Background: The genus Trichoderma is widely spread in the environment, mainly in soils. Trichoderma are filamentous fungi and are used in a wide range of fields to manage plant patho-genic fungi. They have proven to be effective biocontrol agents due to their high reproducibility, adaptability, efficient nutrient mobilization, ability to colonize the rhizosphere, significant inhibitory effects against phytopathogenic fungi, and efficacy in promoting plant growth. In the present study, the antagonist Trichoderma isolates were characterized from the soil of Abha region, Saudi Arabia. Methodology: Soil samples were collected from six locations of Abha, Saudi Arabia to isolate Trichoderma having the antagonistic potential against plant pathogenic fungi. The soil dilution plate method was used to isolate Trichoderma (Trichoderma Specific Medium (TSM)). Isolated Trichoderma were evaluated for their antagonistic potential against Fusarium oxysporum, Alternaria alternata and Helminthosporium rostratum. The antagonist activity was assessed by dual culture assay, and the effect of volatile metabolites and culture filtrate of Trichoderma. In addition, the effect of different temperature and salt concentrations on the growth of Trichoderma isolates were also evaluated. Results: The most potent Trichoderma species were identified by using ITS4 and ITS 5 primers. Total 48 Trichoderma isolates were isolated on (TSM) from the soil samples out of those six isolates were found to have antagonist potential against the tested plant pathogenic fungi. In general, Trichoderma strains A (1) 2.1 T, A (3) 3.1 T and A (6) 2.2 T were found to be highly effective in reducing the growth of tested plant pathogenic fungi. Trichoderma A (1) 2.1 T was highly effective against F. oxysporum (82%), whereas Trichoderma A (6) 2.2 T prevented the maximal growth of H. rostratum (77%) according to the dual culture data. Furthermore, Trichoderma A (1) 2.1 T volatile metabolites hindered F. oxysporum growth. The volatile metabolite of Trichoderma A (6) 2.2 T, on the other hand, had the strongest activity against A. alternata (45%). The Trichoderma A (1) 2.1 T culture filtrate was proven to be effective in suppressing the growth of H. rostratum (47%). The temperature range of 26 °C to 30 °C was observed to be optimum for Trichoderma growth. Trichoderma isolates grew well at salt concentrations (NaCl) of 2%, and with the increasing salt concentration the growth of isolates decreased. The molecular analysis of potent fungi by ITS4 and ITS5 primers confirmed that the Trichoderma isolates A (1) 2.1 T, A (3) 3.1 and A (6) 2.2 T were T. harzianum, T. brevicompactum, and T. velutinum, respectively. Conclusions: The study concludes that the soil of the Abha region contains a large population of diverse fungi including Trichoderma, which can be explored further to be used as biocontrol agents.
