Sclerotinia stem rot is taking a toll on canola crops in parts of the southern cropping region where the disease has infected individual crops by as much as 60 per cent.
The disease, which can result in yield losses of up to 30 pc, has been detected in parts of southern New South Wales, Victoria and South Australia.
Plant Pathologist Kurt Lindbeck, of NSW Department of Primary Industries, says a combination of factors – likened to a “perfect storm” – has led to widespread and damaging occurrences of sclerotinia stem rot this year.
“Conditions over winter favoured spore production and development of the disease in canola,” Mr Lindbeck said. “We had frequent rainfall events through the flowering period for canola, and we know that is an important trigger point for sclerotinia stem rot development.
“In addition, crops have flowered very early this year due to the mild winter temperatures and this has contributed to the levels of disease we observed,” said Mr Lindbeck, who is based at the Wagga Wagga Agricultural Institute where he leads Grains Research and Development Corporation-funded research on the management of pulse and oilseed diseases in southern NSW.
Flowering occurred three to four weeks earlier than normal in some districts where the disease is known to frequently occur.
“The disease levels we are witnessing this year are a result of low levels of inoculum building up over the past couple of seasons. With the right environmental conditions, as we have witnessed, it doesn’t take much for outbreaks of disease to occur.”
Mr Lindbeck said the increased intensity of canola in cropping rotations was adding to the disease pressure on crops.
Mr Lindbeck said that in addition to sclerotinia being an airborne disease, the sclerotinia stem rot pathogen produced hard, black, survival bodies on infected plant tissue called sclerotia which enabled the fungus to survive for up to 10 years in the field.
“At harvest, sclerotia fall on to the soil surface and can reside there for many years. The reservoir of sclerotia is being topped up every two years if canola is being grown in rotation with wheat.”
To minimise the risk of sclerotinia next season, Mr Lindbeck advises growers to consider where canola is to be sown and avoid paddocks which have had moderate to high levels of sclerotinia over the past three years. Optional low-risk break crops such as field peas and faba beans should be considered
“If retaining seed for sowing next season, it is also important to ensure the seed is graded and free of sclerotia.
“And if you are farming in a high-risk district, have foliar fungicides on hand and be prepared to go in and spray at the 20-30 per cent flowering stage, because the window for spraying can sometimes be only a couple of days.”
Timing of fungicide application is critical. Often the fungicides are applied too late, resulting in poor sclerotinia control.
“A single application of foliar fungicide at 20-30% flower is often effective in reducing yield losses from stem rot by preventing main stem infections,” Mr Lindbeck said. “In some districts with a known history of high levels of stem rot and high yield potential, a second application may be cost effective.
“The objective of the fungicide application is to prevent early infection of petals while ensuring that fungicide also penetrates into the lower crop canopy to protect potential infection sites (such as leaf axils and lower stems).”
In summary, important sclerotinia prevention and management measures include:
- Sowing canola seed that is free of sclerotia. This applies to growers retaining seed on farm for sowing. Consider grading seed to remove sclerotia that would otherwise be sown with the seed and infect next season’s crop.
- Separate next season’s paddock away from the previous year’s canola stubbles. Not only does this work for other diseases such as blackleg, but also for sclerotinia.
- Rotate canola crops. Continual wheat/canola rotations are excellent for building up levels of viable sclerotia in the soil. A 12-month break from canola is not effective at reducing sclerotial survival. Consider other low risk crops such as cereals, field peas or faba beans.
- Follow recommended sowing dates and rates for your district. Canola crops which flower early, with a bulky crop canopy, are more prone to developing sclerotinia stem rot. Bulky crop canopies retain moisture and increase the likelihood of infection. Wider row spacings can also help by increasing air flow through the canopy to some degree until the canopy closes.
- Consider the use of a foliar fungicide. Weigh up yield potential, disease risk and costs of fungicide application when deciding to apply a foliar fungicide.
Further information is available from the new GRDC Fact Sheet, Sclerotinia Stem Rot in Canola, which can be viewed and downloaded via www.grdc.com.au/GRDC-FS-Sclerotinia, as well as the latest GRDC Back Pocket Guide on diseases in canola, www.grdc.com.au/GRDC-BPG-CanolaDiseases. To listen to a GRDC Radio interview with Kurt Lindbeck visit www.grdc.com.au/Media-Centre/GRDC-Podcasts/Southern-Weekly-Update/2013/11/18-south.
Caption: The sclerotinia stem rot pathogen produces hard, black, survival bodies on infected plant tissue called sclerotia which enable the fungus to survive for up to 10 years in the field. Sclerotia can be mistaken for mouse droppings. Photo by Kurt Lindbeck.
Kurt Lindbeck, NSW DPI
(02) 6938 1608
Sharon Watt, Porter Novelli
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