Norway spruce needles were sampled from two series of stand areas located in southern Finland. Both series consisted of five sampling areas in mature managed stands and one in a mature virgin stand. The proportion of spruce varied from 8 to 100% of the basal tree area and the major other species were pubescent birch and Scots pine. From each sampling area (some of which consisted of several sites) 40 mature spruces were randomly chosen and healthy looking needles of the third age class were sampled from heights of 5–8 m and incubated on water agar for isolation of endophytic fungi.

The majority of isolates were identified by their combined fatty acid and sterol profiles (FAST-profiles) as Lophodermium piceae. Tiarasporella parca was less common and Rhizosphaera kalkhoffii, Sclerophoma pythiophila, Lirula macrospora and Thysanophora penicillioides occurred occasionally. For calculations of fungal diversity all isolates were classified by their FAST-profiles into 81 groups (=operational chemotaxonomic units) according to a defined upper variation limit, i.e. an upper FAST-profile mismatch limit.

The highest percentage of endophytically infected needles was found in pure spruce stands and dense virgin stands. Location of the stand, its proportion of spruce and total basal area of trees (i.e. tree density) explained 82% of the variation of the overall infection rate. The effect of location was probably a consequence of differences in air quality between the various sampling areas.

The endophyte diversity, expressed as the number of FAST-groups per 40 spruces of each sampling area, correlated positively and statistically significantly with the percentage of needles infected or with the proportion of spruce in the stand. The highest endophytic diversity, expressed as FAST-groups per tree, was found in pure spruce and mixed virgin stands.