Efficacy and Mechanisms of Action of Essential Oils' Vapours against Blue Mould on Apples Caused by Penicillium expansum

Abstract

Biofumigation with slow-release diffusers of essential oils (EOs) of basil, oregano, savoury, thyme, lemon, and fennel was assessed for the control of blue mould of apples, caused by Penicillium expansum. In vitro, the ability of the six EOs to inhibit the mycelial growth was evaluated at concentrations of 1.0, 0.5, and 0.1%. EOs of thyme, savoury, and oregano, at all three concentrations, and basil, at 1.0 and 0.5%, were effective in inhibiting the mycelial growth of P. expansum. In vivo, disease incidence and severity were evaluated on 'Opal' apples artificially inoculated with the pathogen and treated at concentrations of 1.0% and 0.5% of EOs. The highest efficacy in reducing blue mould was observed with EOs of lemon and oregano at 1.0% after 60 days of storage at 1 ± 1 °C (incidence of rot, 3 and 1%, respectively) and after a further 14 days of shelf-life at 15 ± 1 °C (15 and 17%). Firmness, titratable acidity, and total soluble solids were evaluated at harvest, after cold storage, and after shelf-life. Throughout the storage period, no evident phytotoxic effects were observed. The EOs used were characterised through GC-MS to analyse their compositions. Moreover, the volatile organic compounds (VOCs) present in the cabinets were characterised during storage using the SPME-GC-MS technique. The antifungal effects of EOs were confirmed both in vitro and in vivo and the possible mechanisms of action were hypothesised. High concentrations of antimicrobial and antioxidant compounds in the EOs explain the efficacy of biofumigation in postharvest disease control. These findings provide new insights for the development of sustainable strategies for the management of postharvest diseases and the reduction of fruit losses during storage.

Keywords: Malus x domestica; SPME-GC-MS; antimicrobial; antioxidant; biofumigation; gas chromatography analysis; natural antifungal compounds; postharvest disease; quality factors; sustainable agriculture.

Figure 1

Effect of volatile organic compounds released by EOs on the growth of the mycelium (diameter, cm) of P. expansum (two strains, PEN2, blue bars, and PEN3, orange bars) through the sandwich plates method. Essential oils were applied at different concentrations (0.1%, 0.5%, and 1.0%). The plates were cultured at 25 °C and the measurements were carried out after 24 h, 48 h, 72 h, 96 h, and 11 days. Values of the same strain and time point, followed by the same letter, are not statistically different according to the Tukey’s test (p < 0.05).

Figure 2

Incidence of rot on ‘Opal’ apples treated with EOs’ biofumigation after 60 days of storage at 1 ± 1 °C (blue bars) and 95% relative humidity, and after 14 days of shelf-life at 15 ± 1 °C (orange bars). Values of the same time point, followed by the same letter, are not statistically different according to the Tukey’s test (p < 0.05).

Figure 3

Severity of rot caused by P. expansum on ‘Opal’ apples treated through the biofumigation of EOs (blue bars) at 28 and 43 days, at a temperature of 1 ± 1 °C, and after 50 days, i.e., after 7 days of shelf-life at 15 ± 1 °C. Yellow bars represents apples treated with pyrimethanil, red bars represent inoculated control. Values of the same time point, followed by the same letter, are not statistically different according to the Tukey’s test (p < 0.05).

Figure 4

Chemical structures of α-farnesene and oxidation products.

Figure 5

(a) Tautomerisation process of carvacrol, acid (top) and basic (bottom) catalysed. (b) Homolytic scission of the hydroxyl group of carvacrol and resonance structure.

Figure 6

Chemical structures of carvacrol, thymol, p-cymene, linalool, t-anethole, and limonene.