Nitrogen and carbon dynamics of a foliar biotrophic fungal parasite in fertilized Douglas-fir.

•  We investigated the nutritional dynamics of Phaeocryptopus gaeumannii and the impact of nitrogen (N) fertilization of Douglas-fir (Pseudotsuga menziesii) on the production of P. gaeumannii fungal fruiting bodies. Emergence of P. gaeumannii fungal fruiting bodies (pseudothecia) in Douglas-fir stomata has been directly linked to premature needle loss, a symptom of Swiss needle cast disease. •  Douglas-fir trees (10-yr-old) naturally infected with P. gaeumannii were treated with soil applications of N fertilizer isotopically enriched with N to increase foliar N and track the movement of N from the host to the fungus. Foliar N, free amino acids, percent of stomata occluded by pseudothecia, N isotope and carbon (C) isotope levels were assessed on treated and control trees. •  Higher foliar N resulted in increased %N and %C in P. gaeumannii, as well as increased fungal fruiting and thus disease severity. Comparisons of δ N levels between P. gaeumannii pseudothecia and associated needles indicated an increase in δ N of needles and a simultaneous decline in δ N of pseudothecia coupled with increased levels of foliar and fungal percentage N. •  These findings confirm that P. gaeumannii responds to host nutrient status and that increased N availability inside Douglas-fir needles is linked to increased severity of Swiss needle cast disease.