Mango fruits are commercialized worldwide for their sensorial and nutritive qualities, antioxidant and dietary properties [1] [2]. Top producers are Asian countries such as India, China, Thailand, Indonesia and Pakistan. These countries produced 15,188,000 to 1,888,449 MT of mango in 2014 (FAOSTAT Database), compared to Reunion Island that produces around 3500T of mango a year, both for exportation and local market use [3]. At preharvest stage, mango fruits suffer attack from fruit fly (Bactrocera dorsalis, Tephritidae), bacterial (Xanthomonas, Xanthomonadae; Ralstonia, Rasltoniaceae) and fungal pathogens (Penicillium, Trichocomaceae; Alternaria, Pleosporaceae; Fusarium, Nectriaceae and Colletotrichum, Glomerellaceae) that induce visual damages like rots and lesions at postharvest stage and cause tremendous loss during storage [4] [5]. One of the biggest challenges for the mango market is to protect stored fruit against anthracnose development. Postharvest anthracnose is a disease caused by phytopathogenic fungi included in the Colletotrichum genera that reduces mango fruit marketability, storability and nutritional value [6]. Current treatments combine one to multiple methods to overcome postharvest loss caused by phytopathogenic fungi: copper sprays can be applied at the preharvest stage, hot water treatment that is for use before storage; carbendazim treatment is used during storage, whereas prochloraz and benomyl are used as postharvest treatments [7]. Most of these treatments are of a chemical nature. There is thus a need for an eco-friendly and biological alternative product that can be used during mango storage to prevent anthracnose incidence and postharvest loss.

Essential oils (Eos) are natural products known through ancient time to have protective abilities against food spoilage and therefore were incorporated in stored food for their antibacterial [8] and antifungal [9] activities against a broad range of animal, human and plant pathogens, thus extending their shelf life of fruits without any negative effects on their physiochemical and sensorial qualities [10]. Eos such as cinnamon Eo [11], thyme Eo [12] [13] [14] and clove oil [15] are among the most investigated and successful against fruit postharvest pathogens. Their effects against Colletotrichum genera are generally focused on their in vitro ability to inhibit fungal growth and spore germination. Clearly, their effectiveness against the anthracnose disease pathogen and its usability as a mango preservative would benefit from further investigation.

Many papers link the ability of Eos to induce fruit resistance against Colletotrichum infection to their capacity to elicit an effect on defense-related compounds such as resorcinol, chitinase enzyme synthesis in tropical fruits [16] [17]. In particular, researches on mangoes linked fruit maturity, resistance to phytopathogenic fungi to a decrease in resorcinol synthesis in mango peel [17] [18]. Such findings support the hypothesis that resorcinol compounds are involved in mango resistance to phytopathogenic fungi-caused disease.

Malagasy essential oils have recently been reported to have antifungal properties, present paper focused on two of them: clove and ravensara Eos. Clove trees were introduced into Madagascar just before the colonial period. These Eo-producing trees are actually well domesticated and play a predominant role in the economy of eastern Madagascar [19], whereas Ravensara aromatica Sonnerat (also known as Cryptocarya agathophylla Van der Werff) is an endemic tree located in the central and eastern parts of the island [20]. Clove Eo ability to inhibit phytopathogenic fungal germination and growth is already established worldwide [21]. In addition to its antifungal properties, clove Eo was also found to have antioxidant [22] and protective abilities against oxidative/nitrosative stress [23]. Similarly, clove Eo and Ravensara Eo from Madagascar are both reported to have a growth inhibition effect on human and plant pathogens such as Aspergillus niger and Saccharomyces cerevisiae [24], in addition to their phytotoxic [25] and antioxydant properties [26], and studies on the chemical composition and physical properties of both Eos [20] [27] [28] reported comparable results with previous investigations on the subject [29] [30].

This study is therefore based on the hypothesis that native Malagasy Eo also has antifungal abilities that can be used to prevent anthracnose development on stored mangoes from Reunion Island. The high variability and volatility of Ravensara Eo [31], on which its anti-germinative intensity depends [25], and the microvariability and high density of clove Eo [32] are important characteristics on which investigations were focused. Experiments were then conducted on the biological activities of these Eos against Colletotrichum and their effect on mango fruit anthracnose development and resorcinol production in view of their eventual use as a potential mango preservative during postharvest storage.

