The interaction between epiphytic algae, a parasitic fungus and Sphagnum as affected by N and P

We report the effects of fertilisation with N and P on the infection of Sphagnum by its fungal parasite Lyophyllum palustre, the expansion of epiphytic algae and the interaction between the latter two from 1998 to 2001. We added 40 kg N ha(-1) yr(-1) or 3 kg P ha(-1) yr(-1) in a full factorial design at 4 field sites. In a greenhouse experiment we reinoculated Sphagnum to verify the identity of the fungus and its necrotic effect on Sphagnum. Lyophyllum palustre was responsible for the necrosis of Sphagnum in our experiments. Adding N induced complete necrosis of Sphagnum cuspidatum by L. palustre whereas adding P decreased the area of necrotic tissue. Disease severity was related to the N concentration in the Sphagnum capitula. In Sphagnum magellanicum and S. papillosum, infection with L. palustre resulted in defoliation of stem sections. Adding N stimulated the expansion of algae at all sites, reducing the volume of photosynthetic tissue in Sphagnum. The density of the film of algae in the treatments receiving N was a function of the frequency of defoliated Sphagnum steins. We conclude that infection with parasitic fungi and, in humid environments, the expansion of epiphytic algae, may aggravate the impact of increased N deposition or of other ecosystem disturbances that affect nutrient availability.