Negative effects on survival and performance of Norway spruce seedlings colonized by dark septate root endophytes are primarily isolate-dependent Christoph Tellenbach, 1,2 * Christoph R. Grünig 1 and Thomas N. Sieber 1 1 ETH Zürich, Institute of Integrative Biology, Forest Pathology and Dendrology, Universitätstr. 16, CH-8092 Zürich, Switzerland. 2 Eawag, Aquatic Ecology, Überlandstr. 133, CH-8600 Dübendorf, Switzerland. Summary 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 the Phialocephala fortinii s.l.–Acephala applanata species complex (PAC) are ubiquitous root colonizers of conifers and Ericaceae, but their ecological func- tion is largely unknown. Responses of Norway spruce seedlings of two seed provenances to inoculations with isolates of four PAC species were studied in vitro. In addition, isolates of Phialocephala subalpina from two populations within and one outside the natural range of Norway spruce were also included to study the effect of the geographic origin of P. subal- pina on host response. The interaction of PAC with Norway spruce ranged from neutral to highly virulent and was primarily isolate-dependent. Variation in viru- lence was much higher within than among species, nonetheless only isolates of P. subalpina were highly virulent. Disease caused by P. subalpina genotypes 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 quanti- tative real-time PCR. Introduction Symbiotic fungi of plants are widespread and highly diverse contributing to adaptation and selection in plant communities (Dobson and Crawley, 1994; Clay, 2001; Gilbert, 2002; Selosse et al., 2004). While research has been focused largely on the two extremes of these sym- bioses (i.e. mycorrhizae and pathogens), seemingly neutral associations such as endophytes, which live inside the host tissue, are of increasing interest (Carroll, 1988; Saikkonen et al., 1998; Sieber, 2002; Rodriguez et al., 2009). Endophytic fungi colonize functional plant tissues of aerial plant parts (Carroll, 1988; Petrini, 1991; Rodriguez et al., 2009) as well as roots (Stoyke et al., 1992; Varma et al., 1999; Sieber, 2002; Addy et al., 2005; Mandyam and Jumpponen, 2005; Schulz and Boyle, 2005; Summerbell, 2005), but apparent disease symp- toms do not develop at all or for prolonged periods of time (Carroll, 1988; Saikkonen et al., 1998; Sieber, 2007; Rod- riguez et al., 2009). Endophytes can have beneficial effects on their host plant by promoting plant growth (Varma et al., 1999; Mandyam and Jumpponen, 2005; Schulz, 2006; Rodriguez et al., 2009), by increasing tol- erance to abiotic stress (Varma et al., 1999; Clay, 2001; Mandyam and Jumpponen, 2005; Rodriguez et al., 2009), by the production of compounds that are toxic for herbi- vores (Carroll, 1988; Clay, 2001; Arnold et al., 2003; Selosse et al., 2004; Schulz, 2006; Sieber, 2007; Miller et al., 2008), and by protecting the plant either directly by occupation of infection sites, by hyper-parasitism and antibiosis, or indirectly by inducing the plant’s defence system (i.e. acquired resistance) against harmful patho- gens (Arnold et al., 2003; Selosse et al., 2004; Schulz, 2006; Sieber, 2007; Oelmüller et al., 2009). In contrast, weak pathogens might slow down plant growth by allocat- ing resources that are taken up or produced by the plant, or penetrate living host cells and kill them. They might remain latent in the host until the host’s physical status or environmental conditions change and trigger pathogenic- ity in the endophyte (Sinclair and Cerkauskas, 1997; Saikkonen et al., 1998; Sieber, 2007; Barrett et al., 2009). However, the true nature of interaction with their host and the ecological significance of endophytic fungi is still not fully understood and it is assumed that they stretch a Received 20 January, 2011; accepted 8 May, 2011. *For correspon- dence. E-mail ctellenb@ethz.ch; Tel. (+41) 44 632 69 11; Fax (+41) 44 632 13 80. Environmental Microbiology (2011) 13(9), 2508–2517 doi:10.1111/j.1462-2920.2011.02523.x © 2011 Society for Applied Microbiology and Blackwell Publishing Ltd