Summary1 Experimental additions of N to an old-growth boreal forest resulted in elevated levels of free amino acids in leaves of the dominant dwarf-shrub Vaccinium myrtillus and increased attack from a parasitic fungus, Valdensia heterodoxa.2 Glutamine additions to the leaf surface of V. myrtillus increased disease incidence by an average of almost three times compared to controls and suggested a causal connection between amino acid availability and fungal infection.3 Increased abundance of the grass Deschampsia flexuosa followed N addition but infection by the parasitic fungus, which causes premature leaf loss of its primary host V. myrtillus, explained four times as much of the variation in grass abundance as N did.4 Nitrogen deposition can have marked effects on vegetation by affecting the interaction between dominant hosts and their natural enemies. A shift in abundance of dominating species occurred within 3 years of treatment, with nitrogen loads similar to those deposited over large areas in Europe and North America, suggesting that such effects may by important for the vegetation of large areas subjected to low levels of nitrogen input.