| Summary: | Many root diseases can cause serious damage to tomato (<i>Solanum lycopersicum</i> L.) production during the seedling growth period, particularly <i>Rhizoctonia solani</i>, <i>Pythium</i> spp., and <i>Alternaria</i> spp. Paclobutrazol (PBZ) has proved to reduce the damage of the Alternaria blight disease on tomato seedlings. The present investigation was designated to evaluate the growth of infected tomato seedlings with <i>R. solani</i> under applications of <i>Trichoderma harzianum</i> alone, PBZ alone, both PBZ and <i>T. harzianum</i>, and comparing with applied fungicide. PBZ enhanced the chlorophyll system in tomato seedlings, enzymatic antioxidants, and the total antioxidants, besides the vegetative parameters. The combined application of both PBZ and <i>Trichoderma</i> significantly improved growth parameters, decreased the damping off percent of <i>R. solani</i>, as well as increased the enzymatic antioxidants and the total antioxidants of tomato seedlings. It was found that applying 100 mg L<sup>−1</sup> PBZ did not affect the effectiveness of the biocontrol agent of <i>T. harzianum</i>. The images of the scanning electron microscope (SEM) confirm that PBZ showed twisting, shrinking, and collapsing of lysis and hyphae in cultures of <i>R. solani</i>. This is the first report that confirms the biochemical control of <i>R. solani</i> on tomato seedlings using the combined application of PBZ and <i>T. harzianum.</i> Further studies are still needed to test more <i>Trichoderma</i> species and different phytopathogens.
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