The current study aimed at developing diverse Trichoderma fusants for fungicides, drought, and salt tolerance with enhanced antagonistic activity against Sclerotium rolfsii Sacc. Trichoderma virens NBAII Tvs12 (mycoparasitic) and Trichoderma koningii MTCC796 (multistress tolerant) were used as parental strains for development of interspecific protoplast fusants. A total of 36 stable fusants were used for mycoparasitism, fungicides, and abiotic stresses (drought and salt) tolerance. The results revealed 20 homozygous progenies showing characteristics of either one parental strain and 14 heterozygous mutants depicting traits of both parental strains. A novel concept of inhibition coefficient was established using growth-related key parameters that represent the pathogen biology and the biocontrol-related biophysics of Trichoderma fusants. The results indicated a differential inhibition coefficient of the test pathogen and the highest (92.88%) inhibition coefficient of S. rolfsii was observed by interstable fusant Fu21. It also grew better under fungicides and abiotic stress (drought and salt) conditions. The molecular characterization and heterozygosity analysis evidenced the highest observed heterozygosity (0.5441) and gene flow (0.3872) in stable heterozygous Fu21. Principal coordinates analysis exhibited 62.7% of total variability. The ecofriendly heterozygous Trichoderma fusant (Fu21) might be useful for biocontrol of stem rot disease under adverse conditions or as a part of integrated disease management. K E Y W O R D S inhibition coefficient, molecular heterozygosity, protoplast fusion, Sclerotium rolfsii, Trichoderma