Viruses in spring-sown dual-purpose canola in the high rainfall zone of southern Victoria

Take home messages

• Spring-sown canola provides a virus reservoir for Turnip yellows virus (formerly Beet western yellows virus), Cauliflower mosaic virus and Turnip mosaic virus.
• There is a high risk of transmission of Turnip yellows virus from spring- to autumn-sown canola under favourable conditions.
• There is a need to manage Turnip yellows virus in spring-sown canola and protect autumn-sown canola from transmission of the virus by aphids.

Background

Canola is the third most important winter grain crop following wheat and barley in Australian dryland farming systems. In southern Victoria, canola has gained acceptance in mixed farming enterprises as a dual-purpose grain/graze crop. Particularly, the introduction of high yielding canola from Europe with winter dormancy has provided the opportunity for spring sowing. This extends the period for grazing over the summer and into the autumn, filling the feed gap at these times of harsh summer conditions, with minimum grain yield penalties (Paridaen and Kirkegaard, 2015). However, the adaptation of fodder canola and turnip crops into southern Australia is a matter of concern because they may host pests and diseases (Kirkegaard 2007), especially viruses.

Three significant viruses are reported in canola grown in Australia. They are Turnip yellows virus (TuYV) (formerly Beet western yellows virus) (Aftab et al 2015), Cauliflower mosaic virus (CaMV) and Turnip mosaic virus (TuMV) (Coutts and Jones 2000). The most important and widespread of these viruses is considered to be TuYV (Dreyer et al 2001).

TuYV is a persistent virus that belongs to the Poleroviruses within the family of Luteoviruses. In field experiments, early infection of TuYV caused yield losses of up to 46% (Jones et al 2007). While field observations made during the 2014 TuYV epidemic have seen early infections cause up to 75% yield loss (Coutts et al 2015). Earlier infection with TuYV can result in high yield loss, but yield loss decreases with later stages of crop development (Coutts et al 2006). However, canola remains susceptible until mid-podding. Infection after mid-podding can affect oil quality, but yield loss is minimal (Jones et al 2007).

CaMV a non-persistent virus, genus Caulimovirus family Caulimoviridae, which has been shown in field trials to reduce seed yield per plant by 92% (Walsh and Tomlinson 1985). In glasshouse experiments, TuMV, a non-persistent virus genus Potyvirus family Potyviridae, reduced seed yield by 64% (Walsh and Tomlinson 1985).

Transmission of canola viruses is exclusively through their aphid vectors. The green peach aphid (Myzus persicae) is the principal vector of TuYV (96% transmission efficiency), while the cabbage aphid (Brevicoryne brassicae) can also transmit at a lower rate (14% transmission efficiency) (Schliephake et al 2000) as can turnip aphid (Lipaphis pseudobrassicae) (van Leur et al 2013). Autumn is the critical time for virus infection in canola, as aphids move from the ‘green bridge’ into the earliest-sown crops. Infections can also occur with spring aphid flights, but these probably have little effect on yield (Marcroft et al 2008).

The potential for carryover of canola viruses on self-sown canola and alternative hosts from one season to the next on the ‘green bridge’ has been identified in south-western Australia (Coutts and Jones 2000). Despite this, little research has been undertaken to determine the potential for carryover of canola viruses on spring-sown dual-purpose canola in southern Victoria. The aim of this study was to determine virus incidence in the ‘green bridge’, to assess the potential for the carryover of viruses, and subsequent level of transmission to autumn-sown crops by aphid vectors.

Methods

Canola field survey

A survey of 12 canola crops was conducted during March and April 2016 to detect viruses that may be present in crops acting as a ‘green bridge’ (Figure 1). These crops were sown in spring 2015 for use as dual-purpose crops. One hundred random leaf samples were collected from each canola crop by walking in an ‘M’ pattern and removing individual leave petioles at random every five metres to avoid bias. To assess the possible spread of viruses by aphids from the ‘green-bridge’ into new season (2016) autumn canola crops, 10 random leaf samples from each of six nearby crops (<1.5 km) were collected in mid-August 2016 and tested for all three viruses.

Figure 1. Location of 12 paddocks of dual-purpose winter canola sown in spring 2015 and surveyed for canola viruses in the autumn of 2016, across south-eastern Victoria.

