Joop van Leur¹, Mohammad Aftab², Murray Sharman³, Kurt Lindbeck⁴

Plant / virus pathosystems are dynamic

Farming systems in Australia’s northern grain region are continuously evolving with new crops introduced and rotation patterns changed. The past decade has seen large increases of winter pulses, particularly chickpea and faba bean, and canola throughout the region. Increasing use of broadleaf crops can contribute to the sustainability and profitability of broadacre farming through higher soil fertility, better weed control and less stubble and soil borne disease in the following wheat crops. However, a further increase of broad leaf crops, either through shorter rotations between pulse crops or through expansion in new areas, will aggravate its own disease problems. Viral diseases are of particular concern as curative control is not possible and, unlike most fungal diseases, viruses are generally not species specific. Extending pulses and canola cultivation in the northern grain zone, a sub-tropical region characterised by a short and relatively warm winter growing season, will aggravate virus problems. Rain can occur throughout the year in this region and, together with fertile soils with good water holding capacity, allows cultivation of a range of summer crops, rainfed or under irrigation. These summer crops, as well as weeds and volunteers, provide a ‘green bridge’ for pathogens and pests of winter crops between growing seasons. This is especially of importance to virus diseases as most require living plant tissue to survive and need insect vectors (mainly aphids) to spread. Viruses can be classified based on their mode of vector transmission; ‘persistently transmitted’ viruses that require feeding of the vector on the host plant both to acquire and transmit the virus; ‘non-persistently transmitted’ viruses that can be transmitted by brief probing of the vector. Consequently, the vector range of persistently transmitted viruses is smaller than for non-persistently transmitted viruses and generally restricted to host-specific aphids. On the other hand, non-persistently transmitted pulse viruses can even be transmitted by cereal aphids. A good understanding of virus vector ecology is a prerequisite to the development of virus control packages.

Chickpea viruses

Aphid colonisation is not often observed in chickpeas and while high levels of virus symptoms are common in pulse crops that are favoured by aphids, like faba bean, field pea and lentils, virus incidences in chickpeas are generally low. However, the impact of virus infection in chickpeas can be far more severe than in other pulse crops. A wide range of viruses are capable of infecting chickpeas, but – based on regular surveys throughout the Australian winter pulse growing region – three viruses (BWYV, AMV, CMV) are the most common encountered (Table 1). During 2012 unusually high virus incidences were observed in chickpea crops in the northern region, particularly on the Liverpool Plains. Diagnostic tests of symptomatic plants showed mainly BWYV positives (van Leur et al. 2013), but closely related viruses could account for up to 30% of these BWYV-like infections in some paddocks (Moore et al. 2013).

Canola viruses

Unlike with chickpea, severe aphid colonisation is common in canola and aphicide sprays are commonly applied to avoid aphid-feeding induced yield losses. Three viruses are reported on canola in Australia (Table 1), but – until now – only high infection levels of BWYV have been found in commercial canola crops. Of the three aphid species that are predominant on canola, the green peach aphid (GPA) is the most BWYV efficient vector, but cabbage and turnip aphids are capable of transmitting BWYV at a lower efficiency. All three aphid species can transmit TuMV and CaMV. Compared to pulse viruses, canola viruses have received very little research attention in Australia so far.

Interaction between chickpea and canola viruses

The rapidly expanding canola cultivation in the northern grain region could potentially become a source of BWYV infection for chickpea crops: The virus could build up in early sown canola crops and move to the later sown chickpea crops in early spring. Green peach aphid feeds on a large number of plant species, including chickpeas. TuMV has as well been reported in chickpeas (Schwinghamer et al. 2007).

The possible interaction between chickpea and canola viruses and the lack of research on canola viruses prompted GRDC to fund a short term project; ‘Scoping Study on Canola Viruses in Northern Australia: Occurrence and Variety Performance’ (DAN00179).

Development of canola and winter pulse viruses during the 2013 season in the northern grain zone.

Similarly to the preceding season, extended drought conditions prevailed during July – September throughout the northern region in 2013. Little or no virus was found in early sown faba bean crops, but - similarly to 2012 – high incidences of severe symptoms were observed in a number of chickpea paddocks by mid September. Diagnostic tests showed BWYV (or a closely related virus) associated with symptoms in the severely affected parts of the northern part of the region. Chickpea paddocks in the southern part of the surveyed region only showed low levels of symptoms, mainly caused by AMV with a lower incidence of CMV (Sharman et al. 2014). Testing of virus symptomatic chickpea plants for TuMV did not yield any positives.

Table 1. Priority viruses for winter pulses and brassicas in the northern grain region

Highly variable TuMV incidences were found between turnip weed populations, with 83% infection already by the end of July in a population near North Star. However, other turnip weed populations sampled in that area on the same day were virus free. TuMV infection of canola was rare; out of the 51 canola paddocks surveyed during the 2013 season, 44 were TuMV free. All 7 TuMV infected paddocks were located on the Liverpool Plains with one paddock showing 100% infection on October 1. Low levels of CaMV were found in a few turnip weed populations.

