Impact and management of viral diseases in chickpeas

Murray Sharman¹, Kevin Moore², Joop van Leur², Mohammad Aftab³, Andrew Verrell2

Take home message

Minimise risk of virus by retaining standing stubble, planting on time and at optimal seeding rate for rapid canopy closure. Control weeds in and around crop and ensure adequate plant nutrition

Viruses in chickpea crops

Chickpea and other winter pulse crops are susceptible to many plant viruses. The effects on plants include stunting, reddening, chlorosis, distortion, shoot tip wilting, reduced yield and grain quality and for chickpeas, often premature death. Infections occurring early in the cropping cycle generally result in more severe disease outbreaks and yield losses. All are spread by flying insect vectors and almost all can be separated into two main groups, those that are transmitted by aphids persistently and those that are transmitted non-persistently. Persistently transmitted viruses (eg Beet western yellows virus - BWYV, Bean leaf roll virus - BLRV) include the luteoviruses and poleroviruses where the aphids can retain and transmit the viruses for many weeks but require up to 1-2hrs of feeding to transmit. Pea aphid, green peach aphid and cowpea aphid are considered to be important vectors of chickpea viruses. Non-persistently transmitted viruses (eg Alfalfa mosaic virus – AMV, Cucumber mosaic virus – CMV) are only carried by aphids for a few hours but can be transmitted in less than a minute of feeding. Some chickpea viruses are also transmitted by leaf hoppers. Virus disease outbreaks in chickpeas are sporadic and difficult to predict from season to season or between locations. Major outbreaks of virus diseases in chickpeas occurred in the early 1990s (when losses in many chickpea crops on the Liverpool Plains reached 100%) and most recently in 2012 in several regions of NSW.

Impact of viruses in chickpea crops in northern region in 2013

In 2013, virus infection was found in almost all chickpea crops inspected from southern QLD to Wellington in the south.  The incidence of virus infection was generally lower than observed in 2012 with most crops inspected having <5% plants with symptoms but it was as high as 30-50% in several crops from the Breeza / Werris Creek area and Edgeroi. Overall, the most prevalent virus was BWYV and in some locations more than 90% of symptomatic plants were infected with BWYV (Table 1). There are related virus species that also react with the BWYV assay as is discussed further below, so it is likely there was a mix of BWYV-like viruses present at many locations. Some of the main outcomes from the chickpea surveys in N-NSW were:

• Higher proportion of BWYV infections found at, and north of the Liverpool Plains. Higher proportion of AMV infections in the south (Table 1). Very low levels of BLRV and CMV.
• Up to 15% of non-symptomatic plants still had BWYV infection from the Liverpool plains.
• Accurate identification by PCR has shown the aphid transmitted luteovirus species to have a wide geographical range in a number of alternative weed hosts (Table 2).
• Soybean dwarf virus (SbDV) was the major virus affecting several crops in the Edgeroi region in Oct 2013 and was confused with BWYV in the antibody test (Table 1).

Using the virus species-specific PCR described below, 49 virus affected plants from 2013 were screened consisting of 38 SbDV, 5 PhBV, 3 BWYV, 2 BLRV and 1 mixed SbDV/BWYV. From the 45 samples that were not BWYV by PCR, 33 were false positives in the BWYV antibody assay. This demonstrates the BWYV antibody used (from DSMZ) was not useful for identifying BWYV and PCR indicated that SbDV was the dominant virus from the samples tested.

During this work, a new polerovirus referred to Phasey bean virus – PhBV (previously thought to be a strain of BWYV) has been identified from many hosts and locations in the northern region (Table 2). It is transmitted efficiently by cowpea aphid. Although the relative importance of PhBV in chickpea crops is still uncertain, it appears to have been responsible for approximately 30% of the infections thought to be BWYV in the 2012 virus outbreaks (Moore et al 2013).

Table 1. The percent infection of BWYV, AMV, BLRV and CMV from chickpeas displaying virus symptoms in northern NSW as determined by TBIA diagnostic. Virus identification based on antibody reaction. Sample locations shown roughly from north to south. Note that the BWYV infections may be a complex of related viruses. Samples from most locations were also tested for Turnip mosaic virus (TuMV) but no positives were detected.

Location	Plants tested	% BWYV	% AMV	% BLRV	% CMV
Boomi	6	100	0	A n/t	n/t
North Star	12	67	17	n/t	n/t
Moree	19	79	0	0	0
Edgeroi	32	62	0	n/t	n/t
Edgeroi	17	47	0	n/t	n/t
Tamworth	15	60	20	n/t	0
Tamworth	30	87	10	n/t	0
Breeza	18	89	0	5	6
Breeza	25	88	8	4	8
Breeza	26	77	0	0	0
Breeza	19	53	5	0	5
Liverpool Plains	20	90	10	5	0
Liverpool Plains	21	90	10	5	0
Werris Creek	15	73	13	n/t	0
Pine Ridge	15	93	7	n/t	0
Pine Ridge	15	80	13	n/t	0
Blackville	15	13	67	n/t	0
Gilgandra	14	7	78	n/t	n/t
Gilgandra	38	21	71	0	3
Gilgandra	49	12	88	0	2
Wellington	30	10	73	n/t	n/t
Wellington	16	19	63	0	0
Wellington	15	7	60	0	0
Wellington	20	5	55	0	0

