Anti-oomycete activities of 5 essential oils against 3 genotypes of Phytophthora infestans the causing agent of late blight disease

Abstract

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, resistant to some fungicides or for having recently emerged in the agroecosystems of northern Europe. To fight this disease, farmers still heavily rely on the use of synthetic pesticides in conventional agriculture. Given the actual environmental context, alternative management are encouraged. For this reason, plants secondary metabolites (PSM) such as essential oils (EOs) become widely studied and seem promising to develop innovative biopesticides. This study aims at screening the anti-oomycete potential of 5 essential oils on 3 genotypes of P. infestans (EU-13, EU-36 and EU-37): bergamot (Citrus bergamia), rosemary (Rosmarinus officinalis), oregano (Origanum vulgare), clove (Syzygium aromoticum) and Ceylan cinnamon (Cinnamomum zeylanicum). EOs were characterized by GC-MS and have been tested according to a range of concentration. The inhibition of the mycelium was evaluated on solid medium both by fumigation and by contact. Fungistatic or fungicidal properties were then established. In addition, the median inhibitory concentration (IC50) of each oil in liquid medium was calculated in regard of spores’ germination. Inhibitory tests by contact in solid medium showed total inhibition with clove, oregano and cinnamon EOs from 0,5 µL/mL. An inhibition of up to 70% compared to control (non-treated) was obtained with both clove and oregano EOs. Conversely, higher inhibition is achieved by fumigation with bergamot and rosemary oils (which are monoterpenes-rich EOs with respectively limonene and camphor/α-pinene as major compounds) than with oregano, clove or cinnamon oils (mainly containing respectively carvacrol, eugenol and cinnamaldehyde). Lower vapor pressure prevented those from causing significant inhibition of the mycelium even at 1 µL/mL of air. In addition, bergamot and rosemary EOs have proven themselves fungistatic whereas the other three exhibited fungicidal properties. In liquid media, the smallest IC50 (from 0,08 to 0,013 mg/mL) were associated with cinnamon EO and did not vary significantly among the three genotypes. This indicates the greatest activity against spore germination. Hence, although the three isolates of P. infestans globally respond the same, all 5 EOs demonstrated distinct activities towards either the mycelium or the spores. Further experiments must be carried out to evaluate not only the phytotoxicity on potato plants but also the biofungicidal properties of the formulated EOs on late blight disease.PHYTOIL12. Responsible consumption and productio