natural-products-in-plant-pest-management

Suppressive Effects of Compost Tea on Phytopathogens 245Boehm, 1999), less than 10% of the compost induced systemic resistance inplants (Zhang et al., 1996, 1998; Hoitink and Boehm, 1999).There are two primary mechanisms by which the colonies of biocontrolorganisms in compost combat disease, general suppression and specificsuppression.General suppression occurs when a high-microbial activity environmentis created in which the germination of pathogen propagules is inhibited.General suppression occurs when many different organisms compete withpathogens for nutrients (as root or seed exudates) and/or produce generalantibiotics that reduce pathogen survival and growth. In compost there is aslow release of nutrients which supports beneficial activity of this microbiotic.Biocontrol agents that colonize composts include bacteria such asBacillus, Enterobacter, Flavobacterium balusstinum, and Pseudomonas; actinomycetessuch as Streptomyces; and fungi such as Trichoderma and Gliocladium(Hoitink et al., 1991b).General suppression owes its activity to the total microbial biomass inthe soil or substrate and is not transferable between them. Whether soilorganic matter can support biological control depends on its decompositionlevel and the types of biocontrol agents present on the substrate (Hoitink andBoehm, 1999). The carrying capacity of organic matter in the substrate limitssuppressiveness to pathogens that depend on exogenous sources of nutrients(root exudates) for germination and infection. Excessively stabilized organicmatter such as dark peat has a limited ability to sustain activity of the generalmicrobial biomass in soil (Hoitink et al., 1993). Dark decomposed sphagnumpeat is consistently conductive to Pythium root rot, whereas light sphagnumpeat harvested from the surface of peat bogs is less decomposed and has ahigher microbial activity (Hoitink et al., 1991a). Light peat moss is suppressiveagainst Pythium for a short time (6–7 weeks).Specific suppression involves the action of one or a few specific microbialagents in suppressing a specific pathogen (Hoitink, 1993). They exert hyperparasitismon the pathogen or induce systemic resistance in the plant to specificpathogens. Specific suppression owes its activity to the effects ofindividual or select groups of microorganisms and is transferable. This canbe achieved by inoculating the compost with the desired microbial agent(Hoitink, 1993).The suppression effect on pathogens is linked to the type of pathogens.Those pathogens that have a small propagule size, such as Pythium and Phytophthoraspecies, are susceptible to general suppression (‘nutrient-dependent’pathogens). They have small nutrient reserves and need to rely on an externalcarbon source and other nutrients. Pathogens with a large propagule size,such as Sclerotium rolfsii and R. solani, are susceptible to specific suppression.Structures like sclerotia are less susceptible to microbial competition but specifichyperparasites such as Trichoderma species will colonize the sclerotia.Composting conditions as well as the materials that are composted arecritical, with the type of the material impacting on the sort of active microflora.Thus the composting of lignocellulosic wastes will induce a specificsuppression of Rhizoctonia by Trichoderma species, while Penicillium fungi