Protecting Australian grain crops against fungal disease requires the development of safe and sustainable pesticides suited to modern practices and markets. Our aim is to develop non-toxic RNA-based biopesticides for the control of rust pathogens of major cereal crops through RNA interference (RNAi). We have shown that exogenous double-stranded RNA (dsRNA) sprays can effectively prevent and cure disease caused by myrtle rust that threatens many of Australia’s iconic and indigenous Myrtaceae species, including genera such as Eucalyptus, Psidium, and Syzygium (Degnan et al., 2022, 2023). Building on this work, we developed a detached leaf assay featuring a nylon membrane reservoir to assess wheat rust disease control at the seedling stage. This semi-automated assay ensures continual contact of the RNA treatment with the pathogen during germination and into early infection while minimising variation in the experimental conditions. With this assay, we have shown that Puccinia triticina-specific dsRNA at high concentrations (1 µg/µL) can provide a significant but inconsistent reduction in leaf rust disease coverage at seven days post-inoculation. Importantly, we observed no evidence of non-specific effects upon disease progression with the GFP non-specific control. We are now using this assay to test dsRNA and siRNA as control agents of stem rust.
In experiments to test exogenous dsRNA sprays to control leaf rust on mature wheat, we found that moderate doses of dsRNA (25 µg per plant) provided no visible control of rust disease when applied pre- and post-infection. In similar experiments at high doses of dsRNA (250 µg per plant) we observed a ~50% reduction in disease coverage, but also saw a significant reduction in the non-specific (GFP dsRNA) control group. These findings highlight some of the limitations when scaling to spray-delivered treatments, and the necessity for more efficient treatment delivery. Accordingly, we have also investigated the uptake and processing of exogenous dsRNA in wheat rust spores. Our findings constitute important groundwork for the development of RNA biopesticides against agronomically critical pathogens.
Degnan R et al. (2022) Mol. Plant Path. 24:191; Degnan R et al. (2023) Comm. Biol. 6:1234.