natural-products-in-plant-pest-management

48 J.C. Pretorius and E. van der Wattassociated with its use in plant disease control systems that have beenencountered in the past, and that have probably contributed to it not beingregarded as a viable strategy in the agrochemical industry. First, claims havebeen made that the efficacy of plant extract compounds are not comparableto that of synthetic fungicides and lack consistency (Benner, 1993). Second,natural products have, in some cases, been reported to be phytotoxic to cropplants (Benner, 1993). Third, some natural compounds are unstable and canbe broken down by UV-light or oxidation before the desired biological effectshave been produced (Seddon and Schmidt, 1999). However, these argumentscannot be generally accepted as less than 10% of natural plants have beenscreened for their application potential and in most cases only towards onetarget pathogen (Hamburger and Hostettman, 1991). In light of the vast numberof plant species known in the world today, the subsequent chemicaldiversity should allow for the identification of desired biologically activecompounds with sufficient stability.In contrast to the arguments against the use of natural plant compounds,other arguments towards its potential beneficial attributes should be considered.These include the possible reduction in the risk of fungicidal resistance,and the fact that they are potentially less toxic to humans and animals( Ganesan and Krishnaraju, 1995). Most importantly, it is envisaged that crudeplant extracts might be more affordable to subsistence farmers as they arereadily available and are probably cheaper to produce. Hence, attempts todevelop plant-derived natural products and the consideration of its applicationpotential in disease management systems in both developed anddeveloping countries does not seem to be out of line.In developing countries in particular, the consideration of applying naturalplant products in their crude form should be high on the agenda. Theapproach has long been used in most traditional farming systems in manydeveloping countries. Most African farmers possess substantial indigenousknowledge of insect and pathogen control. Although this knowledge is probablynot scientifically based, some examples of natural insecticides appliedin Ethiopian subsistence farming systems are worth mentioning. Extracts ofchinaberry (Melia azedarach), pepper tree (Schiunus molle) and endod (Phytolaccadodecandra) are used to control insects in both organized agricultureand home gardens (Gebre-Amlak and Azerefegne, 1998). Crude extracts ofall three plant species were reported by the authors to be effective againstBusseola fusca (maize stalk borer) larvae. With regard to disease management,a crude extract of Dolichos kilimandscharicus L. (Bosha) has been used as aslurry to treat sorghum seed in the control of covered (Sporisorium sorghi;Ehrenberg) and loose (Sphacelotheca cruenta, Kuhn) kernel smuts in Ethiopia(Tegegne and Pretorius, 2007). Experimentally, treatment of sorghum seedwith D. kilimandscharicus, in a powder form, provided excellent control ofboth pathogens and was as effective as the standard chemical, Thiram ® .However, this has been practised on a small and isolated scale. It seems necessaryto obtain a more scientific base through additional research in order toconsider an expansion of these practices as well as to consider the economicpotential of this approach.