Dark septate endophytes (DSE) are a group of endophytic fungi characterized by their morphology of melanized, septate, hyphae. This group is likely paraphyletic, and contain conidial as well as sterile fungi that colonize roots intracellularly or intercellularly. Very little is known about the number of fungal taxa within this group, but all are in the Ascomycota. They are found in over 600 plant species and across 114 families of angiosperms and gymnosperms and co-occur with other types of mycorrhizal fungi. They have a wide global distribution and can be more abundant in stressed environments. Much of their taxonomy, physiology, and ecology are unknown.