Constituents from Morus root bark against Venturia inaequalis – the causal agent of apple scab

Apple scab, which is caused by infection of Venturia inaequalis (Cooke) Winter, is the most serious disease in the cultivation of apples. It causes yield decrease and loss of fruit quality. Especially in organic apple orcharding effective organic options for synthetic pesticides are being called for [1], of which the use of plant extracts exhibiting antifungal activity seems most promising.

In this study an in vitro assay is presented derived by modification of protocols by Olaya and Köller [2] and Gilliver [3]. It allows for quantitative and qualitative discrimination of the germination inhibitory potential of extracts and pure compounds; it provides cheapness, a minimum requirement for technical equipment, and minute sample amounts.

From a screening of plant extracts, the methanol extract of Morus root bark, which previously showed pronounced cyclooxygenase-1 and -2 inhibitory potential [4], revealed distinct qualities against V. inaequalis. Based on the established assay, a bio-guided fractionation was performed to isolate the active constituents applying different chromatographic methods. Using this approach, the aim of this study was to identity the antifungal components within the active extract and to determine the germination inhibiting effect of the isolated single chemical entities. A further goal was to verify the in vitro activity by application and evaluation of the V. inaequalis inhibiting plant material in vivo on inoculated seedlings.

Among the isolated metabolites from Morus root bark [moracin M, O/P, kuwanon L, sanggenons D, B, G, O, E, and C] all the Diels-Alder adducts, showed an antifungal activity with IC₅₀ values between 10 and 123µM. The in vitro activity of the most active fraction (A5, IC₅₀ 39.0±4.2µg/mL) was tested on inoculated Golden Delicious seedlings, confirming a distinct antifungal activity against V. inaequalis for the tested natural material [5].

References: [1] Holb, I. J., Heijne, B. (2001) Gartenbauwissenschaft 66: 254–61. [2] Olaya, G., Köller, W. (1999) Pestic Sci: 55, 1083–8. [3] Gilliver, K. (1947) Ann Appl Biol 34: 136–43. [4] Rollinger et al. (2005) Planta Med. 71: 399–405. [5] Rollinger et al. (2006) J Agric Food Chem 54: 8432–6.