Courtesy: GRDC – Research fast-tracked to arrest rising blackleg resistance | GroundCover
Research has indicated that fungicide application at 4 to 6-leaf stage in canola crops could be driving fungicide resistance in blackleg disease.
The findings are part of GRDC-invested research conducted after resistance to SDHI (Group 7) fungicides was found in blackleg in canola for the first time.
The resistant population was detected in samples from the Eyre Peninsula (EP) in 2024 as part of routine testing by the University of Melbourne and Marcroft Grains Pathology teams.
Key points
- Resistance to SDHI (Group 7) fungicides was found in blackleg in canola for the first time on the Eyre Peninsula in 2024.
- Fungicide application at 4 to 6-leaf stage in canola crops could be driving fungicide resistance in blackleg disease.
- Strategies to contain and prevent blackleg include choosing resistant varieties, rotating crops and using fungicides with different modes of action.
Caused by fungal pathogen Leptosphaeria maculans, blackleg is a potentially devastating disease in canola that can lead to significant yield loss. The fungus can survive in stubble and can infect the plant at several growth stages including in seedlings and in the upper canopy.
Alec McCallum is investigating resistance in L. maculans as part of his PhD at the University of Melbourne, with support from a GRDC Research Scholarship. He says samples of blackleg taken from 50 paddocks on the EP in 2022 showed no resistance to SDHI fungicides. Within 2 years, in 2024, moderate and high SDHI resistance was detected in 14 of those paddocks.
In 2025 he again detected SDHI resistance from samples from the EP; however, resistance has not been detected elsewhere at this stage.
Impact of overuse
Strategies to contain and prevent blackleg and other diseases include choosing resistant varieties, rotating crops to prevent the carryover of disease, and using fungicides with different modes of action (MOA).
But an overuse of fungicides is thought to be supporting the development of resistant blackleg strains. This is because all susceptible spores are killed if fungicide is applied too often, leaving the resistant strain to dominate.
While 2 to 3 fungicide applications are common (at seed dressing, 4 to 6-leaf stage and 30% bloom), Mr McCallum says there have been reports of 5 applications in a single crop.
University of Melbourne pathologist Dr Angela Van de Wouw, who leads the research team, says low costs and practicality have led to fungicides being viewed by some as a “insurance policy”. But an over-reliance on chemical applications could be contributing to the development of fungicide resistance.
“We do have an increased reliance on fungicides and so this is really becoming quite a hot topic,” she says.
Research insights
A GRDC National Grower Network investment to investigate factors driving resistance was fast-tracked after the SDHI resistance was detected. Led by Dr Van de Wouw, the research combined on-farm fungicide use data with controlled experiments.
Real-world insight was drawn from 6 years of records from 29 paddocks across the EP and Victoria with varying levels of SDHI and other fungicide resistance. Data included all crop rotations and fungicide applications between 2018 and 2024. This was correlated with frequencies of resistance and disease.
The research found that for SDHI resistance, there was a significant correlation with the number of foliar applications at the 4 to 6-leaf stage. For DMI (Group 3) fungicides, there was a correlation between resistance and the average number of applications.
These findings were backed up by controlled trials at Horsham, Victoria, using canola inoculated with blackleg disease containing varying levels of resistance to DMI and SDHI fungicides.
The researchers also tested a blackleg sample with no resistance, and created a novel population (that doesn’t exist in the field) with resistance to both DMIs and SDHIs.
A range of fungicide treatments was then applied from zero fungicide and seed treatment only, to several combinations of seed treatments and 4 to 6-leaf foliar sprays. The plants were then scored for different levels of disease at a variety of growth stages.
Efficacy effect
The research found that the efficacy of DMIs began to break down when more than 10% of the blackleg population contained resistant isolates. And for SDHIs, efficacy began to wane when just 1% of the population was resistant (see Figure 1).
For foliar fungicide applications, the research showed that application of DMI and SDHI fungicides at the 4 to 6-leaf stage significantly restricted disease in susceptible blackleg populations, but was far less effective against populations with resistance.
Foliar applications preventing susceptible populations from surviving meant they couldn’t work to ‘dilute’ resistant populations in the next generation, potentially increasing fungicide resistance, Dr Van de Wouw says.
“What we think is happening is if we’re coming back at the 4 to 6-leaf stage with a foliar spray, that’s essentially stopping any susceptible blackleg isolates from getting through,” she says.
“And this supports the on-farm paddock use survey data that suggests that the 4 to 6-leaf foliar applications might be driving this fungicide resistance selection.”
Field failure findings
Dr Van de Wouw says that because SDHI resistance has been detected early, researchers have the opportunity to look at what proportion of the blackleg population with resistance could lead to field failure (or significant yield reductions).
We don’t think we’re seeing field failure yet, and this has really opened up an opportunity for us to have a look at what’s happening and try to prevent it.
To investigate field failure – and in a similar approach to the resistance research – canola was inoculated with blackleg isolates containing varying levels of resistance to both DMI and SDHI fungicides. Fungicides were applied at a range of rates and times and disease levels monitored at different growth stages.
The research suggested between 1 to 10% of the blackleg population needed to be resistant to DMI fungicides to result in field failure.
But for SDHI fungicides, field failure could occur at a much lower rate of population resistance – between 0.1 to 1%.
These results are being correlated with data from field trials gathered from 25 paddocks in the 2025 season.
Current advice
Dr Van de Wouw says further research is needed into different fungicide MOAs to tease out the relationship between the chemistry, its effectiveness and the development of resistance. This would help refine recommendations around the different fungicides.
In the meantime, growers should think carefully about whether a 4 to 6-leaf foliar application is necessary, she says. And SDHI fungicide use should be limited to twice per season.
The use of tools such as the BlacklegCM app or UCI BlacklegCM app can help to determine whether fungicide application is required, supporting growers to make more informed decisions.
“By letting some disease in at the 4 to 6-leaf foliar stage, while your canola might not look as pretty because you’ve got lesions, you’re probably not going to get yield loss and it is probably going to help with managing fungicide resistance in the long term,” she says.