natural-products-in-plant-pest-management

206 M. PorrasNaturally occurring substances found in fungi, bacteria and plants areimportant sources of molecules with different biological properties. Theymay be developed either as products per se or used as starting points forsynthesis to optimize specific properties. Low mammalian toxicity, low environmentalimpact, low levels of residues in food and compatibility with integratedpest management (IPM) programmes are important considerations inthe election of fungicides for development (Knight et al., 1997).This chapter covers the use of natural products in plant disease management(fungicides and bactericides), and personal experience of Brassicacarinata as a biofumigant in strawberry fields.10.2 Natural Products Used as Fungicides and Bactericides inAgricultureThe natural products used as agrochemicals have been well documented(Knight et al., 1997; Warrior, 2000; Copping and Duke, 2007; Kim and Hwang,2007; Dayan et al., 2009). Microorganisms can synthesize secondary metabolitesof versatile chemical structures with diverse biological activities thatexceed the scope of synthetic organic chemistry (Porter, 1985). Pesticidalpotency is not the only factor for formulating a plant-based product as acommercial pesticide. In-field chemical stability adequate to reduce theapplication times to an economical level, and lowered volatility for sufficientretention on the surface of host plants are also important factors for microbialfungicides to be efficiently applied in agriculture (Kim and Hwang, 2007).Therefore, only a few microbial metabolites have been successfully developedinto commercial fungicides. The excellent fungicidal activity of thesemicrobial metabolites and their potential as lead candidates for further fungicidedevelopment continue to stimulate research and screening for antifungalmicrobial metabolites (Kim and Hwang, 2007; Dubey et al., 2009). Table 10.1summarizes the microbial metabolites, and plant- or animal-derived productsused as fungicides or bactericides in agriculture.Biological control agents (including both bacteriological or fungus origin),plants, natural compounds and preparations have been described withactivity against bacterial or fungal plant pathogens. Furthermore, plants protectthemselves from microbial attacks with both constitutive antimicrobialsand compounds induced by the attacking pathogens (phytoalexins). Naturalproducts have been used to protect plants indirectly from pathogens byinducing systemic acquired resistance (SAR). The SAR-inducing compoundsare termed elicitors. Pathogens cannot evolve resistance directly to the elicitorbecause the activity is indirect, making such products excellent candidatesfor integrated disease management. Nevertheless, elicitors are generallynot as effective as chemical fungicides, partly because the timing of elicitorapplication and threat to the crop by a pathogen is crucial, but difficult tomaximize (Dayan et al., 2009).Disease suppression by biocontrol agents is the sustained manifestationof interactions between the plant, the pathogen, the biocontrol agent, and the