AUSTRALIAN cereal crops are suffering their heaviest fungal disease pressure in over a decade, with severe infestations of stripe rust likely to cause significant yield damage in wheat in places.
While the focus has been on the damage to crops from waterlogging the heightened disease pressure, closely associated with wetter seasons, is also taking a heavy toll, particularly in areas with poor paddock trafficability where farmers have been unable to apply in-crop fungicides in a timely fashion.
Grant Hollaway, Agriculture Victoria plant pathologist, said the makings of the severe outbreak dated back to late last year.
"Particularly in the northern cropping belt we saw a lot of summer rain which in turn led to a green bridge of plants over the summer period carrying that rust inoculum," Dr Hollaway said.
"This meant we entered the 2022 winter cropping season with high levels of inoculum and that in turn, with wet conditions we've seen in the spring, has led to damaging levels of disease," he said.
"We have seen the worst year for stripe rust in most areas since 2011 after the floods of the 2010-11 summer and the resultant green bridge we saw then."
Dr Hollaway said the window for fungicide application was just about closing.
"The critical thing is to protect the crop's flag leaf, once the crop gets to the flowering stage then there is not much benefit in terms of yield from fungicide."
Dr Hollaway said the rust began to mobilise once temperatures rose above 15 degrees.
"From there it has been very challenging with the favourable conditions for rust and the difficulties in getting treatments out."
He said the record wet spring meant the disease was a problem in areas not normally susceptible to rust.
"The rust infestation has been unexpected for some growers, in places like the Mallee there is not normally a need for an in-crop fungicide, or if they do use one, they only need one."
He said those growers who had been able to get on paddocks to apply fungicides in a timely manner were likely to keep yield losse to a minimum but for those who either could not spray or did not notice the disease until too late there could be severe yield penalties.
"Untreated you can see losses as high as 20 to 30 per cent, although with varietal adult plant resistance you'd hope most people would lose less than that."
"The adult plant resistance conferred into modern varieties by plant breeders is very useful, without you could expect far more damage from the disease, but it is not infallible.
"The rule of thumb is the higher the disease pressure then the more you need to up the controls."
The good news for growers was that while they may observe rust in the head at present it was not necessarily a major driver of yield loss.
"You can see some yield loss from rust on the head but it is not likely to be major, the big issues were early uncontrolled infections."
Dr Hollaway said it was important to start thinking of rust strategies for next year.
"Going forward we're virtually certain of having another big green bridge so people need to do their best to reduce the risk, reduce the number of volunteers and weeds present over summer."
"Also things like really looking at disease ratings when making varietal choices that could also prove critical."