Breonadia salicina (Rubiaceae) extracts are as effective as a commercial fungicide in post harvest protection of oranges against Penicillium infections

After examining the antifungal activity of several medicinal and aromatic plants Breonadia salicina (Vahl) Hepper & J.R.I.Wood was selected for in depth study. The main active compound (ursolic acid) was isolated and characterised. Acetone extracts had good in vitro antifungal activity against P. janthinellum (MIC 0.08mg/ml.) but P. digitatum and P. expansum were more resistant (MIC 1.25mg/m)l. Penicillium species cause serious post harvest problems in the citrus industry. To test the in vivo efficacy, oranges were infected with the fungi and treated with the extracts, ursolic acid and a commercial fungicide amphotericin B. The crude leaf extract had the same level of protection as ursolic acid indicating synergistic activities within the crude extract. The acetone extract had an MIC of 0.16mg/ml compared to the MIC of 0.08mg/ml of amphotericin B against P. digitatum. The acetone extract therefore had sufficient antifungal activity against these organisms to consider its use in the citrus industry especially since it could be produced at a very low cost. The extract was however more toxic to the kidney cells that 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.

Keywords: antifungal extract, Breonadia salicina, Penicillium, post harvest, orange, antifungal compound

Acknowledgement: The National Research Foundation provided funding

References: 1. Mahlo SM, McGaw LJ and Eloff JN (2010) Crop Protection 29: 1529–1533