Growers in northern Victoria and southern New South Wales with wheat-on-wheat rotations have a greater chance of mitigating the impact of yellow leaf spot using cultural measures, such as variety selection and stubble management, rather than relying on fungicides to treat the problem.

Research conducted by FAR Australia as part of the GRDC's 'Maintaining profitable farming systems with retained stubble' initiative, in conjunction with Riverine Plains Inc, has shown triazole fungicides provide relatively poor control of yellow leaf spot in a wheat-on-wheat scenario, whereas variety choice and cultural practice can provide up to 90 per cent control of the disease.

FAR Australia research manager Michael Straight says yellow leaf spot has been the principal disease growers in the Riverine Plains region have dealt with following the advent of no-till stubble-retained farming systems.

Trials conducted over four years between 2014 and 2017 using susceptible and moderately susceptible wheat varieties investigated the influence fungicide treatments had against the disease, finding that disease control rarely exceeded 50 per cent.

"Using either Prosaro® (tebuconazole and prothioconazole) or Tilt® (propiconazole) applied at the end of tillering or early stem elongation - between growth stage 23 and 31 - disease control ranged from 25 per cent to 50 per cent," Mr Straight says.

"This level of disease control is poor, relative to traditional control levels observed with fungicides against other diseases, such as stripe rust.

"Despite this, there were small, but consistent, yield benefits across the four years, ranging between 0.17 tonnes per hectare in 2016 to 0.4t/ha in 2015.

"These small yield effects were seen in response to two applications of fungicide and later spray timings during stem elongation, or growth stage 33."

Mr Straight says early infection of yellow leaf spot up to the start of stem elongation - growth stage 30 - was better controlled with stubble management practices, such as burning, rather than with foliar fungicides.

"Fungicides applied at or before early stem elongation (growth stage 31) between 2014 and 2016 gave poor disease control and were rarely economic," he says.

"Across all four years, even though the rotation position and variety have favoured the disease, the yields of the trials ranged from 2 to 4t/ha.

"It was also noticeable that in the large-block stubble management trials, a switch to the more resistant variety Corack (PBR) gave good control of yellow leaf spot, such that differences in yellow leaf spot control as a result of stubble management treatment was not observed.

"This tells us that reducing stubble load, growing resistant varieties and - when necessary - applying fungicides at plant growth stages after stem elongation, can help growers in the Riverine Plains area to reduce the impact of yellow leaf spot."

The trials conducted by FAR Australia in conjunction with Riverine Plains Inc also examined the impacts of row spacing and stubble height on crop development.

Mr Straight says stubble height of the previous crop can have a profound effect on crop development, especially in early growth stages.

"Longer stubble above 30 centimetres in height reduced tiller numbers and dry matter production over four years of research due to shading and in one trial significantly reduced grain yield when compared to short stubble below 15cm high," he says.

"The research also showed that, when planting wheat in mid-April, row spacing in the 22.5 to 37.5cm range had less effect on final grain yield.

"In a no-till system, wider rows allow for better trash flow in high stubble loads at sowing time."

More information: Michael Straight, 0427 142 033, michael.straight@faraustralia.com.au