Sclerotinia stem rot can cause up to 20 per cent yield losses in canola, but outbreaks are sporadic and heavily dependent on weather conditions.

Disease management specialist
Dr Steve Marcroft, of Marcroft Grains Pathology, says Sclerotinia has a complex disease cycle with several key stages. “Favourable weather conditions are needed for the disease to progress,” he says.

When sclerotia (survival structures) in the soil mature in wet conditions and form apothecia (fruiting bodies), they release spores into the air. These spores land on and infect the flowers. As the flowers senesce, the petals drop off and catch on leaves, in leaf junctions and in the branches.

If weather conditions are favourable, the Sclerotinia fungus grows from the petals into the stem or branch, causing the infection.

“Typically, wet weather during spring when the canola is flowering increases the chance of infection, especially if the canopy stays wet for more than 48 hours,” Dr Marcoft says.

GRDC investment

Dr Marcroft says the GRDC Sclerotinia investment is exploring the severity and implications of Sclerotinia in western Victoria.

“Data from 26 paddocks over two years shows Sclerotinia infection is variable between years, seasons and individual paddocks. However, fungicide applications were effective at controlling disease, albeit many of the paddocks had low levels of infected plants and, therefore, may not have benefited from  a fungicide application.”

He says individual paddocks may have had a large economic return from fungicide application in both years and from the Wimmera and Mallee.

“The most severe infection in the paddock at west Willaura in 2022 may have had up to 20 per cent yield loss. Accordingly, we aim to determine if Sclerotinia infection triggers can be determined from weather conditions.”

During the crop bloom stage in southern New South Wales, researchers found when humidity exceeds 95 per cent for more than  48 hours, coupled with petal infection at higher than 75 per cent, Sclerotinia infection will occur.

Infection severity

Dr Marcroft says Sclerotinia infection is rarely as severe in western Victoria, even though extended periods of high relative humidity are regularly experienced.

“We have measured infection events (48 hours at more than 95 per cent relative humidity and more than 50 per cent petal infection) over two years in two different regions in western Victoria.

The data shows a strong relationship between spring rainfall and infection; in 2022, there was five times more disease compared to 2023 (wet compared to dry spring). However, there is considerable variation between individual paddocks.

Dr Marcroft says the data shows that infection events do not correlate with Sclerotinia symptoms in individual crops.

He says this finding supports anecdotal agronomist knowledge that conditions in western Victoria are not as conducive for Sclerotinia development, regardless of  known infection events.

Proactive management

“Although Sclerotinia infection is not as severe in western Victoria, the data shows that 66 per cent of paddocks had more than 10 per cent of plant infection in 2022 (four out of six paddocks) and eight per cent of paddocks had more than 10 per cent of plant infection in 2023 (two out of 20 paddocks),” Dr Marcroft says.

“This indicates that growers and agronomists cannot be complacent and must still manage Sclerotinia, even if it is less likely to occur than in other Australian regions.”

He says the triggers for infection events in western Victoria remain unknown. “Data from this project will be provided to the NSW Department of Primary Industries and Regional Development for further analysis,” he says. “Experiments will continue for the next two years.”

More information: Steve Marcroft, steve@grainspathology.com.au