Abstract
<title>Abstract</title> <p> Spot blotch caused by <italic>Bipolaris sorokiniana</italic> is a major threat to barley production worldwide. Although the genus <italic>Trichoderma</italic> is widely recognized for its biocontrol potential, the extent to which genetic diversity among distinct pathogen isolates influences antagonistic responses remains poorly understood. This study screened eight native <italic>Trichoderma</italic> isolates from threatened Araucaria Forest remnants against six <italic>B. sorokiniana</italic> field isolates using exclusively <italic>in vitro</italic> assays. Pathogen genetic diversity was assessed using inter-simple sequence repeat (ISSR) markers, while biocontrol potential was evaluated through dual culture and volatile/non-volatile metabolite production. Nei’s genetic diversity ( <italic>h</italic> = 0.444) and Shannon’s information index ( <italic>I</italic> = 0.634) revealed high genetic diversity among <italic>B. sorokiniana</italic> isolates, with no clear geographic association. Our results showed that the antagonistic efficacy of <italic>Trichoderma</italic> varied according to the pathogen genotype. While all <italic>Trichoderma</italic> isolates inhibited pathogen growth, their performance was highly isolate-dependent: <italic>T. harzianum</italic> LMA A14 and <italic>T. azevedoi</italic> LMA A22 exhibited the highest inhibition in dual culture (up to 82%), whereas <italic>T. asperelloides</italic> (LMA A20 and LMA A24) and <italic>T. longibrachiatum</italic> LMA A21 excelled differentially in volatile and non-volatile metabolite production, showing distinct suppression profiles across different <italic>B. sorokiniana</italic> genotypes. This study suggests that microbial biocontrol is a dynamic interaction in which pathogen genetic variability may influence antagonist performance, highlighting the importance of evaluating genetically diverse pathogen isolates when screening broad-spectrum native biocontrol candidates for sustainable disease management. </p>