Increased stripe rust pressure

Growers have been urged to focus on being proactive with management, especially prior to flowering, to preserve yield and avoid risking withholding periods at harvest from delayed foliar fungicide applications.

The Grains Research and Development Corporation (GRDC) conducted an online webinar on wheat stripe rust earlier this month to update growers on pathotype frequency and how to manage infections at this stage of the season.

More than 350 growers and advisers tuned into the webinar, which featured updates from the University of Sydney’s Robert Park, FAR Australia’s Nick Poole and NSW Department of Primary Industries’ (NSW DPI) Steven Simpfendorfer.

Dr Simpfendorfer said growers had been highly concerned with infection rates, especially considering the disease was occurring in most varieties at seedling stages, even for those varieties with Adult Plant Resistant (APR) genes.

“This is a social disease, and the high pressure has come from a wet summer fallow favouring ‘green bridge’ survival and early epidemic development whilst in season from more susceptible varieties have continued to expose neighbouring crops to inoculum,” he said.

Seedling infections were widespread this year, with growers advised to support crops until the genetics start 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.”

Growers with susceptible varieties are under immense pressure to keep infections at bay, especially because the number of active spores is making it extremely difficult to time fungicide applications properly.

There have been numerous reports of infections occurring one to two weeks after spraying.

Dr Simpfendorfer said this wasn’t a case of the fungicide not working, but 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.”

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 is less than ideal for disease management.

“A tillering application followed by a flag emergence spray leaves too big of a gap for the disease to cycle and reinfect plants,” Mr Poole said.

“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.

“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 is difficult using foliar fungicide if disease has been actively developing in the canopy all season but can be achieved with a timely proactive management plan where the gap between fungicide timings during stem elongation is no more than three to four weeks.

In the lead up to grain filling, Dr Simpfendorfer said it is crucial growers are proactive with managing susceptible and moderately susceptible varieties and working two to three 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.”

The impact of head infection on yield is dependent on the conditions during grain fill, but more importantly, fungicide applications at this time are ineffective and can risk breaching withholding periods before harvest.

With the Bureau of Meteorology recently confirming Australia will experience the third La Niña in a row this summer, the risk of stripe rust is expected to be high again next season.

The industry anticipates this season will be a one-in-50-year epidemic for stripe rust.

Stripe rust costs the Australian grains industry $120 million on average annually.