Dark septate endophytes (DSE) are a miscellaneous group of ascomycetous anamorphic fungi that colonize root tissues intracellularly and intercellularly. The limited selection of studies quoted here exemplifies the range of host responses to symbiotic DSE fungi. Like mycorrhizal associations, DSE associations vary from negative to neutral and positive when measured by host performance or host tissue nutrient concentrations. This range of host responses is partially attributable to variation between different fungus taxa and strains. Similarly, hosts differ in their responses to a single DSE strain. Experimental conditions may also govern the nature of the symbiotic association. It is concluded that DSE are capable of forming mutualistic associations functionally similar to mycorrhizas. If the variation in host response to mycorrhizal fungi is considered to represent a continuum ranging from parasitism to mutualism, DSE symbiosis must be considered mycorrhizal, at least under some conditions.
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