Trichoderma harzianum as a biological control agent

Biological control of soil-borne plant pathogens is an attractive approach to control fungal diseases (Chet 1990). When effective, it is a significant method of pest control not only because it eliminates the use of fungicides, but also because if the introduced biocontrol agent becomes properly established, it does not require repeated applications. Trichoderma spp. is among the most studied biocontrol agents (Chet 1990). Several species of Trichoderma spp have been isolated and found to be effective biocontrol agents of various soil-borne plant pathogenic fungi under greenhouse and field conditions (Chet and Inbar 1994). Trichoderma harzianum has proved to be the most effective species, and it has been shown to attack a range of economically important soil-borne plantpathogenic fungi (Chet 1990). Trichoderma can be added to the soil as a powder, wheat bran or a peat-wheat bran preparation. It can be sprayed or injected; painted on tree wounds; inserted in pellets in holes drilled in trees; and conidia can be applied directly to the ground or as seed coating (Chet 1990). In addition, Trichoderma may contribute to the overall plant defence response as it has been recently shown that Trichoderma application induces systemic resistance mechanisms in cucumber plants (Yedidia et al. 1999).

Trichoderma may have three modes of action as part of its antagonic activity, antibiosis, competition and mycoparasitism. However, mycoparasitism has been suggested as the main mechanism involved in the antagonism as a biocontrol agent (Haran et al. 1996). In order to attack a fungal cell, Trichoderma must degrade the cell wall. Given the composition of most fungal cell walls, it has been suggested that chitinases, proteases and β-1,3 glucanases are the main enzymes involved in the mycoparasitic process (Elad et al. 1982; Haran et al. 1996). However, it is likely that the co-ordinated action of all hydrolases produced by Trichoderma is required for a complete dissolution of the cell wall. Indeed, a number of T richoderma isolates are able to secrete different kinds of hydrolytic enzymes into the medium when grown in the presence of cell walls of phytopathogenic fungi (Geremia et al. 1993). In addition, the production of several of these lytic enzymes by Trichoderma is induced during the parasitic interaction (Haran et al. 1996; Flores et al. 1997).

As of late, people have been trying to increase the effectiveness, stability, and biocontrol capacity of Trichoderma spp by altering the levels of different hydrolytic enzymes. Several improved Trichoderma strains have, therefore, been obtained which display an increased antifungal activity by overexpression of a proteinase (Flores et al. 1997) or a 33 kDa chitinase (Limon et al. 1999). For that reason, considering the activity and specificity of many fungal enzymes, mycoparasitic fungi may serve as excellent sources of genes for disease resistance (Lorito et al. 1998). In spite of the success achieved, treatments with Trichoderma harzianum have usually not been as effective as the use of some fungicides. Treatment with T. harzianum has, therefore, been combined with other cultural practices to implement integrated pest management (Hall 1991).