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.
Diagnosing beet western yellow virus in canola
Beet western yellows virus (BWYV) that has been renamed Turnip Yellows virus (TuYV) 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.
What to look for
- Discoloured sometimes stunted plants that occur in patches, in thinner crop areas or the edge of the paddock, and gradually spread.
- 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.
What else could it be
|Diagnosing nitrogen deficiency in canola||Purple red colours spreading down from the ends of oldest leaves||However nitrogen deficient plants are smaller and thinner rather than stunted. Paddock distribution varies according to soil type rather than in patches or edges.|
|Diagnosing Group A herbicide damage in canola||Reddish mottling of older leaves||However herbicide damage causes discolouration and distortion of emerging leaves, and multiple growing points|
|Diagnosing glyphosate damage in canola||Reddish mottling of older leaves||However herbicide damage causes discolouration and distortion of emerging leaves, and multiple growing points|
Where did it come from?
- BWYV is mainly found in high and medium rainfall wheatbelt zones but also occurs in low rainfall zones.
- It is not seed-borne, but survives in weeds or volunteer canola host plants outside the growing season and is spread from these infected plants into crops by aphids which act as vectors for virus transmission.
- Weeds include wild melon, fleabane, stinkweed, and blackberry nightshade. It also persists year-round in infected lucerne pastures and some native legume species. The main source of BWYV for canola crops is infected wild radish weeds growing along fence lines and road verges.
- It also infects other crop and pasture species including mustard, chickpea, faba bean, field pea, lucerne, medic and subterranean clover.
- Green peach aphid is the principle vector, but cabbage aphid acts as a minor vector in late infections.
- BWYV is transmitted persistently. When an aphid feeds on an infected plant it acquires the virus permanently. When the infective aphid then probes the phloem of a healthy plant it infects the plant and continues transmitting BWYV for the rest of its life.
- Autumn is the most critical infection period, so the earliest-sown crops are usually infected most.
- Infections can occur past the rosette stage of canola growth but these probably have little effect on yield
- Control broadleaf weeds (especially over summer) as they act as reservoirs for the viruses.
- Retain stubble at sowing to cover the ground, this reduces the ability of aphids to land on young canola plants thereby reducing virus spread.
- Manipulate sowing dates. Delay sowing to avoid exposure of young canola seedlings to peak aphid flights.
- Sow at high seeding rates. High plant density helps diminish the rate of virus spread and speeds up canopy closure resulting in lower aphid landing rates.
- Sow varieties with infection resistance. While there are currently no canola varieties with resistance to BWYV, varieties do differ in their susceptibility to infection. For example some varieties will only become infected at low levels while others have moderate resistance to infection.
- Use insecticide seed dressing. Imidacloprid seed dressing applied effectively can provide early control of green peach aphid vectors during the vulnerable seedling growth stage. However to achieve effective control it is important to ensure that sufficient insecticide coats each seed. Other common insecticides are ineffective against green peach aphids because of widespread insecticide resistance.
- Consult the web-based forecasting and decision support system for BWYV in canola.
How can it be monitored?
- Check for aphids (usually under leaves) before rosette stage.