Folia Geobotanica 38: 191–200, 2003 INOCULATION OF RHODODENDRON CV. BELLE-HELLER WITH TWO STRAINS OF PHIALOCEPHALA FORTINII IN TWO DIFFERENT SUBSTRATES Martin Vohník 1,2) , Simon Lukanèiè 3) , Edi Bahor 3) , Marjana Regvar 3) , Miroslav Vosátka 1) & Dominik Vodnik 3) 1) Institute of Botany, Academy of Sciences of the Czech Republic, CZ-252 43 Prùhonice, Czech Republic; e-mail martinvohnik@yahoo.com 2) Department of Plant Physiology, Faculty of Science, Charles University, Vinièná 5, CZ-128 44 Praha 2, Czech Republic 3) Department of Biology, Biotechnical Faculty, University of Ljubljana, Veèna Pot 111, 1000 Ljubljana, Slovenia Abstract: The growth response of an ornamental Rhododendron hybrid to the inoculation with Phialocephala fortinii was studied in two pot experiments in order to decide about the effectiveness of the inoculation of young rhododendron microplants. Two different substrates were used in both experiments, either sterilized or non-sterilized: a horticultural substrate and a soil collected from a field site with dominant ericoid vegetation. Two fungal isolates were used for an inoculation: P. fortinii strain P (UAMH 8433) and P. fortinii strain F, a dark septate endophyte (DSE) previously isolated from naturally-infected roots of Vaccinium myrtillus. Both Phialocephala strains successfully colonized the roots of the host plants forming typical DSE (=pseudomycorrhizal) colonization pattern including the formation of intracellular microsclerotia. However, pseudomycorrhizal colonization did not affect the growth parameters of the host rhododendrons. The results from both experiments indicate a neutral effect of the inoculation with DSE fungi on the growth of Rhododendron cv. Belle-Heller. Keywords: Dark septate endophytes, Pseudomycorrhiza, Ericoid mycorrhiza INTRODUCTION Dark septate endophytes (DSE) are conidial or sterile fungi inhabiting roots of a wide range of higher plants. They appear ubiquitous with worldwide distribution and represent a heterogeneous group, functionally and ecologically interacting with other soil biota (JUMPPONEN &TRAPPE 1998a). It is believed that they replace arbuscular mycorrhizal fungi at climatically and edaphically extreme sites (HASELWANDTER &READ 1980, READ & HASELWANDTER 1981). Among DSE fungi, Phialocephala fortinii WANG et WILCOX (1985) is the most studied due to its frequent isolation from roots. Although DSE fungi are frequently isolated from roots or rhizosphere of higher plants, their identification has been difficult for a long time. The relationship between fungal strains s now often analyzed using ribosomal DNA (rDNA) amplified by the polymerase chain reaction (PCR) (MCLEAN et al. 1999). The universal primers are used to amplify the region of rDNA containing the two highly variable internal transcriber spacers (ITS) and the 5.8S subunit between them. These sequences can be analyzed phylogenetically in order to