Abstract :
[en] Venturia inaequalis, the causal agent of apple scab disease, is known to be one of the most economic threats to apple production without proper management (Caffier et al. , 2022 ; Patocchi et al. , 2020). Ten to fifteen chemical treatments are usually required to prevent severe infections and this number can even double during years favorable for apple scab development (Patocchi et al. , 2020). Developing innovative phytopharmaceutical products based on natural active substances is stimulated by the issue of pesticides resistance of the pathogen but also their potential negative impacts on ecosystems and on human health. Essential oils (EOs) count many interesting molecules that can take up this challenge. Initially, an extensive collection of EOs was screened thanks to bibliographic studies and laboratory analyses. A selection of nine EOs was tested on infectious apple scab conidia in microplates experiments. To evaluate and choose EOs with the greatest fungicidal potential, the concentrations that inhibited pathogen growth by 50% (IC50) were determined. The chosen methodology demonstrates the fungistatic effect of the studied EOs on Venturia inaequalis. To further refine this selection, another in vitro methodology was developed on the pathogen. It consists of an evaluation of the death rate of conidia exposed to EOs. Lethal doses of EOs causing 50% of mortality (LD50) was calculated with fluorescein diacetate coloring. Numerical analyses of epifluorescence microscopy images allowed to calculate an estimation of the LD50 of three EOs by non-linear regressions. IC50 and LD50 are good indicators to evaluate the EOs potential to be used as an active substance against V. inaequalis. Based on this in vitro screening, preventive and curative fungicidal properties of three EOs experimental formulations were assessed directly on apple plants, in a controlled growth chamber. An imagery analysis of scab symptoms was implemented to precisely study EOs efficiency. Such an unbiased method should allow rigorously studying the effects of co-formulants within the framework of future product development. Bibliography: Caffier, V., Shiller, J., Bellanger, M.-N., Collemare, J., Expert, P., Gladieux, P. P., Pascouau, C., Sannier, M., & le Cam, B. (2022). Hybridizations between formae speciales of Venturia inaequalis pave the way for a new biocontrol strategy to manage fungal plant pathogens. Phytopathology, 2022(7), 1401–1405. https://doi.org/https://doi.org/10.1094/PHYTO-05-21-0222-SC Patocchi, A., Wehrli, A., Dubuis, P. H., Auwerkerken, A., Leida, C., Cipriani, G., Passey, T., Staples, M., Didelot, F., Philion, V., Peil, A., Laszakovits, H., Rühmer, T., Boeck, K., Baniulis, D., Strasser, K., Vávra, R., Guerra, W., Masny, S., … Bus, V. G. M. (2020). Ten Years of VINQUEST: First Insight for Breeding New Apple Cultivars with Durable Apple Scab Resistance. Plant Disease, 104(8), 2074–2081. https://doi.org/10.1094/PDIS-11-19-2473-S
