Section of Ecology, Department of Biology and Kevo Subarctic Research Institute, University of Turku, FIN-20014 Turku, Finland

Backround and objectives
Systemic fungal endophytes of some grasses increase the resistance of their host plants against herbivores or plant pathogens by producing toxic compounds, by inducing host plant's resistance mechanisms or by competing for resources [1]. This kind of a mutualistic relationship is often considered to exist also between endophytes and other host plants, for example, trees. However, fungal endophytes are a heterogeneous group both taxonomically and ecologically, for example by their infection cycle, and therefore the nature of the interaction between a fungal endophyte and a host plant may vary [2]. We studied the pattern of occurrence of endophytic fungi and a pathogenic fungus, Melampsoridium betulinum (Fr.) Kleb., in mountain birch (Betula pubescens subsp. czerepanovii (Orlova) Hämet-Ahti) leaves. Melampsoridium betulinum causes birch rust which is the most important leaf disease of birch.

Materials and methods
We analysed the frequency of endophytic fungi and the rust fungus on 120 mature mountain birch trees during three years in Finnish Lapland. The analyses were done at the end of the growing season when both endophytic fungi and the rust fungus are most abundant. Study trees were situated in two different tree gardens (altitudinal difference 180 ;m) and they represented 10 different mountain birch families. For endophyte analyses, leaf discs were cut from surface-sterilized leaves and the discs were placed on a nutrient medium. Outgrowing endophyte colonies were counted and identified. Birch rust frequency was analysed by counting the percentage of leaves infected with the birch rust fungus.

Results and conclusions
The two most commonly isolated endophytic fungi were Fusicladium sp. and Melanconium sp. which composed 70% and 10% of all endophyte colonies, respectively. Phenotypic correlations at tree-specific level among the frequencies of the two most common endophytic fungi and the birch rust fungus indicated that these three fungi were not the most frequent on the same trees. Birch trees with high frequencies of Melanconium sp. endophytes were less infected with Fusicladium sp. and the birch rust fungus. When we analysed the relationships among Melanconium sp. and other fungi at birch-family-specific level (genetic correlations), the frequencies of Melanconium sp. were high in birch families where Fusicladium sp. and the birch rust fungus frequencies were low. The negative phenotypic and genetic correlations appeared more clearly in the tree garden with higher altitude, where Melanconium sp. endophyte was more frequent; it might therefore be antagonistic to Fusicladium sp. and the birch rust fungus. However, the difference in their frequencies might also be caused by differences in their responses to tree characters. The endophytic fungi of mountain birch leaves probably infect only a small leaf area because several endophyte colonies emerged from a single leaf disk of 20 ;mm2. Therefore, antagonistic relationships among endophytic and pathogenic fungi in mountain birch leaves may not be as probable as, for example, in grasses having systemic endophytic fungi growing through the host plant. To draw deeper conclusions about the nature of the relationships among these fungi, experiments manipulating the frequencies of each fungal species should be done.

1. Clay K, 1990. Annual Review of Ecology and Systematics 21, 275-297.
2. Stone J, Petrini O, 1997. In: Carroll GC, Tudzynski P, eds. The Mycota V, Part B. Plant Relationships. Berlin: Springer-Verlag.