Sample processing and virus detection

Sap from canola leaf petioles or stems were blotted onto nitrocellulose membranes in bundles of 10. The bundles were blotted on three copies of membrane and the blots were tested for TuYV, TuMV and CaMV using tissue blot immunoassay (TBIA) as described by Freeman et al. (2013). The strains of Beet western yellows virus that infect oilseed rape are now referred to as TuYV (Stevens et al 2008) but antisera to detect TuYV is not available. Instead, Luteovirus group antisera was used to detect TuYV in TBIA. This antisera was obtained from DSMZ Germany.

Aphid monitoring

During both the autumn and spring virus surveys, the same plants sampled for TBIA virus identification were also assessed for the presence of aphids within each paddock. In live colonies, only turnip aphid was identified. Additionally, aphid flights were monitored from June to November 2016 fortnightly during the growing season using four sticky traps on the edge of a canola crop near Hamilton.

Results

Virus incidence in spring-sown canola crops acting as a ‘green bridge’

Viruses were widespread in spring-sown dual-purpose canola crops assessed in autumn 2016 (Table 1). TuYV was detected in 92% of crops sampled, with an average in-crop incidence of 47%. TuMV was found in 50% of crops surveyed and CaMV was detected in 83%, however, the in-crop incidence was low with 2% and 8% of plants infected respectively. Combinations of viruses were also detected. TuYV in combination with TuMV was found in 50% of crops and with CaMV in 75% of crops. TuMV and CaMV were found together in 42% of crops. All three viruses were found together in 42% of crops.

Table 1. Detection of Turnip Yellows Virus (TuYV), Turnip mosaic virus (TuMV) and Cauliflower mosaic virus (CaMV) in 12 spring-sown dual purpose (graze/grain) canola crops in southern Victoria in autumn 2016. One hundred plants were tested for virus per paddock.

Virus incidence in autumn-sown crops

In mid-August 2016, TuYV was detected in three autumn-sown canola crops with an average in-crop incidence of 7%. TuMV and CaMV were not detected.

Aphid monitoring

Aphids were counted and identified to species level during the autumn survey. Turnip aphid was the only aphid detected in canola crops during 2016. It was found in eight of the twelve crops surveyed. Turnip aphid colonies were also found in eight of the spring-sown canola crops. On yellow sticky traps green peach aphid, turnip aphid, oat aphid (Rhopalosiphum padi), blue green aphid (Acyrthosiphon kondoi), ornate aphid (Myzus ornatus) and corn aphid (Rhopalosiphum maidis) were trapped in low numbers (data not shown).

Discussion

TuYV, TuMV and CaMV occur throughout grain production areas in Australia (Hertel et al 2004), but TuYV is considered the most important virus in south eastern Australia due to its high incidence (Aftab et al 2015), wide host range (Coutts and Jones 2000) and persistent transmission. Autumn is the critical period for virus infection because aphids invade crops from the weed hosts and self-sown canola, and canola is more susceptible to yield loss at seedling stage (Coutts et al 2006). Therefore, dual-purpose canola crops growing over the summer period are considered to be high risk management option for virus due to ‘green bridge’ for infection of autumn crops.

Results from the survey confirm spring-sown canola provides a virus reservoir for not only TuYV, but also for TuMV and CaMV (Table 1). Some crops also had multiple virus types, the implications of which need further investigation as TuMV and CaMV are considered more destructive, reducing canola yield by up to 90% (Coutts and Jones 2000) while TuYV reduced yields by 46% (Jones et al 2007). In this study no yield loss studies were conducted.

There was no clear evidence for the movement of viruses from spring-sown dual-purpose canola to the autumn-sown canola. Possible factors that could contribute to the lack of transmission are (i) the presence of turnip aphid in spring-sown dual purpose canola which is considered a less efficient vector (van Leur et al 2013) and (ii) a low number of alate (winged) green peach aphids in newly sown autumn canola crops.

The low number of green peach aphids detected during the crop survey and subsequent low virus transmission was probably related to dry summer (decile 4-7) and autumn (decile 4-7) conditions (Source: www.bom.gov.au). This agrees with Coutts et al (2006) who found that regional differences in TuYV incidence was related to the amount of rainfall in the two months preceding the growing season, with low rainfall leading to low aphid flights and hence low virus transmission.

