Green peach aphids and beet western yellows virus

Although green peach aphid numbers in canola crops have now declined in most districts, crops may remain susceptible to yield losses from BWYV infection until the mid-podding stage. This article draws on the expertise of national specialists.

 

Incidence of beet western yellows virus (BWYV) in canola crops continues to cause concern in several districts. This follows widespread infestations of green peach aphid (GPA) (Myzus persicae), the principal vector of BWYV, during autumn and early winter. BWYV has been found across numerous canola crops in the Victorian Mallee and the Victorian Wimmera. Some reports suggest that greater than 50% of all canola crops in the Mallee are now infected with BWYV. Plant samples from a large number of crops have been sent to VicDEPI for testing. South Australia has suffered considerable damage due to BWYV.

Unusually high BWYV autumn infections have also been found in several canola paddocks in NSW. Plant samples with virus-like symptoms have been received from all over the state and are currently being tested. High levels of damage, similar to the South Australian situation, have not occurred.

BWYV infection can result in significant losses in seed yield and oil content. Symptoms of the virus may not be evident for 2-5 weeks after infection. Symptoms may initially resemble nutrient deficiencies, herbicide damage or other disorders. Leaves may turn yellow and purple, starting from the lower leaves. Other symptoms may include leaf mottling, leaves become thickened and cupping inwards, and premature bolting. In addition to the virus, it is very likely that GPA have caused significant damage to many crops as a result of their direct feeding on plants.

Canola is most susceptible to BWYV at the rosette stage, when infection can lead to high yield losses. Generally, the yield consequences of BWYV decrease with infection at later stages of crop development. However, canola crops remain susceptible to yield losses from BWYV infection until approximately the mid-podding stage. To some extent, this depends on the strain of BWYV present.

Why did this happen?

Heavy rainfall in cropping districts during February and March is likely to have created a ‘green bridge’ of weed hosts for both BWYV and GPA. Host plants of BWYV include weeds, such as wild radish, wild melon, fleabane, stinkweed, blackberry nightshade, marshmallow, bedstraw, thistles and volunteer canola, and pastures including lucerne, medic and sub-clover. GPA persist over-summer on a variety of hosts, including wild radish, turnips, mustard, volunteer canola and many other broadleaf weeds.

Warm temperatures continuing through autumn and early winter created ideal conditions for aphid population development. High population densities combined with warm temperatures are likely to have increased aphid flight activity, resulting in early and widespread colonisation of canola crops. Winged aphids carrying the virus from infected weed hosts will have transmitted the virus to young canola plants before spreading it to other healthy plants.

Compounding these issues is the presence of insecticide resistance in many GPA populations. High levels of resistance to carbamates (e.g. pirimicarb) and pyrethroids are now widespread across Australia. Moderate levels of resistance to organophosphates have been observed in many populations, and there is evidence that resistance to neonicotinoids (e.g. imidacloprid) is evolving. GPA populations can now be screened for resistance by cesar using DNA diagnostic tools. Contact us for further information.

Where to from here?

The onset of cold and wet weather conditions has caused GPA populations to decline in many crops. Aphid flights over the next few weeks are unlikely due to the cool weather conditions. The risk of further virus spread is therefore currently low, and will remain so while these conditions persist. As temperatures rise from late winter onwards, GPA will become more active and numbers will increase once again. This will increase the risk of further virus spread in unaffected canola crops, particularly in the Victorian Mallee. Generally speaking, the extent of spring infestations are more influenced by climatic and crop conditions in late winter-early spring than the number of aphids persisting in the crop during winter.

Pulse crops may also be at risk in spring. Annual surveys in south-eastern Australia always find BWYV in pulses. Due to the higher than normal aphid numbers this year, there may be an increased risk, particularly in warmer areas where aphid populations may build-up early.

Our advice

Experts agree that spraying to control GPA while winter conditions persist is probably not warranted in most regions. However, growers should remain vigilant, monitor for colonising (winged) aphids, and be prepared to apply insecticides in late winter or early spring to limit virus spread into canola and pulse crops. Although unlikely in most regions, if aphids are actively flying at present, chemical control should be considered.

It is important to only use insecticides registered for the crop and situation, and to comply with the label directions. The use of unregistered products may result in residue detections in a cell or shipment of canola. This would cost much more than just the value of the product; it could result in the loss of an important market, the loss of our premium food reputation, prosecution(s) and more stringent chemical use requirements for the future.

In cases where sprays are needed, the use of ‘softer’ insecticides such as pirimicarb should be considered. Fortunately, many GPA populations in NSW and Victoria (unlike in many other states where resistance is widespread) can still be successfully controlled with pirimicarb. Please note, pirimicarb is more effective if applied when temperatures are above 20°C. Transform® is also registered in canola and field reports suggest very good control of GPA. Paraffinic spray oils can also be used for suppression of GPA.

Monitor aphid populations (and their flights) over the coming weeks using a combination of yellow sticky traps and directly searching for aphids on plants. GPA are typically (but not always) found on the undersides of leaves. On rare occasions in canola, GPA can be found on the flowering spikes in spring; often in mixed populations with cabbage aphids and/or turnip aphids.

 

*Sources of field reports and pest information

We thank Kym Perry and the PestFacts SA & Western Victoria (SARDI) team for preparing much of this article, after wide consultation.

Brett Atkin – Agronomist, Elders (Victorian Mallee)

Greg Baker – Entomologist, SARDI (Adelaide)

Jenny Davidson – Plant Pathologist, SARDI (Adelaide)

David Eksteen – Agricultural Consulting (NSW Riverina)

Angela Freeman – Senior Research Scientist, VicDEPI (Victorian Wimmera)

Frank Henry – Plant Pathologist, VicDEPI (Victorian Wimmera)

Don McCaffery – Technical Specialist (Pulses & Oilseeds), NSW DPI (NSW Central Tablelands)

Andrew McMahen – Agronomist, Landmark (Victorian Mallee)

Adam Pearce – Agronomist, Clovercrest Consulting (Victorian Wimmera)

Joop van Leur – Plant Pathologist, NSW DPI (NSW North West Slopes & Plains)

Kate Wilson – Agronomist, AgriVision Consultants (Victorian Mallee)

Matthew Whitney – Agronomist, Dodgshun Medlin (Victorian Mallee)