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

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 1000 μl/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 (1000 μl/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 750 μl/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 . [ABSTRACT FROM AUTHOR]