Virus development in canola crops during 2014 in New South Wales and implications for the oilseed and pulse industry

Joop van Leur1, Kurt Lindbeck1, Mohammad Aftab2, Angela Freeman2and Don McCaffery1

1New South Wales Department of Primary Industries; 2Department of Environment and Primary Industries Victoria.

GRDC project codes: DAN00179, DAN00202, DAV00134  

Keywords: viruses, BWYV, TuYV, TuMV, GPA, canola, pulses

Take home messages

  • Beet western yellows virus (BWYV, syn. Turnip yellows virus, TuYV) was found in most tested canola crops throughout NSW. However, substantial yield losses appeared to be limited to a few paddocks where early infection occurred.
  • BWYV (syn. TuYV) is a persistently transmitted virus that infects a wide range of crops and weeds. Its main vector is the Green Peach Aphid (GPA, Myzus persicae).
  • Virus control strategies should be based on preventing infection as infected plants cannot be cured. Preventive measures to avoid BWYV infection in canola include seed treatment with systemic insecticides that are effective for GPA control and sowing in standing wheat stubble.
  • Growers are advised to check canola crops early in the season for aphid presence and, if found, apply an effective insecticide.
  • There is no indication to date that the occurrence of BWYV (syn. TuYV) in canola poses a threat to neighbouring pulse crops.
  • High infestations of aphid species other than GPA were also observed in other broad acre crops during June-July 2014. Widespread cowpea aphid infections throughout Queensland and NSW resulted in very high levels of infection of Bean yellow mosaic virus (BYMV) in faba bean and, possibly, lupin crops.

Background

Three viruses have been reported on canola (Brassica napus) and mustard (B. juncea) in Australia: Beet western yellows virus (BWYV), Turnip mosaic virus (TuMV) and Cauliflower mosaic virus (CaMV). BWYV is a persistently transmitted virus, with the Green Peach Aphid (GPA, Myzus persicae) as its main vector, while TuMV and CaMV are non-persistently transmitted and can be vectored by a wide range of aphid species. Earlier surveys in Western Australia and NSW recorded generally low infection levels, except for occasionally high incidences of BWYV in canola and TuMV in B. juncea (Coutts and Jones 2000, Hertel et al. 2004). Research in Western Australia on the potential impact of BWYV on canola production indicated that the virus could cause significant yield losses if infection occurred at an early plant development stage, although it caused no plant death (Jones et al. 2007).

International research has shown that BWYV is diverse and the virus has been re-classified in several species, the most important being Beet western yellows virus (BWYV) and the closely related Turnip yellows virus (TuYV). Recent molecular research in the virology laboratories of the Queensland Department of Agriculture, Fisheries and Forestry (QDAFF) and the Department of Agriculture and Food, Western Australia (DAFWA) on strains originating from canola as well as pulse crops showed these to be TuYV. Biologically, both virus species are very similar in vectors and host range; in order to avoid confusion we will continue the use of the acronym BWYV for industry publications rather than BWYV (syn. TuYV), or (more correctly) TuYV.

Unusually high GPA infestations levels were recorded in South Australia during the 2014 autumn, causing extensive BWYV infection. The severe plant stunting observed after the early virus infection in a large number of paddocks caused widespread concern to canola growers in other states and also prompted a number of initiatives to quantify the scale of virus infections in canola crops, to estimate risks to other crops and to advise on possible control strategies.

Canola virus survey results - NSW 2014 season

Methodology

Following reports of severe BWYV infection in South Australian and Victorian canola crops, a sampling protocol was developed in July 2014 and distributed to NSW agronomists and canola growers. Participants were requested to submit samples (youngest fully expanded leaves or flower stem tips) of both virus-symptomatic (reddening and/or yellowing of the leaves, stunting) as well as healthy looking plants in order to relate field symptoms to virus infection. Tissue blot Immunoassay (TBIA) was used to determine virus presence as it is an economic and reliable diagnostic test for large numbers of individual plant samples: Samples were blotted on three duplicate nitrocellulose membranes, which were processed for BWYV, TuMV and CaMV at the DEPI-Vic virology laboratory at Horsham or at the NSW DPI laboratory at Tamworth.

