AVOIDING exposing pathogen populations to high levels of fungicides is a key tool for limiting the development of resistance in barley crops, according to the Centre for Crop and Disease Management (CCDM).
The necessity of avoiding over spraying was highlighted by CCDM senior researcher Fran Lopez-Ruiz at the Liebe Group Crop Updates in Dalwallinu on March 11.
Dr Lopez-Ruiz said development and spread of fungicide resistance was quite concerning, especially in a pathogen strain which is found in Australia's South West.
"It's a hybrid strain, between net form net blotch and sport form net blotch, which are both pathogens to barley," Dr Lopez-Ruiz said.
"We were very unlucky and got a nasty cross in the south which is very resistant to some compounds within the group three fungicides and is spreading very fast."
The Department of Primary Industries and Regional Development ran a trial last year which tested different group three and non-group three fungicides in an area where the hybrid was prominent.
In that trial, the majority of DeMethylation Inhibitors (DMI) group three fungicides for control of the disease did not work when used as a solo application, it was only when there were combinations with different modes of actions that a better outcome was managed.
Dr Lopez-Ruiz said the trial illustrated the real nature of the risk growers were facing with the hybrid spreading.
"The main mechanism to avoid resistance is to avoid exposing the pathogen populations to very high levels of fungicides," he said.
"That means you have to limit applications and you have to avoid spraying prophylactically, so if you don't need a fungicide, you don't use it.
"On most farms, fungicides are sprayed as part of following a spraying program and often whether the disease is there or not is not relevant and the fungicide is sprayed anyway, but that's putting an unnecessary selection pressure on the population."
If a spray is necessary, mixing different modes of action is the best strategy.
Dr Lopez-Ruiz said if the same mode of action was used every time, the pathogen evolved very quickly and would build resistance towards that particular mode of action, but if two or more modes of actions are used at the same time, that evolution becomes more complicated.
However he did also recognise that sometimes growers' access to different modes of actions is not that fast or easy.
"One strategy is to alternate fungicides within the group three mode of action group," he said.
"So instead of using the same fungicide every season and really several times per season, try to rotate compounds from the same mode of action within the season.
"It's not ideal but it's better than using the same compound every time."
While it's important to not spray prophylactically, it is necessary to take action when symptoms are first seen.
"We should be spraying before wide infection, so not once the disease has already taken over the crop, because that increases chances of resistance developing," Dr Lopez-Ruiz said.
"We also have to stay within the correct rates and cutting them down is not a good idea as the rates have been developed by the chemical companies to provide the best control possible.
"If we're cutting rates all we're doing is exposing the population to a sub-lethal dose of the chemical and allowing those populations to survive and potentially evolve resistance."
Farm hygiene, crop rotation, use of varieties with high disease resistance rating and strategic burning to control high disease levels in the paddock, are key measures already being implemented as part of good integrated disease management (IDM) plans.
These tools are equally effective for the control of fungicide resistance and their use contribute to preserving the lifespan of the chemicals currently available for the control of net blotch diseases.
