Unprecedented populations of green peach aphid (GPA) descended on canola crops last winter and with no definitive consensus as to the best course of action, the decision whether to spray proved challenging.
Of concern for growers was Beet Western Yellows Virus (BWYV) infection which is transmitted to canola crop by the GPA.
BWYV infects the phloem of plants and can result in significant losses in yield and oil content. Crops suffering the infection typically develop a ‘purpling’, the leaves become thickened and cup inwards: the crop generally looks ‘unhealthy’.
Symptoms may not be evident for two to five weeks after infection and may initially resemble nutrient deficiencies, herbicide damage, physiological stress or other disorders.
In 2014, these symptoms were observed in a number of canola crops in South Australia, Victoria and New South Wales. Subsequent analysis conducted by the Department of Economic Development, Jobs, Transport and Resources (DEDJTR) (formerly DEPI) on more than 600 crop samples returned a positive result for BWYV infection in almost 60 per cent of cases.
But did spraying prove effective?
Anecdotal evidence from growers was mixed, with some standing by their decision to spray while others remaining sceptical about whether spraying was worth it.
With the very dry finish to last season, and largely disappointing canola yields in the Mallee and Wimmera, the value of spraying against BWYV was difficult to quantify. In light of 2014s poor canola results, spraying may now seem to have been a futile exercise. But would this have been the case in a more ‘normal’ season?
Last year’s outbreak was thought to have been prompted by a combination of good opening rainfalls, a green bridge and warm autumn to early winter temperatures. Nevertheless, despite the heavy infestation of aphids and the subsequent widespread contraction of BWYV in 2014, this year’s risk is unlikely to be any greater than any other year because BWYV is not a seed-borne disease.
Management in 2015 will involve observing any build-up of a green bridge, monitoring aphid numbers over the summer to 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 (such as allowing a time lapse between weed control and sowing, sowing into standing stubble and using a higher seeding rate). In high risk seasons, growers should monitor the seedling crop for aphids and apply a foliar insecticide if warranted.
