There is concern a crop virus could be re-emerging two years after causing serious damage to a number of regions in South Australia.
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In 2014, the Beet Western Yellows Virus hit thousands of hectares of canola in regions including the Mid North, Eyre Peninsula and Murray Mallee.
Eyre Peninsula agronomist Nigel Myers said aphids associated with the virus had been found on some local farms. He said the warm, mild conditions of 2014 could be repeated.
"If we get similar conditions this year then we might have the right environment for the aphids to build up the numbers and infest into emerging canola crops, if the host plants have not been controlled well before sowing," he said.
Myers said seeing the aphids associated with the virus was a concern.
"It is a risk because we have got the host plants out there growing now. Our concern is that we do not want a repeat of 2014, because aphids like those warm, mild conditions."
Beet western yellows virus is an aphid-borne virus that causes yield and quality losses in canola. It also infects other crop and pasture species including mustard, chickpea, faba bean, field pea, lucerne, medic and subterranean clover.
With early infection yield losses can be up to 50 per cent. For every one per cent increase in the level of BWYV in a canola crop there is a 6-12 kg/ha decline in yield resulting from formation of fewer seeds.
In addition to yield losses, BWYV infection diminishes seed quality by decreasing oil content and increasing erucic acid levels. If the crop is infected late, yield and quality losses are minimal.
First signs are red, yellow or purple colours at the ends or edges of older leaves, then yellowing in the middle of the leaf. Colours are more intense between leaf veins and on the upper side of the leaf.
Petioles and leaf veins are green or pale. Discoloured leaves become thickened and may cup inwards. Infected are often stunted and pale, and produce few flowers or seeds. Late infected plants show leaf symptoms but are not stunted and have lower yield loss. ■