Natural plant volatiles as an alternative approach to control stem-end rot in avocado cultivars

Stem-end rot is a postharvest disease associated with multiple important fungal pathogens including Lasiodiplodia theobromae. The incidence of stem-end rot in avocado during postharvest storage affects the shelf life, quality and marketability of the fruit. This study is aimed at the investigation of the antifungal activities of selected natural plant volatiles (vapour phase): citral, octanal, hexanal and thymol against L.theobromae (causal pathogen of stem-end rot) in vitro and in vivo in Hass and Fuerte avocados. Hexanal showed a lower inhibitory effect on the radial mycelial growth of L.theobromae in vitro. However, citral at a minimum concentration of 4l/L revealed fungicidal activity and completely inhibited the spore germination of L.theobromae. Artificially inoculated Hass and Fuerte avocados with L.theobromae were exposed to citral (768l) and commercial fungicide prochloraz and stored for 6days at 20 degrees C and 14days at 10 degrees C separately and thereafter held at 20 degrees C for 3days to simulate the retail shelf conditions. Although citral in a volatile phase effectively reduced the development of stem-end rot in both cultivars, its effect was significant in Fuerte with 75% reduction in the incidence of stem-end rot. The biochemical analysis demonstrated an increase in total phenol contents, phenylalanine ammonia-lyase, chitinase and -1, 3 glucanase activity in fruit exposed to citral when compared to the reference treatment prochloraz and the untreated control for both cultivars. Furthermore, fruits exposed to citral retained the ready-to-eat firmness and therefore could be considered a potential alternative treatment to control stem-end rot at the postharvest stage.