Comparison of anti-oomycete activities of essential oils and fungicidal substances against the agent of potato late blight disease Phytophthora infestans

The Oomycete Phytophthora infestans causing late blight disease represents one of the major threats for potato crop. Particular attention is given to specific genotypes known to be either persistent or having recently emerged in the agroecosystems of northern Europe. To fight this disease, farmers in conventional agriculture still heavily rely on the use of synthetic pesticides. As a consequence, some populations appear with reduced sensitivity to commonly employed molecules. Given these environmental concerns, alternative methods of management are encouraged. In this context, plants secondary metabolites (PSM) such as essential oils (EOs) have become extensively studied and seem promising to develop sustainable and efficient biopesticides. This work aims at comparing the anti-oomycete potential of 5 essential oils (bergamot - Citrus bergamia, rosemary - Rosmarinus officinalis, oregano - Origanum vulgare, clove - Syzygium aromoticum and Ceylan cinnamon - Cinnamomum zeylanicum) with 3 fungicidal active substances (including two organic molecules: mandipropamid, fluazinam and one mineral: copper sulfate) on 3 genotypes of P. infestans (EU-13, EU-36 and EU-37 that has been listed as less sensitive to fluazinam). EOs chemical compositions were determined by GC-MS. All products were tested according to an adjusted range of concentrations. The inhibition of the mycelium was evaluated on solid medium by contact for all substances and by fumigation for essential oils only. Then, fungistatic or fungicidal properties were assessed. On the other hand, the median inhibitory concentration (IC50) of each product in liquid medium was calculated in regard of spores’ germination.
Inhibitory tests by contact on solid medium showed the best inhibition of the mycelium with clove and oregano EO. In liquid media, the smallest IC50 are associated with cinnamon which indicates its greatest activity against spore germination of all the three genotypes. Finally, mandipropamid showed by far the highest activity both on solid and in liquid media followed by fluazinam and copper sulfate although significant sensitivities were identified between EU-13, EU-36 and EU-37.
In conclusion, the three isolates of P. infestans globally follow the same trends toward the inhibition of their mycelium caused by the essential oils, both by contact and fumigation. However, radically different levels of activities on spore germination have been quantified through the IC50. Further experiments must be carried out to evaluate not only essential oils phytotoxicity on potato leaves but also their biofungicidal properties of the formulated EOs on late blight disease.