Stop the rot – early intervention key to disease control


Canola leaves infected with sclerotinia

Early infection of canola leaves by sclerotinia often starts at the base of the leaf.

PHOTO: Kurt Lindbeck

Northern region canola growers are being warned that early intervention and disease management are critical to minimising the risk of sclerotinia stem rot disease this season.

The disease, which can cause yield reductions of 30 to 40 per cent in heavily infested crops in high-rainfall years, is top of the 2016 season watchlist for plant pathologist Dr Kurt Lindbeck, from the New South Wales Department of Primary Industries.

Dr Lindbeck, who is based at the Wagga Wagga Agricultural Institute, where he leads GRDC-funded research on the management of pulse and oilseed diseases, says prolonged wet weather through winter and early spring are ideal for the development of apothecia, the fruiting structures of the sclerotinia fungus.

portrait of Dr Kurt Lindbeck

Dr Kurt Lindbeck says early intervention is critical if growers are to minimise the production impact of sclerotinia stem rot.

PHOTO: Toni Somes

Indicators sclerotinia stem rot could be a problem in 2016

  • Spring rainfall: Epidemics of sclerotinia stem rot generally occur in districts with reliable spring rainfall and long flowering periods for canola.
  • Frequency of sclerotinia outbreaks: Use the past frequency of outbreaks in your district as a guide; paddocks with a recent history of disease are a good indicator of risk. Infections in high-risk regions have become more severe in recent years, with intensive wheat/canola rotations suggesting higher levels of residual inoculum (sclerotia) in the soil.
  • Start of flowering: Flowering time can determine the severity of an outbreak. Canola crops that flower earlier in winter, when conditions are cooler and wetter, are more prone to disease development.

Sclerotinia is the cause of canola stem rot and Dr Lindbeck is warning growers and advisers to be alert for early signs of the disease this year.

He is also advocating careful planning and disease management in northern canola-growing regions to minimise production risks.

His warning comes on the back of sporadic outbreaks of the disease during 2015, across regions of southern NSW with a history of sclerotinia, high intensity of canola and reliable spring rainfall.

Growing conditions last season were initially highly conducive to a sclerotinia stem rot outbreak.

“The first warning signs appeared in early August, with apothecia observed in canola crops in southern NSW,” he says.

“Continued wet weather through August provided periods of extended leaf wetness and opportunities for disease epidemics to develop early.”

Dr Lindbeck’s research shows extended periods of leaf wetness (approximately 48 hours) are ideal for triggering epidemics of stem rot. Weather conditions play a major role in the development of the disease, with the presence of moisture during flowering and petal fall critical.

The onset of drier conditions in September 2015 spared many canola crops from what he describes as “potential disaster”.

“Dry conditions during flowering and petal fall last season quickly prevented widespread outbreaks of the disease. Even if the flower petals are infected dry conditions during petal fall will prevent stem infection,” Dr Lindbeck says.

“Leaf wetness within the crop canopy is the driving factor behind the development of stem rot.”

He says the complexity of the sclerotinia stem rot cycle means outbreaks are more sporadic than other diseases, because weather conditions must be right for the pathogen at each stage of development.

Using a foliar fungicide

There are no commercial canola varieties with resistance to sclerotinia stem rot available to Australian growers. So Dr Lindbeck says disease management relies on the use of cultural and chemical controls.

“Foliar fungicides should be considered in high-risk regions where the disease frequently occurs, there is a long flowering period and reliable spring rainfall,” he explains.

Tips to minimise sclerotinia disease risk this season

  • Sow canola seed that is free of sclerotia. Clean or grade any retained canola crop seed from the previous season that shows signs of sclerotinia.
  • Rotate canola crops and grow break crops such as cereals, or field peas or faba beans, which are less susceptible.
  • Avoid planting this season’s canola in paddocks where there have been moderate to high levels of sclerotinia during the past three years.
  • Follow recommended sowing dates for your district. Do not be tempted to sow crops early if you are in a sclerotinia-prone district.
  • Consider the use of a foliar fungicide: weigh up yield potential, disease risk and application costs. Apply foliar fungicides at the optimum flowering window (20 per cent bloom to 50 per cent bloom). The optimal spray timing window can be as short as two days.
  • Monitor crops for disease development and identify the type of stem infection. Main stem infections cause the greatest yield loss and indicate infection events early in the growing season. Branch infections can develop later in the season and generally result in a lower yield loss. 
He says the decision to use a foliar fungicide will be governed by disease presence, conditions conducive for disease development (including rainfall and crop biomass), crop maturity, crop yield potential and canola prices.

There are several foliar fungicides to manage sclerotinia stem rot registered for use in Australia.

He suggests growers and advisers consider the following when using a foliar fungicide:

  • most yield loss from this disease results from early infection, which is likely to result in premature ripening and pod loss;
  • plants are susceptible to infection once flowering starts. Research has shown a single fungicide application at 20 to 30 per cent bloom is often effective in reducing yield losses from sclerotinia stem rot by preventing main stem infections (20 per cent bloom is 14 to 16 flowers on the main stem; 30 per cent bloom is approximately 20 flowers on the main stem). Most registered fungicides can be applied up to 50 per cent (full bloom) stage;
  • use bloom stage as a guide for fungicide applications. Bloom refers to the number of flowers developed on individual plants;
  • timing of fungicide is critical. Application is designed to prevent early infection of petals, while ensuring fungicide penetrates into the lower crop canopy to protect potential infection sites, such as the lower leaves, leaf axils and stem;
  • a foliar fungicide is a protectant, and has no curative capabilities, so it is only effective when applied before an infection (before a rain event during flowering);
  • in general, foliar fungicides offer protection for up to three weeks; and
  • use high water rates and fine droplet sizes for good canopy penetration and coverage.
Soil containing Apothecia

When conditions are favourable, sclerotia in the soil germinate to produce an apothecia, which will release ascospores. These can be up to 10 millimetres in diameter and are shaped like a flattened golf tee.

PHOTO: Kurt Lindbeck

More information:

Dr Kurt Lindbeck,
02 6938 1608,
kurt.lindbeck@dpi.nsw.gov.au

Sclerotinia stem rot in canola fact sheet

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