Competitiveness of endophytic Phialocephala fortinii s.l. - Acephala applanata strains in Norway spruce roots.

Dark septate endophytes of the Phialocephala fortinii s.l. - Acephala applanata species complex (PAC) are presumed to be the most abundant root colonizing endophytes of conifers across the Northern hemisphere. To test the competitiveness of different PAC strains, PAC-free Picea abies saplings were inoculated with five different PAC strains by planting them in pre-colonized substrates. Saplings were left to grow for six weeks and then transplanted crosswise into a substrate colonized by one of the other four strains for a further two weeks. PAC were isolated and genotyped using microsatellite markers. The power of colonization, i.e. the ability of colonizing roots already colonized by another PAC strain, and the power of retention, i.e. the ability of a resident strain of not being suppressed by an invading PAC strain, were calculated for each strain in every combination. The experiment was run twice under two different climatic conditions. Our results show that PAC strains differ (1) in their ability to colonize PAC-free, non-sterile roots, (2) in resistance against being suppressed by another PAC strain and (3) in their ability to invade roots already colonized by another PAC strain. In addition, both the PAC-PAC and the PAC-host interactions depend on the climatic conditions.