Acetone leaf extracts of Breonadia salicina (Rubiaceae) and ursolic acid protect oranges against infection by Penicillium species

Abstract

The activity of acetone leaf extracts of Breonadia salicina and the main antifungal compound isolated from the extract, ursolic acid, was determined against three important plant fungal pathogens (Penicillium expansum, P. janthinellum and P. digitatum) to evaluate their potential use in combating post-harvest infections of oranges. In an in vitro assay, acetone extracts had good antifungal activity against P. janthinellum with an MIC (minimum inhibitory concentration) of 0.08 mg/ml. P. digitatum and P. expansum were more resistant both with MICs of 1.25 mg/ml. Weevaluated the potential use of an acetone extract and ursolic acid against these fungal pathogens in artificially infected oranges. A crude leaf extract at a concentration of 1 mg/ml gave the samelevel of protection as 1 mg/ml ursolic acid indicating synergistic activities within the crude extract. The acetone extract had an MIC of 0.16 mg/ml compared to the MIC of 0.08 mg/ml of amphotericin B against P. digitatum. Cytotoxicity of the crude extract and ursolic acid was determined using a tetrazolium-based colorimetric assay (3-(4,5-dimethylthiazol)-2,5-diphenyl tetrazolium bromide (MTT)) against Vero monkey kidney cells. The acetone extract had sufficient antifungal activity in vitro against these organisms to consider its use in the citrus industry after it has been tested under production and natural infection conditions and if it does not affect the fruit quality. The extractswere howevermore toxic to the kidney cells than to the fungi. The results show the potential use of plant extracts to combat plant fungal infections if extracts with lower cellular toxicity can be found or if the toxicity of the extract can be decreased without changing the antifungal activity.