurs naturally in greater levels than BYDV-MAV, but prevalence could be completely different in SA. In 2023, visual symptoms were observed across southwest Victoria, with aphicides applied in response to those observations in late August. Further, areas of the Mid North SA have observed yield responses (for example, in 2016) where early protection from aphids is provided using seed treatments, as per current best practice, where symptoms were observed early. Further long-term monitoring of BYDV is needed outside regions that have traditionally been thought of as areas of high risk.

Conclusion

  • Regular foliar applications of aphicides saw a significant increase in yield of a winter and a spring type cultivar susceptible to BYDV-PAV in 2022, over and above seed treatments alone.
  • Late inoculation of wheat in trials with infected aphids was successful in establishing BYDV-PAV in both 2022 and 2023, with the virus detected in stem samples from Struan, SA in 2023.
  • Plots not treated with aphicides had a higher percentage of stems infected with BYDV-PAV, compared to treated plots in 2023.
  • In addition to BYDV-PAV, a BYDV-MAV-like isolate was identified in the surrounding crop in 2023.
  • Cultivar differences in stems infected with BYDV-PAV were detected in 2023, with Accroc, RGT_Cesario, Anapurna and RGT Calabro having higher levels compared to Vixen, Manning and RockStar.
  • 2023 trial results do not inform industry regarding the threat BYDV-PAV poses to wheat yields in the HRZ.
  • Further research is needed, but BYDV management complex.

Acknowledgements

Mackillop Farm Management Group would like to thank SAGIT for funding the trial work and growers for providing a field site in 2022. A special thank you to Narelle Nancarrow for advice and providing oat aphids infected with BYDV-PAV.

Contact details

Michael Nash
29 Jikara Dr, Glen Osmond SA 5064
0417 992 097
whatbugsyou@gmail.com
@merindie1 (whatbugsyou)