Application of citral to control postharvest diseases of oranges.

Abstract

Green mould, blue mould and sour rot, caused by the fungi Penicillium digitatum, P. italicum and Geotrichum citri-aurantii, are postharvest diseases which cause significant losses to the citrus industry worldwide. Current control of the diseases raises some problems, such as development of fungicide resistance, concerns about residues harmful to humans, and also restrictions on the use of certain fungicides. Those problems have led to a need to develop alternative fungicides, including exploitation of some natural products such as essential oils. Application of the essential oil, citral (3,7-dimethyl-2,6-octadienal) to control the fungi and the diseases was assessed in this study. In vitro, citral incorporated into agar at 2%, 6% and 15% prevented germination of spores of the fungi, and no mycelial growth was observed by microscopic observation after 17 days of incubation. When citral was applied as a solution on agar, spore germination of P. digitatum and G. citri-aurantii was inhibited at concentrations of 6% and 15%. However, germination of P. italicum spores was not affected. Vapour of citral and its individual isomers, geranial and neral, generated from 6 and 15% aqueous solutions, inhibited spore germination and growth of the three pathogens. Vapour generated from 15% aqueous solutions of citral and geranial were fungicidal to P. digitatum and G. citri-aurantii, and fungistatic to P. italicum, while neral was fungicidal to G. citri-aurantii and fungistatic to the other two fungi. The result suggested that method of application and citral concentration affected the efficacy of citral in controlling the fungi. In the three methods of applications examined, citral was effective in controlling G. citri-aurantii, especially at high concentration. As an α , β-unsaturated aldehyde, citral may be degraded over time due to oxidative reactions, resulting in change in its composition, and this may affect its antifungal activity. Storage of citral may result in the oxidation of neral and geranial to produce neric acid and geranic acid. GC/MS results showed that neral, geranial, neric acid and geranic acid were detected, while the related compounds, nerol, geraniol, citronellal, citronellol and citronellic acid were not detected either for citral stored at 5°C or at room temperature. At room temperature, geranial and neral content declined more quickly than at 5°C. The effect of citral on the incidence of disease on fruit was studied by applying citral as a fumigant. Wounded oranges inoculated with spore suspension (10⁶ spores mL⁻¹) of the fungi were placed in 5-litre plastic boxes, fumigated with 2, 6, or 15% citral, and incubated at 5°C or room temperature. Fumigation of oranges with citral in this closed system delayed the onset of sour rot at room temperature by 7 – 10 days and at 5°C, by 13 – 30 days, suggesting that volatile citral controlled G. citri-aurantii on fruit as well as in vitro. The effects of fumigation with citral on green and blue mould were more variable. Fumigation delayed the onset of green mould and blue mould at 5°C by 2 days at the higher concentrations (6 and 15%) tested, while at room temperature, spoilage was not delayed even at the highest concentration tested. Measurement of citral in the headspace of boxes containing fruit and citral-soaked pads showed that the concentration above the fruit was higher than that measured below the fruit both at 5°C and at room temperature. Phytotoxicity symptoms were observed on the upper surface of some fruit that was close to or in direct contact with the citral-soaked pad at concentrations of 6% and 15%, suggesting that phytotoxicity may have been associated with high volatile citral concentration. However, citral residue was not detected in oranges irrespective of treatment with citral, which suggested that little citral had penetrated into the peel. During storage the citral content decreased due to oxidation of geranial and neral to produce geranic acid and neric acid both at 5°C and room temperature. This may have had an impact on the efficacy of citral against the pathogens.Findings may contribute to a better understanding of the efficacy of citral when applied to the pathogens in vitro and to the development of effective control methods when applied on fruit. The possibility of combining citral treatment with other commonly used practices is also worthy of consideration. For example, citral could be combined with heat treatment to increase the volatility of the citral. In addition, incorporation of citral in a wax formulation may allow a low concentration of citral to be used in direct contact with the pathogens on fruit. Fumigation of fruit with citral may offer potential as a means to control development of sour rot of oranges, and its effects on fruit quality, flavour and nutritional aspects require further investigation.