Results and Discussion

By the end of August a total of 3,909 plant samples from commercial canola crops were processed, representing 150 sites (including 14 from Victoria and two from Tasmania). The majority of the NSW samples originated from the major canola growing districts of the southern region. BWYV was detected in 119 out of the 134 NSW sites sampled. There was a clear trend of the higher infection levels in southern NSW as compared to central and northern regions of the state (Table 1). TuMV was common on the Liverpool Plains and found to have a severe impact in several canola paddocks. Apart from the Liverpool Plains, TuMV was also detected in commercial crops in a paddock near Forbes and one near Coolah and in an experimental field near Goulburn. Canola virulent TuMV strains are still rare in Australian commercial crops but have the potential to spread and cause significant damage to the Australian canola industry (van Leur et al. 2014).

Table 1. BWYV incidence in NSW canola samples submitted for virus testing, July – August 20141.

NSW

Region2

Number3

Number of samples / Infection class

Average %

Infection

0%

>0

≤10%

>10

≤25%

>25

≤50%

>50

<100%

100%

South

86

3

6

4

15

35

23

68

Central

31

4

3

7

4

12

1

43

North

17

8

4

1

1

2

1

17

1  Incidence based on TBIA.

2  South; sampling sites south of West Wyalong, North; sampling sites north of Dubbo

3  Paired samples (symptomatic and non-symptomatic plants from the same paddock) were combined.

Comparing BWYV incidences between symptomatic and non-symptomatic canola plants from the 58 NSW sites from which paired samples were submitted showed generally a higher BWYV incidence in the symptomatic samples. However, non-symptomatic samples were rarely found to be free of BWYV (Figure 1).

Figure 1. Comparison of paired (virus symptomatic v non-symptomatic) samples from 58 canola paddocks in three NSW regions for BWYV incidence, 2014.

Figure 1. Comparison of paired (virus symptomatic v non-symptomatic) samples from 58 canola paddocks in three NSW regions for BWYV incidence, 2014.

The results highlight the difficulties in distinguishing BWYV symptoms from those symptoms caused by other, abiotic stress factors, like nutrient deficiencies or drought stress.

Evaluation of canola germplasm for BWYV resistance

Methodology

Initial sampling of Canola National Variety Trials (NVT) in different locations in central and northern NSW was carried out during August. The Gilgandra and Wellington trials were selected for further sampling as the initial sampling showed moderate levels of infection; sites with very high levels of infection were unlikely to show varietal differentiation. On 8 and 9 September all entries in both locations were sampled by 10 random plants in each of the three replicates. The samples were blotted on a single membrane and tested for BWYV presence. An additional 15 random plants taken from each replicate of the varieties ATR-Gem and Pioneer®45Y86 (CL) were blotted over three duplicate membranes and tested for BWYV, TuMV and CaMV.

Testing for BWYV presence was also carried out at the blackleg testing sites of the National Canola Pathology Project.  Project collaborators sampled the varieties ATR-Stingray and Thumper TT with 10 random plants in each of the three replicates. The samples were mailed to Tamworth and blotted over a single membrane for BWYV testing only.

Results and Discussion

Table 2 summarises the % BWYV positive plants of the 18 named varieties tested at both sites. BWYV incidences between both sites were significantly correlated (r = 0.50, P = 0.01), but large differences could be observed. ATR-Stingray was the only variety found free of BWYV in both sites. This result supports the observations made in South Australia where ATR-Stingray appeared to be the best performing variety under severe BWYV pressure (Coutts et al., 2015). However, the testing results of the sampling of the varieties ATR-Stingray and Thumper TT in sites throughout NSW, Victoria and Western Australia showed no difference between these two varieties and BWYV incidences of up to 60% in ATR-Stingray.

Table 2. BWYV incidence1 in 18 canola varieties tested in the National Variety Trials at Wellington and Gilgandra, September 2014.

