The profusion of dark septate endophytic fungi in non-ectomycorrhizal fine roots of forest trees and shrubs

Endophytic fungi were isolated from surface-sterilized non-ectomycorrhizal fine roots of 14 shrub and forest tree species of the northern temperate zone. With the exception of three host species, between 70 and 100% of the roots of each host yielded endophytic fungi. Species diversity was low. Thirty-five taxa were found, nine of them in more than 1% of the 421 tree/shrub individuals examined. Dark septate endophytes (DSE; Mycelium radicis atrovirens-complex) dominated the fungal assemblages of coniferous and ericaceous hosts. About one fifth of the DSE sporulated after 1 yr of incubation at 4 degrees C and in darkness, and could be identified as Phialocephala fortinii. P. fortinii could be isolated from a wide range of hosts and sites (Abies alba, Calluna vulgaris, Fagus sylvatica, Picea abies, Pinus sylvestris and Vaccinium myrtillus in Switzerland; Alnus rubra and Gaultheria shallon in Canada; Picea abies and Pinus sylvestris in Germany; Pinus sylvestris in Finland). Thus, P. fortinii seems to be neither host- nor site-specific. Four morphological types were recognized among the DSE-isolates. Type 1 was distinctly different because its serial mycelium was very sparse or absent. In contrast to other types, type 1 never sporulated. Thus, type 1 isolates probably belong to a separate taxon. Most isolates were of type 2. About one quarter of the type 2, type 3 and type 4 isolates sporulated and could be identified as P. fortinii. Types 3 and 4 are supposed to be variants of type 2. The relative proportion of each type seemed to depend on the site, but a clear pattern of host or site preference could not be recognized. Differences between types in micromorphology of colonies grown on water agar overlaid with cellophane sheets were minimal and therefore cannot be used for routine differentiation. The features of the DSE observed in root squashes corresponded well with those detected in synthesis experiments or pathogenicity tests with P. fortinii in other studies. In this study DSE were never seen in root tissues proximal to the innermost phellogen. Cryptosporiopsis radicicola was isolated most frequently from roots of F. sylvatica but constituted only a minor component of the fungal assemblages of A. alba, Pic. abies, and Pin. sylvestris.