Conclusion

There was a large reservoir of viruses, particularly the more common TuYV, detected within the 12 crops of spring-sown (2015) dual-purpose canola surveyed. Although spread of viruses into autumn-sown canola was not detected, probably due to the dry summer and autumn conditions, there is a high risk of transmission under more conducive conditions to aphid survival in summer (decile 8-9) in combination with an autumn favourable for aphid multiplication. This could significantly reduce yield and oil quality in autumn-sown crops if the ‘green bridge’ is not managed and autumn-sown canola is not protected from transmission of viruses by aphids. Further work will focus on the factors that lead to yield loss from CaMV and TuMV.

References

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Bureau of Meteorology (BoM). Victorian rainfall deciles for the pre-season (January to March).

Coutts, BA and Jones, RAC (2000) Viruses infecting canola (Brassica napus) in south-west Australia: incidence, distribution, spread and infection reservoir in wild radish (Raphanus raphanistrum). Australian Journal of Agricultural Research 51:925-936.

Coutts, BA, Hawkes, JR and Jones, RAC (2006) Occurrence of Beet western yellows virus and its aphid vectors in over-summering broad-leafed weeds and volunteer crop plants in the grainbelt region of south-western Australia. Australian Journal of Agricultural Research 57:975-982.

Coutts BA, Jones RAC, Umina P, Davidson J, Baker G, Aftab M (2015) Beet western yellows virus (Synon: Turnip yellows virus) and green peach aphid in canola. South Australian GRDC Updates February 2015.

Dreyer F, Graichen K, Jung C (2001) A major quantative trait locus for resistance to Turnip yellows virus (TuYV syn Beet western yellows virus, BWYV) in rapeseed. Plant Breeding 120, 457- 462.

Freeman A, Spackman M, Aftab M, McQueen V, King S, vanLeur J, Loh M, Rodoni B (2013) Comparison of tissue blot immunoassay and reverse transcription polymerase chain reaction assay for virus-testing pulse crops from a South-Eastern Australian survey. Australasian Plant Pathology 42. 675-683.

Hertel K, Schwinghamer M, Bambach R (2004) Virus diseases in canola and mustard. Agnote ISSNH 1034- 6848, 1-6 pages.

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Kirkegaard J, (2007) Evaluating the potential for dual-purpose (graze/grain) canola in the mixed farming systems of southern Australia. Assessment Report GRDC Project CSP00085

Marcroft S, Bluett C and Hind-Lanoislet T (2008) Canola diseases. AgNote No: AG1354. Victorian DEPI.

Mendham, N.J., P.A. Shipway and R.K. Scott. 1981. The effects of seed size, autumn nitrogen and population density on the rapeseed to delay sowing oilseed rape (B. napus). J. Agric. Sci. Camb. 96 417.

Paridaen A and Kirkegaard J (2015) Forage canola (Brassica napus): spring-sown winter canola for biennial dual-purpose use in the high-rainfall zone of southern Australia Crop & Pasture Science, 66, 275–286.

Schliephake E, Graichen K, Rabenstein F (2000) Investigations on the vector transmission of the Beet mild yellowing virus (BMYV) and the Turnip yellows virus (TuYV). Journal of Plant Diseases and Protection 107. 81–87.

Stevens M, McGrann G, Clark B (2008) Turnip yellows virus (syn Beet western yellows virus), an emerging threat to European oilseed rape production. Research Review No 69. Brooms Barn Research Centre, Higham, Bury St Edmunds Suffolk IP28 6NP UK.

van Leur J, Aftab M, Sharman M, and Lindbeck K (2013). Viral diseases in canola and winter pulses. Queensland GRDC Updates Goondiwindi 2013.

Walsh JA, Tomlinson JA (1985) Viruses infecting winter oilseed rape (Brassica napus ssp. oleifera). Annals of Applied Biology. 107, 485–495.

Useful resources

GRDC FactSheet: Reducing aphid and virus risk

GRDC The Back Pocket Guide: Crop aphids 

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 author would like to thank them for their continued support.

Contact details

Frank Henry
915 Mt Napier Road, Hamilton 3300
0447 777 480
frank.henry@ecodev.vic.gov.au
@HamiltonAgro