Name

Gilgandra

Wellington

Average

ATR-Stingray

0

0

0

Hyola®450TT

7

10

8

Hyola®474CL

3

17

10

Hyola®577CL

10

10

10

CB Atomic HT

17

13

15

Pioneer®44Y87 (CL)

7

27

17

MonolaTM515TT

20

13

17

Pioneer®44Y89 (CL)

13

23

18

Hyola®559TT

7

30

18

Hyola®650TT

27

13

20

Pioneer®45Y88 (CL)

23

20

22

ATR-Bonito

24

20

22

ATR-Gem

21

27

24

ATR-Wahoo

27

23

25

Hyola®575CL

23

30

27

Pioneer®45Y86 (CL)

30

33

32

Archer

17

50

33

Hyola®750TT

43

43

43

1  % BWYV positive plants as determined by TBIA, averaged over 10 randomly collected plants in each of three replicates

Development of viruses in NSW pulse crops during 2014

Surveys of virus development in pulse crops in northern NSW are undertaken yearly through random and selected sampling of commercial and experimental crops. At the start of the 2014 season unusually high infestations of cowpea aphids (Aphis craccivora) were observed in early sown faba bean crops in southern Queensland and north and central NSW. Shortly after severe aphid feeding damage, as well as in a number of paddocks where high incidences of Bean yellow mosaic virus (BYMV) were found. However, aphid presence during the remainder of the season was limited and little or no virus development was observed in later sown pulse crops like chickpea.

In view of the high levels of BWYV in canola, particular attention was given to the development of this virus in pulse crops as BWYV can infect both. Results of surveys during both 2013 and 2014 of pulse crops grown in the vicinity of canola do not support the hypothesis that later sown pulse crops are at risk from BWYV inoculum originating from early sown canola crops.

Earlier reports contributed severe crop losses in pulse crops to BWYV. However, recent findings by Dr Murray Sharman (QDAFF) demonstrated that in some cases this was due to the newly described Phasey bean virus (PhBV). PhBV was previously thought to be a strain of BWYV and the two cannot be distinguished by polyclonal antisera (Sharman et al., 2014). However, the ecology of PhBV is completely different and although PhBV virus affects a wide range of pulses, it does not infect canola. Also, its main vector appears to be the cowpea aphid rather than the green peach aphid, the main BWYV vector.

Conclusion

Although differences in BWYV incidences between varieties were found in the NVT trial sites, these are preliminary results and should be confirmed in follow-up trials.  At this stage there is no irrefutable evidence that useful differences in BWYV resistance are present among Australian canola varieties. Future plant testing for BWYV presence in variety resistance evaluation trials should also be done earlier in the season (at the beginning of flowering) as early infections have the potential to inflict most damage.

Virus control strategies should be preventive. Poorly growing, low density crops are more vulnerable to aphid infection. Recommended agronomic practices should be followed that result in good crop establishment and vigorous early plant growth. Sowing in standing wheat stubble lowers aphid landing rates. Proper weed control will reduce BWYV inoculum as the virus has a very wide host range.

It is of particular importance that canola seed is treated properly using neonicotinoid insecticides. The seed treatment will provide a certain level of protection during the early, critical, plant growth stage. Growers should carefully monitor canola crops during this period and apply effective insecticides when aphids are noticed in the crop.

References

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

Coutts, B.A., R.A.C. Jones, P. Umina, J. Davidson, G. Baker and M. Aftab, 2015. Beet western yellows virus (synonym: Turnip yellows virus) and green peach aphid in canola. GRDC Update, February 2015, Adelaide.

Jones, R.A.C., B.A. Coutts and J. Hawkes, 2007. Yield-limiting potential of Beet western yellows virus in Brassica napus. Australian Journal of Agricultural Research 58, 788-801.

Hertel, K., M.W. Schwinghamer and R.W. Bambach, 2004. Virus diseases in canola and mustard. Agnote 495, NSW Department of Primary Industries, Orange, Australia.

Sharman, M., K. Moore, J. van Leur, M. Aftab and A. Verrel, 2014. Impact and management of viral diseases in chickpea. GRDC Update, February 2014, Goondiwindi.

van Leur, J.A.G., M. Guerret,  E. Nyalugwe, M. Aftab, M, Barbetti and R.A.C. Jones, 2014: Emergence of a canola-virulent Turnip mosaic virus strain on the Liverpool Plains, northern New South Wales. Proceedings 11th Australasian Plant Virology Workshop, 13-15 August 2014, Brisbane, Australia.

Contact details

Joop van Leur
Tamworth Agricultural Institute, NSW DPI, Calala, NSW 2340
02 6763 1204
0427 928 018
joop.vanleur@dpi.nsw.gov.au

GRDC Project Code: DAN00179, DAN00202, DAV00134,