Biocontrol agents are non-pathogenic plant-associated microbes that suppress disease or enhance plant growth. Trichoderma harzianum is a fungus, naturally found in soils or associated to other fungi in aerial portions of plants, which has proved to be an efficient biocontrol agent of fungal plant pathogens. The biocontrol activity of Trichoderma on phytopathogens is accomplished by different mechanisms: a) competition for space and nutrients in order to exclude the presence of the phytopathogen from a certain portion of soil or tissue; and/or b) mycoparasitism in order to use the metabolic machinery of the phytopathogen for its benefit; and/or c) production of antibiotics that interfere with cellular functions of the phytopathogen; and/or d) production of hydrolytic enzymes that degrade the cell wall of the phytopathogen. Therefore, using one or more of these biocontrol mechanisms, Trichoderma interfere with the development of the phytopathogen thus preventing or suppressing disease. Several products, that include T. harzianum in their formulations, have been developed. These could replace at least in part, the use of chemicals for the control of phytopathogens preventing or decreasing the environmental pollution and toxic effects on animal and human beings, of chemical pesticides.

Nevertheless, the biocontrol activity of Trichoderma could be affected by other components used at the field level to improve crop productivity, such as plant nutrients and growth regulators. The latter include plant hormones, and are commonly used in agriculture to stimulate budding of tubers, growth and ripening of fruits and development of foliage of plants, among others. The effect of these growth regulators on the biocontrol ability of T. harzianum was tested against Alternaria alternata, a known phytopathogen of several crops, using pure hormones (Indolacetic acid, IAA; Gibberelic acid, GA₃ and Benzylaminopurine, BAP) and selected commercial products purchased in the local market (Bayfolan^R, Bayer 2T005 S from Bayer, Activol^R from BASF, liquid Giberelic acid from Hoescht and NAA-800^R from Shell), which contain these hormones in their formulations.

Pure hormones and most of the commercial products at concentrations up to 80 ppm, did not affect germination of conidia or fungal growth of T. harzianum or of the phytopathogen A. alternata; while Bayfolan^R slightly increased germination of the phytopathogen, probably due to the presence of micro and macro elements in its formulation. The in vitro biocontrol activity of T. harzianum on A. alternata was not affected by the presence of any of the compounds tested. Moreover, they reduced the development of the phytopathogen and increased the growth of the biocontroller. Therefore, plant growth regulators, either alone or included in commercial formulations do not alter germination and/or growth of both fungi and improve the biocontrol activity of T. harzianum on A. alternata suggesting that a similar behaviour could be expected at the field level, as a consequence of the use of these chemicals.

The effect of pure hormones and commercial products was tested on: a) the ability of A. alternata to secrete endo-polygalacturonase, an enzyme the fungus use to infect plant tissues, and b) the ability of T. harzianum to secrete endo-chitinase, an enzyme this fungus use to degrade the cell wall of phytopathogens. 40 ppm of GA₃ or IAA or BAP reduced in 20 - 25% the secretion of endo-polygalacturonase activity from A. alternata and did not affect the secretion of endo-chitinase from T. harzianum. The sole presence of T. harzianum decreased in 46% the secretion of endo-polygalacturonase from A. alternata; and the presence of the phytopathogen increased in 14% the secretion of endo-chitinase from T. harzianum. Therefore, the presence of the biocontrol agent results in a decrease of the infective ability of A. alternata; which in turs stimulate one of the mechanisms the biocontrol agent uses to degrade the cell wall of the phytopathogen. i.e. secretion of endo-chitinase.

It may be concluded that the presence of growth regulators do not affect the ability of T. harzianum to control A. alternata, and that results obtained in vitro could represent what could be happening at the field level.