Beschreibung:
<jats:p>• <jats:italic>Premise of the study:</jats:italic> Cool‐season grasses infected by endophytic fungal symbionts sometimes show improved tolerance of herbivory and abiotic stresses, but the potential effects of endophytes on host competitive ability are unclear. This study examined intraspecific competition among variable genotypes of <jats:italic>Lolium perenne</jats:italic> with and without the endophyte <jats:italic>Epichloë festucae</jats:italic> var. <jats:italic>lolii</jats:italic> (Clavicipitaceae, Ascomycota) to assess the effect of infection on competitive outcomes.</jats:p><jats:p>• <jats:italic>Methods:</jats:italic> Eleven host genotypes distributed among infected plants of four cultivars of <jats:italic>L. perenne</jats:italic> were used for two pairwise competition experiments. In the greenhouse, an infected or uninfected ramet of each genotype was grown alone and paired in competition with a ramet of an uninfected genotype of a native accession of <jats:italic>L. perenne</jats:italic>. Tiller numbers and root and shoot dry mass were determined for targets and competitor plants after 15 wk.</jats:p><jats:p>• <jats:italic>Key results:</jats:italic> Competition and host genotype had highly significant effects on all measured variables. However, endophyte‐mediated effects on hosts were minor, and those that occurred depended on genotype. In addition, endophytic status of target plants did not influence tiller number or dry mass in 10 of 11 competitor genotypes, although these native genotypes did significantly vary in their competitive performance.</jats:p><jats:p>• <jats:italic>Conclusions:</jats:italic> Genotypic variation among cultivars and accessions from native populations of <jats:italic>L. perenne</jats:italic> determines the effect of both intraspecific competition and infection by a fungal endophyte. However, in the absence of herbivory, asexual clavicipitaceous endophytes may not be as important to host success in competitive environments as previously thought in this globally important grass species.</jats:p>