natural-products-in-plant-pest-management

44 J.C. Pretorius and E. van der Wattitself. Plants have evolved highly specific chemical compounds thatprovide defence mechanisms against attack by disease-causing organisms,including fungal attack, microbial invasion and viral infection (Cowan,1999). These bioactive substances occur in plants as secondary metabolites,and have provided a rich source of biologically active compoundsthat may be used as novel crop-protecting agents (Cox, 1990). In naturesome wild plants have the potential to survive both harsh biotic and abioticenvironmental conditions. This has initiated the postulate that suchplants might be utilized as sources for the development of natural productsto be applied in agriculture by man as natural herbicides, bactericides,fungicides or products with bio-stimulatory properties in crude orsemi-purified form.It is estimated that there are more than 250,000 higher plant species onearth (Cowan, 1999) offering a vast, virtually untapped, reservoir of bioactivechemical compounds with many potential uses, including their applicationas pharmaceuticals and agrochemicals. It is generally assumed thatnatural compounds from plants pose less risk to animals and humans andare more environmentally friendly than their synthetic counterparts( Johnson, 2001). As in pharmacology, biochemicals isolated from higherplants may contribute to the development of natural products for the agriculturalindustry in three different ways (Cox, 1990): (i) by acting as naturalpesticides in an unmodified state (crude extracts); (ii) by providing thechemical ‘building blocks’ necessary to synthesize more complex compounds;and (iii) by introducing new modes of pesticidal action that mayallow the complete synthesis of novel analogues in order to counter theproblem of resistance to currently used synthetic products by bacterial andfungal pathogens.However, it is quite common that a natural compound isolated from aplant may be of great biological interest but may not be sufficiently robust foruse (Steglich et al., 1990). Subsequently, a need for the modification of naturalproducts into synthetic analogues that will give the desired effect may stillexist. One can, for example, isolate a natural compound with promising antimicrobialactivity and, by introducing a stable chemical structure with higheractivity synthetically, develop a commercial product (Steglich et al., 1990).The alternative that is now more vigorously being pursued in organic farmingsystems is the application of the plant material itself in a natural form.However, to date and despite statements such as these made almost twodecades ago, the use of natural plant extracts as pesticides to control pathogensin crops, for example, are not widespread while synthetic chemical pesticidesstill remain the major tool in pest management systems. Nevertheless,some natural products have been commercialized recently and the use of acombination of synthetic chemicals and natural products in IPM programmeswill probably become more popular in future.Another related area of organic farming systems is the potential to applynatural plant extracts as either plant growth regulators or natural herbicides.A plant growth regulator is an organic compound, either natural or synthetic,that modifies or controls one or more specific physiological processes within