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Root endophytes are common and genetically highly diverse suggesting important ecological roles. Yet, relative to above-ground endophytes, little is known about them. Dark septate endophytic fungi of thePhialocephala fortiniis.l.–Acephala applanataspecies complex (PAC) are ubiquitous root colonizers of conifers and Ericaceae, but their ecological function is largely unknown. Responses of Norway spruce seedlings of two seed provenances to inoculations with isolates of four PAC species were studiedin vitro. In addition, isolates ofPhialocephala subalpinafrom two populations within and one outside the natural range of Norway spruce were also included to study the effect of the geographic origin ofP. subalpinaon host response. The interaction of PAC with Norway spruce ranged from neutral to highly virulent and was primarily isolate-dependent. Variation in virulence was much higher within than among species, nonetheless only isolates ofP. subalpinawere highly virulent. Disease caused byP. subalpinagenotypes from the native range of Norway spruce was more severe than that induced by genotypes from outside the natural distribution of Norway spruce. Virulence was not correlated with the phylogenetic relatedness of the isolates but was positively correlated with the extent of fungal colonization as measured by quantitative real-time PCR.