Application of botanical and biological methods in plant disease controls are unique techniques in reducing environmental pollutions. Trichoderma species are effective agents in biocontrol strategies and identifying their useful isolates for further studies seems to be helpful. The purpose of this study was to assess the in-vitro biocontrol efficacy of some native Trichoderma isolates against Pythium aphanidermatum an important causal agent of sugar beet root rot disease in North Iran. Evaluations were conducted in Shahrood Agricultural Research Center, North Iran during 2007. Collected Isolates of Trichoderma belonging to four species (T. harzianum, T. longibrachatum, T. erinaceum and T. koningii ), among which T. erinaceum is a new fungus for Irans' mycoflora. Results of lab tests including dual culture technique, assessment of volatile and non-volatile effects on pathogen' colony growth indicated the effectiveness of all ten isolates against P. aphanidermatum. In dual culture T. harzianum-2733 and T. longibrachatum-2734 were best in reducing the colony growth of P. aphanidermatum by 57% compared to control. Volatile antibiotic compounds of T. longibrachiatum-2737 and non-volatile antibiotic compounds of T. harzianum-2739 were best among Trichoderma isolates and controlled the colony growth of the pathogen by 66 and 87% respectively.

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