The essential Eo compositions established on the basis of GC-MS are in accordance with previous reports on Malagasy clove Eoand Ravensara aromatica Eo compositions [27] [28], as well as with reports on other clove Eo compositions [29] [30]. The clove oil was mostly constituted of Eugenol and Caryophyllene and Ravensara oils exhibited similar components but varying amounts of each component. Each R. aromatica oils collected had one major component. Therefore, the collected Eos belong to 4 chemotypes.

Similar to its phytotoxic effects [25] and to the antibacterial effect of Pimenta racemosa var. racemosa leaf Eo against tomato wilt [34], the antifungal properties of Ravensara aromatica Eo also vary with the Eo chemotypes. Therefore, the antifungal activity of different doses of major components should be compared in order to confirm such a hypothesis. Since most antifungal activity reported is dose-dependent [35] [36] [37], and minor components of Eos are also known to have strong toxic abilities [9], this Eo chemotype-dependence of Ravensara aromatic Eo cannot solely be attributed to major Eo components, especially in view of Prakash’s findings on the negative effects of minor compounds on eugenol

toxicity [38]. Difference between clove oil in vitro toxicity and ME-typed ravensare Eo corroborated the previous findings on the decreasing effect of methylation of eugenol on its biological activities [39].

Many authors found Eo treatment to be significantly effective against phytopathogenic fungi-caused diseases [40] without altering the physico-chemical properties of mangoes [41]. Moderate preventive effects of sprayed clove oil were reported by Santamarina et al. on stored rice grain [42] while complete control of Aspergillus flavus, Penicillium cinitricum caused disease on oranges and jujube fruits were reported by Xing et al. [43]. Bill et al. [33] also reported strong curative effects of thymol oil fumigated on artificially inoculated avocadoes and demonstrated that such an effect can be partially attributed to the oil’s ability to elicit resistance compounds release such as chitinase, glucanase and total phenolic compounds without altering fruit marketability. Our investigation revealed similar inhibitory effects concerning the ability of clove oil to prevent anthracnose on mango fruit in storage conditions, but no ability to induce the synthesis of resorcinol compounds was found in mango peel although our results corroborate previews statement on resorcinol content’s decrease with ripening process [44]. Such findings are in agreement with the in vitro effects of clove oil treatment where mycelial growth and conidial germination were totally inhibited with 20 µL of clove oil. The present results suggest that inhibition of anthracnose development in ripening mangoes was mainly due to the toxic effect of clove Eo on C. asianum growth since more research is needed to prove that clove Eo has no effect on the internal resistance of mango to anthracnose. Some researchers directly applied the Eo on the fruit using the pulverization method or by incorporating the Eo into an edible coating such as aloes vera gel and chitosan [45]. Bautista-Banos et al. [46] and Bill et al. [33] reported that these techniques led to a greater reduction in lesion area than current commercial fungicides on fruit anthracnose.

Some works found in the literature also report that essential oil treatments have strong antifungal activity in vitro but weak in vivo and thus do not induce significant inhibition on disease severity or defense-related enzyme activity. Shao et al. [47] reported similar findings when using clove oil on citrus green mold. Itako et al. also found that cymbopogon oil strongly inhibited spore germination in vitro, whereas sporulation and appressorium formation was not significantly reduced on sprayed leaves [48].

The present work attributes the toxicity of clove oil against mango anthracnose and its pathogen development to its major active compound, eugenol. Most research on the identification of the active compounds of a product is in agreement with such findings [42] [49] [50] [51] even if rare synergistic effects between components of the Eo have been reported. On the contrary, Prakash et al. [38] found antagonistic activities between eugenol and the remaining compounds of Piper betle L. essential oil to combat moisture in some edible commodities. They reported that eugenol showed better antifungal activity alone than when it was incorporated into Piper betle L. essential oil.

The authors are grateful to UMR QUALISUD of CIRAD REUNION, DP Forêts et Biodiversité (a joint program between the University of Antananarivo, CIRAD and FOFIFA), the French Embassy, the PARRUR Project and the AFS4FOOD Project for providing the necessary funds and facilities to conduct this research.

The authors declare no conflicts of interest regarding the publication of this paper.

Andrianjafinandrasana, S.N., Chillet, M., Ramonta, I.R., Tsy, J.M.L.P, Minier, J. and Danthu, P. (2020) Biological Activity of Syzygium aromaticum and Ravensara aromatica Essential Oils from Madagascar and Their Possible Use against Postharvest Mango Anthracnose. American Journal of Plant Sciences, 11, 1682-1697. https://doi.org/10.4236/ajps.2020.1111120