Inhibitory effect of essential oils against Colletotrichum gloeosporioides and Rhizopus stolonifer in stored papaya fruit and their possible application in coatings

The aim of this study was to evaluate the fungicidal effect of the thyme and Mexican lime essential oils studies against Colletotrichum gloeosporioides and Rhizopus stolonifer, and to determine the possibility of incorporating them in edible coatings to control postharvest diseases of papaya fruits. For in vitro studies, both essential oils were tested to evaluate their effect on mycelial growth of C. gloeosporioides and R. stolonifer during given incubation times. For in vivo tests, fruit were dipped in the thyme and Mexican lime essential oils before and after inoculation. Non-inoculated fruits were similarly treated. A further experiment was carried out by dipping papayas in a coating formulated with both essential oils. Results indicated that the fungicidal effect was more evident with essential thyme than with Mexican lime oil. For the essential thyme oil, concentrations up to 0.060% stopped mycelial growth for both C. gloeosporioides and R. stolonifer. Papaya fruit dipped in both the essential oils experienced reduced decay caused by C. gloeosporioides and R. stolonifer by up to 50% and 40%, respectively, compared with the 100% infection observed in non-treated papayas. It was also observed that concentration was not a key factor in reducing the development of these two fungi as it occurred in in vitro studies. In papayas immersed in mesquite gum emulsion and formulated with both the essential oils, it was possible to reduce the disease incidence caused by C. gloeosporioides by 100% with the thyme and Mexican lime essential oils at 0.1% and 0.5%, respectively. Conclusion: Thyme and Mexican lime essential oils demonstrated inhibitory effects against C. gloeosporioides and R. stolonifer growth in in vitro evaluations and over development of storage rots in papaya fruit. Incorporation of these two oils in mesquite gum-based coating gave excellent control of these two postharvest diseases. Significance and Impact of the Study: Control of postharvest diseases of papayas by using synthetic fungicides is becoming increasingly difficult. Thyme and Mexican lime essential oils are considered as GRAS compounds. This investigation highlights the potential of using these two essential oils to control anthracnose disease and Rhizopus rot of papaya. We also confirmed that the mesquite-based gum coating formulated with thyme and Mexican lime oils accomplished what is expected in a film material: microorganism inhibition and shelf-life extension of papayas.