Summary

Two SARDI researchers helped develop research methods and conducted trials to evaluate treatments for controlling Rhizoctonia solani AG8 in South Australian soils.

Trial locations were selected for sand to sandy loam soils with a high likelihood of evapotranspiration stress between head emergence and early grain fill. Plots were inoculated with Rhizoctonia at sowing to encourage primary and crown root infection.

Although infection was successfully induced at nine of the ten trial sites, yield impacts were only evident at two, indicating high yield potential and heat stress in early spring are also important factors in yield loss.

Background

Rhizoctonia root rot, caused by Rhizoctonia solani AG8, was historically associated with ‘bare patch’ symptoms in paddocks. This effect is now believed to be partly the result of delayed sowing into cold soils. Modern farming practices like early sowing and better nutrition at sowing mean infection more commonly manifests as crown root rot in early sown crops, although it can also infect primary roots. The resulting root damage can lead to significant yield loss in severe cases, even though ‘bare patch’ symptoms may not be obvious. To evaluate the efficacy and economic benefit of different management practices and treatments, researchers need to be able to reliably produce infection in field trials under conditions where yield loss is likely to occur.

In past field trials, uneven distribution of inoculum has made evaluation of treatments difficult. Inoculating trial soils the season before research was found to result in patchy infection that sometimes spread to control plots.

Research Aims

The core objectives of the project were to:

• Develop a research method for Rhizoctonia solani AG8 to reliably simulate crown root infection in randomised, replicated trials.
• Provide a young SA researcher with experience in methods to conduct future industry-leading research.

In The Field

Tara Garrard started research in 2017. In 2019, when she was appointed to a new role within SARDI, her place on the project was taken by Blake Gontar. Both researchers contributed to the recommendations for producing crown root infection in trials.

One and two-year trial sites were established at Loxton, Palmer, Pinery, Owen and Coomandook between 2017 and 2019, with a mix of wheat and barley sown.

The sites were inoculated with Rhizoctonia-infected sterile millet grain. Inoculum was placed in-furrow with the seed at a rate of 144g per plot (1g inoculum/m crop row), or on the furrow immediately after sowing at rates of 72g (0.5g/m row), 144g (1g/m row) and 288g per plot (2g/m row).

Treatment effects were assessed by scoring root disease, measuring NDVI and measuring yield.

The site at Pinery (wheat 2017, barley 2018) was Rhizoctonia suppressive and disease did not develop. Sites at Loxton (wheat, 2018) and Pinery (barley 2018 and 2019) developed severe root disease but were drought affected. These sites were not scored.

Results

Applying artificial inoculum near the soil surface at sowing reliably produced uniform rates of crown root infection. Inoculum applied with seed produced uniform primary root infection and often significant crown root infection.

Inoculum applied with seed generally produced higher seminal infection than surface-applied inoculum in the year of application. Surface applied inoculum generally produced higher rates of infection on crown roots than seminal roots. This effect was generally reduced in the second year.

There was little difference in disease scores between different rates of inoculum and all application rates produced more infection than the control plots.

Two trials showed significant yield losses, Owen in 2017 (0.63t/ha) and Palmer in 2017 (0.4t/ha). Both had good yield potential, with growing season rainfall (GSR) of 276mm at Owen and 180mm at Palmer, and were subject to heat stress in early spring.

In 2018, the Owen site again had good yield potential (+177mm GSR) but the onset of hot weather occurred ten days later than in 2017, so the early maturing barley escaped evapotranspiration stress at a critical time.

The results suggest environment and growing conditions influence yield loss from Rhizoctonia, not just the rate of infection or root disease. Irrigating trial plots may produce more reliable yield results for future Rhizoctonia trials.