Growers are being urged to be proactive with stripe rust management as some in the industry anticipate a 1-in-50-year epidemic this season.
The disease, which costs the Australian grains industry $120 million on average annually, is a huge issue for growers, especially in years like this where inoculum is extremely high across the eastern Australian wheat belt.
In a recent GRDC webinar, NSW DPI senior plant pathologist Dr Steven Simpfendorfer said the high pressure had come from a wet summer fallow favouring 'green bridge' survival and early epidemic development, while in-season, more susceptible varieties had continued to expose neighbouring crops to inoculum.
Dr Simpfendorfer said growers had been highly concerned with infection rates, especially considering the disease had been occurring in most varieties at seedling stages - even for those varieties with adult plant resistant genes.
He said seedling infections were widespread this year, but that didn't mean APR genes had broken down; rather the resistance genes didn't kick in until the plant got older, so growers had to support them until the genetics started taking care of managing the disease.
"Even in resistant varieties, with such an early epidemic, growers should consider applying fungicide to support young crops and ease disease pressure until the APR genes take over management," Dr Simpfendorfer said.
"They should not wait for APR to kick in; be proactive and manage the pressure within a season."
The plant pathologist said there had been numerous reports of infections occurring one to two weeks after spraying, which wasn't a case of the fungicide not working; more that the fungicide was applied outside of its curative activity phase.
If a fungicide is applied more than five days after infection, necrosis and pustule formation can still occur after application, he says.
Fungicide findings
Similarly, FAR Australia managing director Nick Poole said while it was very convenient to add a fungicide to susceptible crops at tillering when dealing with in-crop weeds, it was less than ideal for disease management.
According to Mr Poole, a tillering application followed by a flag emergence spray left too big of a gap for the disease to cycle and reinfect plants.
"In a season with such high pressure, growers need to question whether they're leaving the crop unprotected for too long and consider if additional fungicide sprays are required," he said.
"It's timing fungicides to keep the foliage of the flag leaf and the next two highest leaves clean that will give you the optimum yield."
Mr Poole said keeping the 'money leaves' clean was difficult using foliar fungicide if disease had been actively developing in the canopy all season, but could be achieved with a timely proactive management plan where the gap between fungicide timings during stem elongation was no more than 3-4 weeks.
Looking at the back end of the season
In the lead up to grain filling, Dr Simpfendorfer said it was crucial growers were being proactive with managing susceptible and moderately susceptible varieties and were working 2-3 weeks back from flowering to limit active spores in wheat canopies during this period.
"What growers should be trying to do is limit head infections by keeping pustules out of the crop's canopy as they're coming into flowering," he said.
"The more spores in the crop, the more head infection may occur, so this is about ensuring growers are being proactive and treating the risk two to three weeks prior to flowering."
Dr Simpfendorfer said the impact of head infection on yield was dependent on the conditions during grain fill, but more importantly, fungicide applications at this time were ineffective and could risk breaching withholding periods before harvest.
With the Bureau of Meteorology recently confirming Australia will experience the third La Nia in a row this summer, the risk of stripe rust is expected to be high again next season.
Mr Poole said while it was important growers managed their varieties with fungicide properly using the correct timing, the most significant thing that was going to take pressure off the system was selecting varieties with a higher level of resistance.
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