FARMERS managing the damaging fungal disease blackleg in canola are being urged to consider their options on a case-by-case basis.
Canola pathologist Steve Marcroft, Marcroft Grains Pathology, said there was no clear-cut rule as to whether a fungicide would generate an economic return.
Instead, he recommended growers use management support tools such as BlackCM to identify whether individual paddocks were high risk and what the best strategies to minimise yield loss would be.
Speaking at a Grains Research and Development Corporation event in Bendigo last month Dr Marcroft said trials had shown various economic responses to a blackleg-controlling fungicide.
"It can range from yield increases of 40 per cent in some small plot research trials to nothing, so the trick is to identify where your crop sits within that range of responses," Dr Marcroft said.
"There is no simple method of assessing exactly how much disease will occur, so the key is to identify the blackleg risk and the potential yield loss compared to the costs of application."
However, he said some common sense rules of thumb could help.
"If it is a dry year, your risk is lower, if it is a wet year it will be higher, and with the higher yield potential it is easier to get an economic response to the fungicide."
He said farmers could also assess their need for a seed treatment on their region and rainfall.
"High levels of canola in the rotation and higher rainfall equals high risk," he said.
"The variety's resistance to disease, together with the blackleg population and the time of crop emergence are also all important considerations."
Dr Marcroft said the canola industry had become increasingly reliant on fungicides to control blackleg at the expense of cultural practices.
He said this could work but farmers needed to ensure their crops were protected when plants were infected during early seedling growth.
Dr Marcroft also said growers needed to recognise the two different forms of infection.
"Crown canker and upper canopy infection are separate and fungicide applications to control infections in the upper canola need to be thought of as such."
He said knowledge of the yield risk from upper canopy infection was becoming better understood.
"It is likely decision new making support tools related to UCI will be released when there is sufficient confidence in the findings the tools are making."