Colorado blue spruce (CBS; Picea pungens) is affected by spruce decline throughout the Lower Peninsula of Michigan. Diaporthe spp. are associated with cankers visible in the inner bark on declining trees exhibiting needle loss and branch dieback. The presence of antagonistic endophytes associated with branches may play an important role in management of spruce decline that could be integrated into current management practices. The objective of this research was to identify potential biological control agents inhibiting the growth of Diaporthe spp. associated with declining trees. Endophytic fungi were isolated from surface sterilized sapwood and inner bark tissues cut from the branches of symptomatic and asymptomatic CBS and identified via DNA barcoding of the ITSrDNA region. A total of 43 endophytic fungi were isolated and evaluated against Diaporthe spp. via in vitro dual culture and culture filtrate assays. Out of the 43 endophytic fungi isolated, 14 showed mycelial growth inhibition greater than 50% in dual culture assays with Diaporthe spp. The fungi that demonstrated substantial antagonistic activity against Diaporthe spp. are possible candidates for biocontrol agents. Eppicocum nigrum was identified as a promising biocontrol agent, producing potential antifungal compounds against Diaporthe spp. Further studies will focus on the validation of biocontrol efficacy of candidate fungi through colonization and interaction assays in planta.