• Fungal strains of the 'Hymenoscyphus ericae aggregate' (Helotiales, Ascomycota) were tested for their ability to form ecto-(ECM) and ericoid (ERM) mycorrhizas. • Twelve fungal test isolates were inoculated in vitro onto sterile ectomycorrhizal and ericoid host plants. • Five isolates of ectomycorrhizal origin formed ECM, three isolates of ericoid origin formed ERM, but none of the tested isolates was able to form both ECM and ERM. Confirmed ectomycorrhizal strains formed the characteristic black-and-hyaline mantles of the Piceirhiza bicolorata morphotype. This morphology represented an intermediate stage, where mantle-colour changed from light brown to black. The progressive darkening appeared to result from a gradual acropetal melanization of the outer mantle layer. • The synthesis results confirm that the ability to form both ecto- and ericoid mycorrhizal symbioses has evolved within the H. ericae aggregate. In addition, a phylogeny based on ITS1-5.8S-ITS2 rDNA sequences showed that the aggregate includes the ect(endo)mycorrhizal dark septate fungus Phialophora finlandia, grouping with four of the ECM-forming strains. The H. ericae aggregate therefore contains a number of closely related taxa with wide ecological attributes.