natural-products-in-plant-pest-management

62 J.C. Pretorius and E. van der Wattin the pharmaceutical industry (Naseby et al., 2001) and folk medicine( Duncan et al., 1999), their potential for plant disease management has notyet been fully realized. Crop production still depends heavily on syntheticbasedproducts (Philip et al., 1995) despite consumer resistance towards theirpossible residual effects. For this reason the switch to organic farming,including the use of natural products in disease management systems, hasbecome a priority (Benner, 1993; Michael, 1999). The next section provides anoverview of selected plants containing compounds with antimicrobial properties,identified during the past two decades, as well as the current status ofnatural products that have recently been commercialized.Plants with antifungal propertiesTwo decades ago Naidu (1988) assayed young and mature leaf extracts ofCodiaeam variegatum for antifungal activity. All extracts inhibited Alternariaalternata and Fusarium oxysporum in vitro, with the young leaves being moreactive against A. alternata and the old leaves more active against F. oxysporum.The active secondary metabolites from the leaves extracts were identified asphenolic compounds by chromatographic analysis. Phytochemical screeningof the leaves also revealed other metabolites that may be responsible forantifungal activity.In another study in the same year Bandara et al. (1988) reported thatsteam distillates of the leaves of Croton aromaticus and C. lacciferus and rootextracts of C. officinalis inhibited mycelial growth of Cladosporium cladosporioidesin vitro. Root extracts of C. lacciferus were moderately active while thoseof C. aromaticus were inactive. Of the six compounds isolated from rootextracts showing antifungal activity, only 2,6-dimethoxybenzoquinoneobtained from the chloroform extract of C. lacciferus was significantly active.Comparatively small quantities of this compound were required to inhibitgrowth of the pathogens Botryodiplodia theobromae and Colletotrichumgloeosporioides (Glomerella cingulata).Also in the same year Gonzalez et al. (1988) extracted Alnus acuminatanodules with either 5% NaOH or water while constituents were separated bysilica gel column chromatography. The following compounds were isolated:xylose, ribose, an aromatic carboxylic acid, a fatty acid, a phenolic biarylheptanoidand a flavonoid glycoside. The flavonoid glycoside was found toinhibit the growth of Fusarium oxysporum and Pythium species.In the 1990s a number of contributions by natural product researcherswere published. Only a few are mentioned here. Ajoene, a secondary metabolitederived from garlic (Allium sativum), was shown to inhibit spore germinationof some fungi including Alternaria solani and other Alternaria spp.,Collectotrichum spp., Fusarium oxysporum and other Fusarium spp. that causeserious diseases in some important crop plants in India (Singh et al., 1990). Ina study conducted by Hoffmann et al. (1992) a methanol extract of Castela emoryiwas active as both a preventative and curative agent against grape downymildew caused by Plasmopara viticola. An active secondary metabolite was