natural-products-in-plant-pest-management

72 J.C. Pretorius and E. van der WattA fairly new approach to microbial disease control is via systemicacquired resistance (SAR) or, in other words, the activation of natural defencemechanisms within the plant via elicitors (Dayan et al., 2009). Harpin proteinthat induces SAR in plants is sold as Messenger or ProAct. Brassinosteroidsthat induce the synthesis of PR proteins that resist fungal infection incrops, are sold as ComCat ® (Roth et al., 2000). The latter product seems to beused more often as a bio-stimulatory agent; an aspect that will be dealt withlater in this chapter.These past successes and the public’s current concern over the impact ofsynthetic pesticides on the environment ensures a continued, if not anincreased, interest in searching nature for environmentally friendlier pestmanagement tools (Dayan et al., 2009).3.5 Natural Bio-herbicidesThe herbicide component of all pesticides sold increased from less than 20%in the 1950s to almost 50% of the market in the 1980s (Jutsum, 1988) and thishas since risen to over 60%. Today, herbicides account for more than half ofthe volume of all agricultural pesticides applied in the developed world andtheir use has also contributed to the concern of consumers towards the potentialhealth and environmental impact they may have (Dayan et al., 2009).However, almost three decades ago, McWhorter and Chandler (1982) estimatedthat weeds contribute to a significant 12% loss in worldwide cropproduction, emphasizing the need to control weeds. These two perspectives,namely the need to control weeds and the negative attitude of consumers,have in part fuelled the current elevated interest in the discovery of naturalproducts to be applied as an environmentally friendly method of weedcontrol.However, the complexity of the environment in terms of weed controland especially the possible side effects of herbicides or biological con trolagents on non-targeted plants, calls for a comprehensive study oncontrol strategies, whether biological, chemical, cultural or a combination ofthese in an integrated weed management system. The biological approach toweed control can be separated into three distinct strategies: conservation,inundative and classical depending on the setting (e.g. range, row crops,urban), the extent of the infestation and the biology of the system (McFadyen,1998; Goeden and Andrés, 1999). According to the authors, conservationbiocontrol refers to situations where a natural biocontrol agent is alreadypresent and is able to control the weed but requires assistance in the form ofcultural practices or integrated management decisions that enable the agentto thrive. Although conservation strategies are less common in the biologicalapproach to weed control than in the biocontrol of insects, examples do exist(Goeden and Andrés, 1999).Inundative biocontrol involves the release of large numbers of a biologicalcontrol agent at a time when weed populations are expected to escapecontrol or exceed a critical economic or competitive threshold. When the