Application of essential oils as multi-target fungicides for the control of Geotrichum citri-aurantii and other postharvest pathogens of citrus

Abstract

Together with Penicillium moulds, Geotrichum citri-aurantii (sour rot) is a major cause of citrus fruit decay. Fifty-nine commercially available essential oils and some of their major components were screened to determine their effects on the mycelial growth of G. citri-aurantii using toxic medium assays. Nine of these essential oils totally inhibited the mycelial growth of the pathogen at 1000μl/l. Gas chromatographic analysis revealed the compositions of the most effective oils. Exposure to the vapours of selected essential oils (Thymus vulgaris, Citrus aurantium var. amara, Cymbopogon citratus, Cymbopogon martinii, Origanum vulgare, Geranium graveolens roseum Bourbon) proved fungistatic, rather than fungicidal, towards the pathogen. ‘Valencia’ oranges, inoculated with G. citri-aurantii, were treated with the most promising botanicals. Essential oils (1000μl/l) of C. citratus, C. martinii, O. vulgare and G. graveolens roseum Bourbon, incorporated into coating or applied as a curative dip, resulted in a drastic reduction in decay when compared to the negative control. No physiological breakdown of the fruit was evident at this concentration. C. citratus (lemongrass) was selected as the most cost-effective option for control of G. citri-aurantii. Previous commercial trials indicated the protective properties of Mentha spicata essential oil against Penicillium digitatum. To develop an effective multi-target protection product against G. citri-aurantii, P. digitatum and Penicillium italicum, essential oil of C. citratus was combined with that of M. spicata. A toxic medium, containing both essential oils, each at 750μl/l, totally controlled all three pathogens. Although individual essential oil-amended coatings have previously been applied for postharvest control, this study proposes a mixture of these essential oils for control of two serious citrus pathogens (G. citri-aurantii, P. digitatum), in addition to P. italicum.