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 4 μl/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 (768 μl) and commercial fungicide prochloraz and stored for 6 days at 20°C and 14 days at 10°C separately and thereafter held at 20°C for 3 days 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 β-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.
K E Y W O R D Schitinase, citral, Lasiodiplodia theobromae, phenols, phenylalanine ammonia-lyase
| INTRODUCTIONAvocado (Persea americana Mill.) is predominantly recognized as a rich source of monounsaturated fatty acids and other important vitamins, which are beneficial to human health and nutrition (Donetti & Terry, 2014;Dreher & Davenport, 2013;Ferreyra et al., 2016). Avocado has been reported to possess a reasonable number of phenolic compounds that play an important role in the defence mechanisms of the fruit against potential pathogenic fungi. The phenolic content in avocado contributes to about 60 mg and 40 mg gallic acid equivalents for each 30 g and one-half fruit, respectively (Dreher & Davenport, 2013).Globally, the increase in postharvest losses is attributed to the fruit susceptibility to several fungal attacks, which impacts on the shelf life and commercial value of the fruit during the supply chain (Sivakumar & Bautista-Banos, 2014;Suvama, Hemalatha, Reddy, & Reddy, 2015).
| MATERIAL AND METHODS
| Isolation of the pathogenL. theobromae was isolated from symptomatic avocado ("Hass" and "Fuerte") fruits and maintained on a potato dextrose agar (PDA) at 30°C for 7 days. The spore suspension was obtained from a viable 7-day-old culture by adding sterile distilled water and adjusting the suspension with a haemocytomet...
