Exploring the antimicrobial potential of plant essential oils as preventive or curative agents against pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa), the causal agent of kiwifruit bacterial canker, has become the main threat for yellow and green fleshed kiwifruits (Actinidia chinensis) worldwide. As current control strategies mainly rely on the application of copper-based compounds, which show limited success in later stages of the disease and pose environmental hazards, plant essential oils (PEOs) have been tested as an environmentally friendly alterative. However, the potential of PEOs to inhibit Psa has been mostly evaluated in in vitro conditions and their chemical composition is rarely provided. Therefore, additional studies are needed to better evaluate their potential in mitigating Psa infection. In this work, six plant essential oils (from anise - Pimpinella anisum, basil - Ocimum basilicum, cardamom - Elettaria cardamomum, cumin - Cuminum cyminum, fennel - Foeniculum vulgare, and laurel - Laurus nobilis) were: (i) chemically characterized through gas chromatography–mass spectrometry (GC-MS) and, (ii) evaluated as preventive and curative agents against Psa in planta (using micropropagated A. chinensis var. deliciosa ‘Hayward’ plants). GC-MS analysis revealed that for anise and laurel the principal component was anethole (87.7% and 77.4%, respectively), for basil it was estragole (93.0%), for cardamom, β-himachalene (75.9%), for cumin, cuminaldehyde (41.9%), and for fennel, eucalyptol (68.6%). Application of PEOs as a preventive treatment (i.e., elicitation 14 days before Psa inoculation), significantly decreased Psa endophytic population 14 days post inoculation in four of the studied PEO (by 70% in basil, 54% in anise, 42% in laurel and 35% in cumin), when compared to inoculated non-elicited plants. Application of PEOs as a curative method, 14 days post inoculation, significantly decreased Psa endophytic population by 93% (laurel), 85% (cardamom), 46% (fennel), and 27% (cumin) when compared to inoculated non-treated controls, 14 days after treatment. These results provide strong evidence on the potential use of PEOs in the formulation of more environmentally friendly products for Psa control.