RNAi spray for sclerotinia needs better formulation

Ribonucleic acid (RNA) interference can reduce a targeted gene’s expression through the application of double-stranded RNA (dsRNA), a new sector of spray treatments. A major benefit is that dsRNAs can target specific pests species, and not affect beneficial or non-target species. For this study, researchers identified dsRNA molecules that can inhibit sclerotinia growth, and tested early formulations with one site-year of field trials.

This study completed four objectives:

• Identify and nominate of sclerotinia bioactive dsRNA molecules.
• Synthesize dsRNAs and screen for fungicidal activity against sclerotinia and non-target effects.
• Develop and test topical formulations for dsRNA adhesion to leaves and durability under different environmental conditions. These were randomized complete block design field trials.
• Assess persistence of dsRNAs in the soil.

For the field trials, researchers seeded and fertilized a commercial canola cultivar on June 9, 2022 at a facility near Minto, Manitoba. They applied two test applications of dsRNA – July 15 at the early flowering and July 22 at late flowering. They also oversprayed the trial with disease inoculum on July 16, 21 and 25 and misted the trial with water regularly to promote disease development. Disease levels were high throughout the trial, with sclerotinia being the main disease present.

Results

Of the 100 dsRNA molecules tested on leaves at a dose of 200 nanograms, 63 showed a reduction in lesion size when applied with the adjuvant Silwet-77. The best target, Sclero-1703, showed an 85 per cent reduction in lesion size.

Formulation testing showed that spreader molecules that include Stilwet are compatible with top performing dsRNAs. Penetrant oil based molecules are not compatible, and nanoparticles are compatible but do not result in improved RNAi-mediated transcript knockdown or improved protection against fungal lesion size.

Field trial results did not show strong performance of dsRNA applications. The night following the first dsRNA application, the field sustained heavy rains, lasting for several days, which most likely led to loss of the dsRNA from the plants, as no sticking agent or penetrant was provided in the formulation. The disease severity and incidence on canola plants treated with either of the negative control treatments (Silwet or GS1+Silwet) showed no significant difference to the treatment with the sclerotinia-specific dsRNA (1703+Silwet), regardless of whether the plants were treated once or twice.

Overall, the treatment with Sclero-1703 applied at the first spraying showed the lowest incidence and severity of disease. However, the differences between treatments were slight. There were no significant differences in yield between treatments.

Consequently, researchers observed no significant impacts of the dsRNA on disease severity, incidence or seed yield. The only noteworthy and possibly favourable result was that the highest seed yield, albeit only marginally higher than the controls, was seen in plants that had two Sclero-dsRNA sprayings. For future field trials, adjuvants that improve adhesion onto or penetration into the leaves will be needed.

The persistence test, done in the lab on five Manitoba soils, showed that dsRNAs were not detectable after six hours mixed in the soil. Further tests are planned to evaluate if the dsRNA is detectable at earlier times, to determine the half-life of the dsRNA with the different soil types.