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 0.08mg/ml. P. digitatum and P. expansum were more resistant both with MICs of 1.25mg/ml. We evaluated 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 1mg/ml gave the same level of protection as 1mg/ml 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. 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 extracts were however more 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.
Highlights
We investigate the activity of acetone extracts and ursolic acid from leaves of B. salicina and the potential use on C. sinensis artificially infected with P. digitatum, P. expansum and P. digitatum because these pathogens are relatively resistant to the currently available fungicides and cause postharvest problems in the fruit industry
Ursolic acid was active against P. expansum, P. janthinellum and P. digitatum with MICs of 0.13, 0.13 and 0.25 mg/ml respectively
P. digitatum isolated from the infected orange was 25 times more resistant to amphotericin B than the two other species cultures, possibly explaining why it was not inhibited by the treatment the oranges received prior to marketing
Summary
Its production exceeds that of any other fruit. There are serious problems impeding citrus production. Up to 25% of the total production of harvested fruit is subject to fungal attack in both industrialized and developing countries and the damage may exceed 50% (Spadaro and Gullino, 2004). Infection occurs through injury during the picking or handling of fruit and results in decay during storage or marketing. It is essential to control postharvest diseases in order to maintain the quality and improve the shelf life of citrus fruit. Storage and transportation from producer to consumer may take several weeks to deliver the fruit. The susceptibility of freshly harvested products to postharvest diseases increases during prolonged storage as a result of physiological changes that enable pathogens to develop in the fruit
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