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

The activity of acetone leaf extracts of Breonadia salicina and the main antifungal compound isolated from the \nextract, ursolic acid, was determined against three important plant fungal pathogens (Penicillium expansum, \nP. janthinellum and P. digitatum) to evaluate their potential use in combating post-harvest infections of oranges. \nIn an in vitro assay, acetone extracts had good antifungal activity against P. janthinellum with an MIC (minimum \ninhibitory concentration) of 0.08 mg/ml. P. digitatum and P. expansum were more resistant both with MICs of \n1.25 mg/ml. Weevaluated the potential use of an acetone extract and ursolic acid against these fungal pathogens \nin artificially infected oranges. A crude leaf extract at a concentration of 1 mg/ml gave the samelevel of protection \nas 1 mg/ml ursolic acid indicating synergistic activities within the crude extract. The acetone extract had an MIC \nof 0.16 mg/ml compared to the MIC of 0.08 mg/ml of amphotericin B against P. digitatum. Cytotoxicity \nof the crude extract and ursolic acid was determined using a tetrazolium-based colorimetric assay \n(3-(4,5-dimethylthiazol)-2,5-diphenyl tetrazolium bromide (MTT)) against Vero monkey kidney cells. \nThe acetone extract had sufficient antifungal activity in vitro against these organisms to consider its use in the \ncitrus industry after it has been tested under production and natural infection conditions and if it does not affect \nthe fruit quality. The extractswere howevermore toxic to the kidney cells than to the fungi. The results show the \npotential use of plant extracts to combat plant fungal infections if extracts with lower cellular toxicity can be \nfound or if the toxicity of the extract can be decreased without changing the antifungal activity.