Optimisation of the Postharvest Treatment with Thymol to Control Mango Anthracnose

Anthracnose, caused by the fungus called Colletotrichum gloeosporioides, is the main postharvest disease that affects mango production on Reunion Island. Fruits for the export market are always treated with chemicals. The use of chemical treatment is not in adequation with consumer expectations, and the increasing emergence of fungicide-resistant isolates promotes the development of alternatives methods. The principal objective of this work was to use antimicrobial properties of thymol as an alternative postharvest treatment on mango. Thymol diluated in a penetrating agent solution was effective on mango anthracnose. At a concentration of 0.025%, Thymol limited necrosis development due to pathogens during fruit storage. This treatment can stimulate some of polyphenols biosynthesis involved in the fruit resistance to postharvest disease, particularly thesynthesis of gallic acid and resorcinol. With this final concentration of 0.025% thymol, the treatment did not affect fruit maturation and quality, especially the peel colour and sugar content. Importantly, the treatment did not show any detectable effect on organoleptic qualities of the fruit.

Potential Use of Essentials Oils to Control Fruitlet Core Rot (FCR) in Pineapple (Queen Victoria Variety) in Reunion Island

Fruitlet Core Rot (FCR) is one of the major postharvest diseases of pineapple (Ananas comosus var. comosus), especially on the prone variety Queen Victoria cultivated in Reunion Island. This aggressive disease is generally due to two pathogens: Fusarium ananatum and Thalaromyces stolii, and needs to be controlled during postharvest. In Reunion Island, F. ananatum is the principal causal agent impacting fruit exportation. Fruit produced for the export market is generally treated with chemicals. This type of postharvest treatment is not in line with consumer expectations, as consumers prefer fruits treated with nonharmful and natural products. The objective of this work was to study alternative postharvest treatments using the fungitoxic properties of essential oils and their ability to elicit the resistance mechanisms of the fruit. Six EAs were tested in vitro on mycelial growth of F. ananatum. Whether by volatilization or included in the culture medium, some essential oils such as thymol, eugenol, geraniol and the carvone/menthol mixture have a strong fungistatic effect. But only thymol has a fungitoxic effect. The thymol, a natural molecule synthesized by thyme (Thymus vulgaris L. thymoliferum) is the more effective in vitro and is the strongest potential to be used in postharvest treatment. Thymol, prepared at 0.025% in a terpene solvent that acts as a penetrating agent, was tested in vivo with inoculated fruits. The treatment was effective only on necrosis development from the upper part of the fruits. Pineapple polyphenol biosynthesis appears to have been suppressed by thymol treatment. Results and opportunities for this treatment are discussed. Additional experiments must be carried out in order to decide on the advisability of this type of treatment.

New Bacterial Agents to Limit <i>Colletotrichum gloeosporioides</i>Development on Mango

Mango anthracnose disease forms typical irregular-shaped black necrotic spots on the fruit peel of mature fruit and is caused by Colletotrichum gloeosporioides. In order to improve the disease control with a limited use of fungicides, new microbial agents able to limit the growth of the pathogen were searched in the indigenous natural flora of mango surface. In order to find a suitable biocontrol agent, a screening was applied to 305 epiphytic bacteria isolated from the carposphere of 17 mango cultivars sampled from eight locations on Reunion Island. The screening approach involved a first step based on the ability of the isolates to form a biofilm, to grow under fruit storage conditions, and to interfere with the development of C. gloeosporioides. In a second step, the capability of selected isolates to limit C. gloeosporioides in vitro mycelial growth and conidia germination was assessed and species identified. The most effective bacteria belonged to the Enterobacter, Pantoea, Kosakonia and Leuconostoc genera, but for some of them, their safe use has to be demonstrated. Efficacy in vivo, performed on wounded mature mango fruit, was limited, probably because of the wounding inoculation strategy favoring the pathogen. Future biocontrol treatments should focus on preharvest applications to enhance the protective benefit.

Biological Activity of <i>Syzygium aromaticum</i>and <i>Ravensara aromatica</i>Essential Oils from Madagascar and Their Possible Use against Postharvest Mango Anthracnose

The fungitoxicity of five Malagasy essential oils (Eos) against Colletotrichum asianum was assessed in terms of conidial germination and mycelial growth. Their effect on defense-related compounds content, physicochemical properties and anthracnose lesions in mango fruits was also determined. Four of the tested Eos were from Ravensara aromatica leaves, and the last Eo was extracted from clove leaves. Their chemical compositions were then determined through GC-MS analysis and the active compound of the most fungitoxic Eo was determined by testing the toxicity of its major component to C. asianums spore germination, mycelial growth and its ability to inhibit anthracnose development on mango fruits. The R. aromatica Eos tested were fungistatic to C. asianum, whereas clove Eo was fungitoxic and the 4 chemotypes of R. aromatica Eo exhibited variable inhibiting capabilities: 1) all tested doses of all Eos (112.5 and 225 μL/L of air) were effective against C. asianum mycelial growth (10% - 100% inhibition) but doses of 225 μL/L were more inhibitory than those of 112.5 μL/L, 2) Conidial germination was more resistant to Eos toxicity since only 225 μL/L of methyl eugenol chemotype of R. aromatica Eo, all tested doses of the sabinene chemotype of R. aromatica Eo and clove Eo were found inhibitory toward conidial germination of C. asianum. 30 μL/L of sprayed clove Eoweretested on inoculated mangoes and were found to be effective against anthracnose development without affecting the resorcinol content in mango peel and the physicochemical properties of mango pulp. Tests on the major components of clove Eo showed fungitoxic activities against mycelial growth and conidial germination of C. asianum similar to those of clove Eo.