^A not tested (n/t)

Biology of significant viruses of pulses, particularly chickpeas

Accurate identification of viruses is critical for long term resistance breeding to be successful and for meaningful studies of how viruses survive in weed hosts and move into crops. To this end, we have begun to develop improved accurate diagnostics for the luteoviruses to help overcome uncertainty of virus identifications that can result from cross reactions of viruses to some antibodies. We have used a PCR for Beet western yellows virus (BWYV), Bean leaf roll virus (BLRV), Phasey bean virus (PhBV) and Soybean dwarf virus (SbDV) to investigate host range of the virus species from a range of locations (Table 2). While testing continues, Marshmallow weed is commonly found to be infected with BWYV from many locations and burr medic is a host for BLRV, PhBV and SbDV.

Table 2. The identification of virus species in different plant hosts from different locations in the northern region confirmed by species-specific PCRs. Testing of selected samples from 2012 and 2013 surveys.

Better agronomy – better chickpeas

Field trials from 2012 and 2013 have shown that chickpea crops are at risk of increased damage from viruses when plant density is below about 20 pl / m² (Verrell 2013, Moore et al 2014). Significantly less plants are infected when plant densities are higher and it is recommended to aim for greater than 25 pl / m².

Trial crops deficient in N, K, P or all three have been shown to have significantly more virus affected plants than a crop with adequate nutrition (Verrell 2013).

Inter row planting into standing wheat stubble significantly reduced virus incidence in small trial plots of PBA HatTrick when compared to the same amount of stubble slashed low to the ground (Moore et al 2014). The mechanism for this difference is unclear but these results are in agreement with many field observations by the authors in large crops during virus outbreaks.

While differences have been observed for the virus resistance of different varieties (Verrell 2013, Verrell 2014, Hawthorne 2008), further screening will be needed to strengthen confidence in these results under high disease pressure, from different regions, and to identify for which virus species resistance is effective. Under low virus pressure in field trials, some of the better performing varieties included Flipper, and PBA HatTrick although both these varieties have been observed with high rates of infection under high disease pressure. Variety Gully is very susceptible to Ascochyta but has moderate virus resistance so may be useful for breeding resistance into future varieties.

While a link could not be confirmed in the 2013 season between BWYV infections in canola and subsequent spread into nearby chickpea crops (van Leur et al 2014), the sometimes high incidence of BWYV in canola indicates it may still be prudent to avoid planting chickpea and other pulse crops next to canola.

Conclusions

Visit http://www.pulseaus.com.au/pdf/Virus%20Contol%20in%20Chickpea.pdf for detailed information on reducing losses from viruses in chickpeas.   Currently, the best strategies to manage chickpea viruses are agronomic ones:

• Retain standing stubble which can deter migrant aphids from landing. Where possible, use precision agriculture to plant between stubble rows. This favours a uniform canopy which makes the crop less attractive to aphids. 
• Plant on time and at the optimal seeding rate of greater than 25 pl / m² – these result in early canopy closure which reduces aphid attraction (Verrell 2013)
• Ensure adequate plant nutrition
• Control in-crop, fence-line and fallow weeds – this removes in-crop and nearby sources of vectors and virus. 
• Avoid planting adjacent to lucerne stands – lucerne is a perennial host on which legume aphids and viruses, especially AMV and BLRV survive and increase (van Leur and Kumari 2011).
• Seed treatment with systemic insecticides such as imidacloprid may be effective for reducing early infections of the persistently transmitted luteoviruses such as BLRV in faba bean, but is not effective for non-persistently transmitted viruses or for late infections of either virus type. Unfortunately, local data supporting seed treatment is lacking.
• Given the high incidence of BWYV sometimes found in canola, consider growing chickpeas (and other pulse crops) away from canola.

Acknowledgements

Thanks to Matthew Webb for technical support for molecular screening of field samples.

References

Hawthorne W (2008) Australian Pulse Bulletin PA 2008 #21.

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

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

Schwinghamer M, Knights T, Moore K (2009). Australian Pulse Bulletin PA 2009 #10.

van Leur J and Kumari S (2011). Australasian Plant Pathology, 40: 180-186.

van Leur J, Aftab M, Sharman M, Lindbeck K (2014) Proceedings 2014 GRDC Grains Research Update, Coonabarabran.

Verrell A (2013). Proceedings 2013 GRDC Grains Research Update, Goondiwindi.

Contact details

Murray Sharman
DAFFQ, Ecosciences Precinct, GPO Box 267, Brisbane, 4001
Ph: 07 3255 4339
Fx: 07 3846 2387
Email: murray.sharman@qld.gov.au