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We examined how the interaction between the parasitic fungus Valdensia heterodoxa and its host plant Vaccinium myrtillus was affected by nitrogen (N) additions over 5 years in a boreal forest in northern Sweden. To examine whether the N effect was scale‐dependent we fertilized different sized plots (1, 10, 100, 1000 and 5000 m² in area) with N corresponding to 12.5 or 50 kg N ha⁻¹ year⁻¹. We also examined how the N effect varied with the amount of summer precipitation.

Disease incidence of the parasite increased following N addition and the effect was, on average, stronger in large than in small plots. The effect of plot size was significant for both N addition levels during the final year, but only marginally significant for the entire experimental period. Dispersal distances of ascospores and conidia were short, suggesting that high net emigration probabilities of propagules from small plots could explain …