The localised, but very high, TuMV infection in commercial canola crops is a significant finding. Overseas studies report the destructive potential of this virus and our preliminary tests with TuMV strains isolated from canola crops showed severe growth reduction after inoculation.  While the susceptibility of brassica weeds and Indian mustard cultivars to TuMV is known, surveys carried out a decade earlier in the same region indicated resistance of canola to local TuMV strains (Hertel et al. 2004). The establishment of new, more virulent, virus strains is a possible explanation.

Control options

Chemical and cultural

Virus management through the control of aphid vectors by insecticides is possible for persistently transmitted viruses as they require extended feeding periods, but is of little use for non-persistently transmitted ones. Seed treatment by systemic insecticides can control early virus infections (like BLRV in faba bean), but is unlikely to be effective for late infections. The treatment of canola seed with the systemic insecticide imidacloprid to control soil-borne seedling pests might provide some protection, but treatment rates are likely too low to be fully effective.

Cultural and agronomic practices that are aimed at improving plant growth, like weed control, proper plant nutrition, correct sowing rates and sowing into stubble, will reduce virus risks and/or increase plant resistance and competition ability (Jones 2004).

Genetic resistance

Improved disease resistance is the most economic and environmentally sustainable way to control diseases, fungal or viral. Over the past decade good progress has been made in the Australian faba bean and field pea breeding programs and new varieties with adequate resistance to the priority viruses are now available. Systematic screening for virus resistance in chickpeas started three years ago in collaboration with the International Centre for Agricultural Research in the Dry Areas (ICARDA) as part of a GRDC funded project. Screening in northern NSW is done at the NSW-DPI Liverpool Plains Field Station (LPFS). Virus development in these screening nurseries is monitored through regular diagnostic tests. Both in 2012 and 2013 BWYV caused a high incidence of severe virus symptoms. Figure 1 shows the incidence of virus symptoms in chickpea breeding lines compared to two check varieties, Gully and PBA Slasher . The variety Gully was developed through mass selection made in an Iranian germplasm accession under high virus pressure in 1992 at the LPFS, is not grown commercially because of its high susceptibility to Ascochyta blight. None of the breeding material showed a level of BWYV resistance comparable to Gully and its good performance demonstrates the potential of genetic virus resistance. Screening of a wide range of chickpea germplasm accessions in a different trial at the LPFS during 2013 showed several promising lines. Single plant selections were made in the best accessions and will be re-tested in the coming season.

Figure 1. Incidence of virus symptoms in 91 desi and 60 kabuli chickpea breeding lines and 2 check varieties on two different soring dates, LPFS, 2013^1
1% virus symptomatic plants/plot, averaged over two replicates (check varieties averaged over 5 plots)

The 2013 season was the first season in which canola and Indian mustard (B. juncea) lines were tested for virus resistance. A total of 40 canola and 8 juncea breeding lines from different breeding programs were sown in the LPFS. On 24 September BWYV and TuMV infection was tested on a random sample of 15 plants from each of 3 replicates. BWYV incidence was relatively low and no significant difference between the tested lines was found. However, high levels of TuMV infection were found both in the canola and especially in the juncea lines tested (Figure 2). None of the tested lines was TuMV-free, but two canola breeding lines had < 10% infection.

Figure 2. Infection level of Beet western yellows virus and Turnip mosaic virus in 40 canola and 8 juncea varieties, LPFS, 24 Sep 2013^1
1 Percentage TBIA positive plants, averaged over 15 randomly collected plants in each of 3 replicates.

Conclusions

High incidences of virus symptoms in chickpea paddocks during 2013 showed again the destructive potential of viruses in this crop. Continuous monitoring of crops remains necessary as the virus spectrum can change. Correct identification of viruses and virus vectors is a high priority for the crop protection research programs.

More data are needed to quantify the importance of viruses in canola. However, the high incidence of TuMV in canola paddocks on the Liverpool Plains may have implications for canola cultivation nation wide.

References

Hertel K, Schwinghamer M, Bambach R (2004) NSW DPI Agnote no. 495.

Jones RAC (2004) Virus Research 100: 5–30.

Moore K, Ryley M, Sharman M, van Leur J (2013). Proceedings 2013 GRDC Grains Research Update, Goondiwindi.

Schwinghamer MW, Thomas JE, Parry JN, Schilg MA, Dann EK (2007) Australasian Plant Disease Notes 2: 41–43.

Sharman M, Moore K, van Leur JAG, Aftab M, Verrell A (2014) Proceedings 2014 GRDC Grains Research Update, Goondiwindi.

van Leur JAG, Aftab M, Manning W, Bowring A, Riley MJ (2013) Australasian Plant Disease Notes 8: 49–53.

Contact details

Reviewed by: Guy McMullen