Rhizoctonia root rot caused by Rhizoctonia solani AG8 remains a significant challenge for grain producers in the low and medium rainfall zones of South Australia and Western Australia (WA). Updates to the original $77 million impact assessment by Murray and Brennan (2009) suggest that the economic impact of Rhizoctonia in Australian cereals now stands at approximately $137 million annually, assuming constant disease incidence and severity while accounting for documented inflation in crop values and expanded production volumes. While all broadacre crops are vulnerable, inoculum increases most rapidly in cereal crops. To manage the disease, growers currently rotate crops with non-cereal varieties, apply in-furrow fungicides, and use mechanical tillage, all of which can help improve grain yields.
In 2023 and 2024, we initiated three field trials with grower group across WA to improve the management of the pathogen and boost grower confidence in adopting these strategies. One trial, established in 2023 at Narembeen with a medium level of R. solani inoculum present in the 2022 site selection sample and patches in the lupin crop, involved 2.5 hectares of replicated strips with pre-sowing tillage using a ‘Grizzly™’ offset disc plough to a depth of 20 cm. Two other trials in 2024 at Merredin and Wongan Hills used replicated 10 m plots of two wheat varieties with different coleoptile lengths (Mace and Mace-18), sown at two different depths, 3 and 10 cm, with and without R. solani inoculum incorporated at 0-3 cm depth. Soil inoculum DNA levels were measured in each treated and untreated strip prior to seeding and after harvest to a depth of 30 cm using 10 cm incremental sampling at the Narembeen trial. Plant emergence and root disease assessments at 10-12 weeks after sowing were completed on all 3 trials. Grain yield and quality data was collected from all trials.
At the Narembeen trial, mechanical tillage resulted in a 4-fold reduction in R. solani inoculum at 0-10 cm depth and improved yields by 0.4 t/ha. In the sowing depth and wheat variety trials, planting to a depth of 10cm significantly reduced seminal root infection compared to sowing at 3 cm. In one of the trials, seeding at 10 cm depth translated into a 0.6 t/ha yield increase for Mace-18. An economic cost analysis for each trial will be developed and shared with growers and grower groups to further encourage adoption of these practices.
The findings from this study, along with the broader project research, will provide growers with the confidence to adopt best management practices to mitigate the impact of R. solani AG8 on grain yield, and will also support the development of a decision-support tool for Rhizoctonia management.