As growers in the southern region move closer to the ‘business end’ of the cropping season the true effects of earlier infestations of Green Peach Aphid (GPA) (Myzus persicae) and Beet Western Yellows Virus (BWYV) in canola crops are being realised.
The encouraging news is that although early infected South Australian crops (rosette growth stage) suffered unprecedented damage from the virus, the impact on crops infected at later growth stages may not be as severe.
According to Primary Industries and Regions SA (PIRSA) grains account manager Dave Lewis, the impact of BWYV on South Australian canola crops, which were reported to have been the hardest hit by the virus, is less-than-expected.
Although an area of between 5,000 and 10,000 hectares was estimated to have been infected with BWYV in the lower and mid north of South Australia, Mr Lewis said flowering and pod set had been quite good in crops that weren’t infected early.
“For the later affected crops flowering and pot set has been pretty much normal,” he reported to ABC Rural on September 18, although some of the later infected crops have been estimated to be below optimum yield.
Meanwhile, in Victoria, Department of Environment and Primary Industries (DEPI) extension plant pathologist Frank Henry, said growers were also reporting that many crops had ‘bounced back’ in recent weeks.
Mr Henry, who has been involved in testing and mapping the spread of the disease across the southern region said that although a significant percentage of plants tested for the disease returned positive results – Mallee (61%), Wimmera (62%), South West (39%) and North Central/East (37%) – testing did not provide information on the number of plants infected in the paddocks sample.
“Feedback from Victorian growers would suggest that the virus may not be as widespread in crops as earlier thought,” he said.
The other positive growers can take from this year’s experience is that the current outbreak does not indicate a higher chance of infection in future years.
SARDI senior pulse pathologist Jenny Davidson said that green peach aphids (GPA) require green plants to survive. As weeds generally die off with the onset of ‘normal’ spring and summer temperatures, next year’s risk is unlikely to be any greater than any other year.
GPA & BWYV management options in 2014 and beyond
Frank Henry, Plant Pathologist, and Dr Mohammad Aftab, Virologist, DEPI Horsham
As of 17 September, 618 crop samples and 6,007 plants were tested by DEPI for BWYV with 3,425 (57%) plants testing positive for the disease. The breakdown by state for positive samples was: VIC 49%, SA 71% and NSW 56%. The virus was also identified in one sample from Tasmania (see BWYV map).
There has been much debate on whether to spray for GPA during the winter, but experts agreed it was not warranted in most areas as aphid numbers decrease during colder weather and are less mobile, reducing the risk of further virus spread.
As spring approached growers were advised to remain vigilant and monitor for colonising (winged) aphids as temperatures started to increase (15 to 18°C). The advice was to be prepared to apply insecticides to limit virus spread by winged aphids into canola and pulse crops. A build-up of numbers and the production of significant numbers of winged aphids in a population, is a signal that flights may be about to occur. However, before making a decision to spray, growers should ensure aphids are correctly identified (CropFacts, issue 9, free registration required). To date no winged aphids have been recorded in spring this season.
The range of chemical control options is limited as GPA is resistant to synthetic pyrethroid, organophosphate and carbamate insecticides. The only effective chemical registered for the control of GPA in canola across many regions of south-eastern Australia is Transform (sulfoxaflor) at $14/ha. Organophosphates can also be effective in some situations despite the presence of resistance. This is because the resistance mechanism in GPA is complex it and may confer only low or moderate levels of resistance in some aphids. In areas with widespread resistance it is recommended to use a test strip to see if organophosphates such as dimethoate are still effective before using Transform. Testing for GPA insecticide resistance is available for growers through Melbourne-based company, cesar.
While widespread frosts in August slowed GPA activity, recent warmer weather has hastened development, particularly as some crops become moisture stressed. However, in Victoria, most canola crops are flowering and beyond the critical period for virus infection.
Both the GPA and BWYV have a wide host range and attention in spring has turned from protecting canola crops to monitoring pulse crops, in particular lentils and chickpea crops. A temporary permit for Pirimicarb (now expired) was obtained for use in pulse crops, but not Transform due to no maximum residue limits (MRLs) being available. This has left pulse growers with no registered, effective chemical control for GPA in pulse crops, except for paraffinic spray oils.
Growers are advised to monitor pulse crops for GPA and if spraying for cowpea aphids (Aphis craccivora) to use Pirimicarb which is soft on beneficial insects. Beneficial insects, in particular the parasitic wasp, were effective in helping manage GPA in canola. Cowpea aphids are not believed to be a vector of BWYV.
DEPI met with beekeepers in August, and it was agreed that raising awareness about the impact of insecticides on bees was a priority. Before spraying growers need to consider their proximity to bees and the potential to impact local beehives.
Beekeepers should be given sufficient advanced notice so that bees can be moved if necessary. Growers should read the chemical label for advice on bee toxicity and not spray when bees are foraging.
Beet Western Yellows Virus (BWYV) is not seed-borne. Management in 2015 will involve observing any build up of a green bridge, monitoring aphid numbers over the summer-autumn period and controlling weeds that host GPA and BWYV. If the seasonal conditions indicate a high aphid-virus risk then growers should use effective seed treatments and agronomic practices that limit aphid landings in the crop i.e. allow a time lapse between weed control and sowing the crop, sow into standing stubble, higher seeding rate. In high risk seasons monitor the seedling crop for aphids and apply a foliar insecticide if warranted.