The potential for the use of Trichoderma species as biocontrol agents was suggested 70 years ago by Weindling (1932) who was the first to demonstrate the parasitic activity of members of this genus towards pathogens such as Rhizoctonia solani (Chet 1990; Weindling 1932). Since then, several species of Trichoderma have been tested as biocontrol agents; and have shown to attack a range of economically important aerial and soilborne plant pathogens (Chet 1987). In many experiments, showing successful biological control, the antagonistic Trichoderma was found to be a necrotrophic mycoparasite (Boosalis 1964; Chet and Elad 1982; Elad et al. 1983b). Mycoparasitism is defined as a direct attack on a fungal thallus, followed by nutrient utilization by the parasite (Chet et al. 1997). Necrotrophic mycoparasites, such as
Trichoderma, are those that kill the host cells before, or just after, invasion and use the nutrients released. These mycoparasites tend to be highly aggressive and destructive. They have a broad host range extending to wide taxonomic groups and are relatively unspecialized in their mode of parasitism. The antagonistic activity of necrotrophs is due to the production of antibiotics, toxins, or hydrolytic enzymes in such proportions as to cause death and destruction of their host (Manocha and Sahai 1993). In our view biocontrol by Trichoderma includes: (a) competition, (b) parasitism, (c) antibiosis, and (d) induction of defense responses in host plants, or the combination of some of them. Parasitism is a complex process including: (a) host recognition, (b) secretion of hydrolytic enzymes, (c) hyphae penetration and invasion (Figure 1), and (d) lysis of the host.
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