Hart Winter Walk: virus explained to growers

Image of a man in a paddock with a microphone

SARDI entomologist Greg Baker gives South Australia’s upper north growers an insight into the aphid-borne beet western yellows virus.

PHOTO: Rebecca Jennings

The annual Hart Field Site winter walk in late July gave growers in the upper-north cropping region of South Australia the chance to learn about the devastating beet western yellows virus (BWYV) outbreak that has struck the region this season.

South Australian Research and Development Institute (SARDI) entomologist Greg Baker and senior pulse pathologist Jenny Davidson talked growers through the impact, management and future risk of the aphid-borne virus.

Ms Davidson explained that when growers first started reporting widespread damage in June it was initially put down to incorrect herbicide use, but as more crops started showing yellowing leaves a different picture emerged.

It is now estimated that more than 5000 hectares of severe infection and a much larger area of lesser damage has occurred across SA, Victoria and southern New South Wales.

Ms Davidson said the above-average start to the growing season created a ‘green bridge’ of broadleaf weeds and volunteer plants, creating the perfect conditions for aphids.

“This virus build-up was driven by the weather, with significant rain in mid-February and follow-up rain in March, combined with above-average temperatures in autumn and early winter, which was perfect for aphids to continue multiplying and spreading the virus.”

The issue has been compounded by the presence of insecticide-resistant aphids and the fact that the pest in question – the green peach aphid (GPA) – is a highly efficient vector.

“While other species, such as cow pea aphid, can transmit BWYV, the green peach aphid is the most effective,” Mr Baker said. “If this species of aphid finds an uninfected host, there is a 97 per cent chance it will infect that plant, whereas for other aphids it might be 14 to 15 per cent. Adding to the challenge is that the green peach aphid is also the most difficult to control, with increasing levels of resistance to insecticide groups.”

Ms Davidson said growers should work on the premise that their canola has the virus if aphids are present.

“If a canola plant gets infected at the rosette stage, there can be up to 50 per cent yield loss,” she said. “The later a crop is infected, the less yield loss. Canola plants are susceptible until mid-podding, if they are infected after this stage there will be no yield loss but oil quality may be affected.”

For now, growers can test for the virus by sending leaf samples (25 leaves from infected plants and 100 leaves from non-infected plants) to Frank Henry at the Victorian Department of Environment and Primary Industries in Horsham. Farm advisers at the Hart Winter Walk were also asked to contribute by placing sticky traps around the region to help monitor aphid movement.

Growers will now be monitoring aphid movement, ahead of potential virus spread in spring.

“Many canola crops show widespread virus infection and it is questionable whether insecticides are warranted in these cases unless aphids are causing direct feeding damage,” Ms Davidson said. “If crops show patchy infection it may be necessary to control the aphids to prevent further spread of the virus into unaffected parts of crops during late winter and spring.”

Crops such as chickpeas, lentils and field peas had not emerged when the first round of the virus hit, but Ms Davidson told growers that pulses will also be susceptible to BWYV in spring.

In the face of resistance to existing insecticides, there is a new aphid control product on the market (Transform®, from Dow), but Mr Baker urged growers to exercise moderation: “Remember insecticide resistance and think responsibly about the stewardship of any new product.”

Transform® is registered on canola but will not be available for pulse crops.