Cinnamomun zeylanicum oil and trans-cinnamaldehyde against Alternaria brown spot in tangerine: direct effects and induced resistance

Abstract

Cinnamon essential oil (CEO) has proven activity against several plant-pathogenic fungi and can reduce fungal diseases to an extent comparable to that achieved using commercial fungicides. Thus, the goal of the present work was to identify the active constituents of this essential oil and determine the direct fungicidal mode of action as well as putative indirect action by activation of citrus plant resistance against Alternaria brown spot (ABS). Gas chromatography–mass spectrometry (GC-MS) of essential oil extracted from Cinnamomum zeylanicum leaves resulted in the identification of the active constituents eugenol and trans-cinnamaldehyde, which had, respectively, minimal inhibitory concentrations (MICs) of 250 and 62.5 μg mL−1 against Alternaria alternata, while, under the same conditions, the MICs for a commercial fungicide and CEO were 1250 and 500 μg mL−1 respectively. Fungal growth inhibition assays showed that eugenol and trans-cinnamaldehyde (CNE) prevent fungal growth by direct fungicidal action and proved that CNE is mainly responsible for the anti-fungal activity of C. zeylanicum essential oil (CEO). CEO decreased the incidence of ABS disease to a greater extent than a commercial copper fungicide and was comparable to a commercial plant activator. Both CEO and its active constituent showed the ability to activate plant defense enzymes, demonstrating a high capacity to induce the citrus plant defense response. Therefore, both the essential oil and its active constituent have potential use in the development of new organic structures and analogues to control ABS disease caused by A. alternata.