CROP pathologists and fungal disease experts are raising concerns about the increasing use of precautionary fungicide applications in Australian canola crops.
Associate Professor Fran Lopez-Ruiz, project lead for the Australian Fungicide Resistance Extension Network, said repeated use of a single fungicide as “cheap insurance” against disease could have serious long-term consequences.
“Repeated use of fungicides from the same chemical group, especially when there’s no clear evidence of yield benefit, can drive the development of fungicide resistance within disease populations,” he said.
Resistance can spread quickly
Professor Lopez-Ruiz said spores released by resistant fungi could spread over large areas in a short time. Poor disease management on one farm could soon become a regional issue.
“We’re seeing this now with net form net blotch of barley in parts of South Australia, Western Australia and Victoria,” he said.
“NFNB pathogen populations with triple resistance to all three registered fungicide groups have left growers with very limited options for protecting barley yields.”
Canola at similar risk
Dr Steve Marcroft of Marcroft Grains Pathology said blackleg in canola was showing a similar trend.
Blackleg with reduced sensitivity to Group 3 fungicides — prothioconazole, fluquinconazole, flutriafol and tebuconazole — has been detected in SA, Victoria, New South Wales and WA.
“There is a high likelihood of resistance developing, with potential for field failure in pathogen-conducive environments,” Dr Marcroft said.
He also noted mutations for reduced sensitivity to Group 7 SDHI fungicides and Group 12 fungicide fludioxonil have been found in SA.
“There is potential for reduced sensitivity to these two groups under high disease and fungicide selection pressure.”
Off-label use poses risks
Dr Marcroft said he had received reports of growers applying double rates or spraying outside label timing recommendations in an attempt to control blackleg.
“Both practices are illegal — label rates and requirements must be followed,” he said.
“They also contribute to increased fungicide resistance. Fungicides should only be applied when disease pressure is high and there is a clear risk of yield loss.”
Early sprays increase resistance risk
Preliminary results from GRDC-supported research by Marcroft Grains Pathology, using blackleg as a model, suggest that fungicide applications between the four and eight leaf stages are a key driver of resistance.
“Growers should avoid spraying fungicides during this growth stage, if possible,” Dr Marcroft said.
“It’s important to remember that canola can tolerate significant disease before any yield loss occurs — typically up to 20 per cent crown canker infection.”
Skipping early foliar applications may lead to some crown canker but is unlikely to reduce yields and can significantly reduce the risk of resistance.
Low levels of upper canopy infection are also unlikely to impact yields, and applying fungicide after 50 per cent bloom could breach maximum residue limits.
Use only when needed
“If fungicides are overused when disease pressure is low, they may not be effective when we actually need them,” Dr Marcroft said.
“What seems like cheap insurance now could become a long-term threat to canola production. Weighing the risks and benefits of a fungicide application with your agronomist is a much better investment.”
AFREN is supported by the Grains Research and Development Corporation.
More information visit afren.com.au
AFREN Fungicide Resistance Five
Principles recommended by the Australian Fungicide Resistance Extension Network to help prevent the development of resistance:
- 1. Avoid susceptible crop varieties
- 2. Rotate crops — use time and distance to reduce disease carryover
- 3. Use non-chemical control methods to reduce disease pressure
- 4. Spray only if necessary and apply strategically
- 5. Rotate and mix fungicides/MoA groups