Essential oils as alternative for sustainable management of Phytophthora infestans causing potato late blight disease : From in vitro to in vivo experiments | Nouvelle méthode de gestion du mildiou de la pomme de terre par les huiles essentielles : Chemin expérimental du in vitro au in planta

Introduction: Phytosanitary products are increasingly restricted due to environmental and health concerns, yet farmers still require solutions to combat phytopathogens. Among biocontrol strategies promoted for integrated pest management, the use of natural substances, such as essential oils, holds significant promises. They consist of complex mixtures of hydrophobic secondary metabolites with diverse biological activities, including antifungal properties. Late blight, caused by the oomycete Phytophthora infestans, remains a major threat to potato crop, with emerging strains showing increased virulence and resistance to conventional pesticides. Objectives: This study aimed to develop an effective biofungicide against potato late blight as well as investigate the molecular mechanisms by which essential oils disrupt pathogen metabolism. Methodology: Six essential oils—Citrus bergamia, Rosmarinus officinalis, Origanum vulgare, Syzygium aromaticum, and Cinnamomum zeylanicum—were selected and their chemical composition was analysed by GC-MS. The anti-oomycete potential was first assessed in vitro by evaluating mycelial growth (both by contact and fumigation on Petri dishes) and spores’ germination (in microplates) against three P. infestans genotypes (EU-13-A2, EU-36-A2, and EU-37-A2). Ex vivo assays were then conducted on detached potato leaves to assess the pathogenicity of these strains on three potato varieties (Bintje, Fontane, and Carolus) with varying sensitivity to late blight. Eventually, phytotoxicity threshold of microemulsions was also evaluated through chlorophyll fluorescence. Results and perspectives: In vitro results indicated that oils rich in phenylpropanoids (i.e oregano, clove, and cinnamon) were more effective by totally inhibiting pathogen growth by contact (from 1µL/mL), while terpenoid-rich oils (bergamot, rosemary) were more effective through fumigation due to their higher volatility. Sensitive varieties (Fontane and Bintje) showed severe late blight symptoms, while resistant one (Carolus) did not. No significant difference in chlorophyll content was observed on potato leaves under 2,5µL/mL, but chlorosis appeared from higher concentrations, indicating phytotoxicity thresholds. These findings highlight promising essential oils for biofungicide development and set the stage for evaluating their in vivo effects on potato late blight pathosystem.