The main objective of this study was to determine the impact of selected Trichoderma strains added to the growing medium on defense reactions in tomato and cucumber plants and, consequently, to increase their resistance to pathogenic infection by Rhizoctonia solani. Changes in concentrations of hydrogen peroxide (H2O2), total phenolics (TP), ortodihydroxyphenolics (oDP) and of lipid peroxidation and the activities of enzymes: phenylalanine ammonia lyase (PAL), guaiacol (POG) and ascorbate (APX) peroxidases in the leaves of tomato and cucumber plants were studied as hallmarks of defense reactions. Basing on the results, a few strains of Trichoderma, causing significant changes in the parameters characteristic of plant defense mechanisms mobilization, were chosen. These results may demonstrate the participation of these fungi in the activation of systemic induced resistance (ISR) pathways. At the moment Polish Trichoderma strains seem to be a promising source of natural resistance inducers of crop plants, which in the future could could be an alternative to commonly used chemical pesticides.
